ANNUAL REPORT OF REGIONAL RESEARCH PROJECT W-188January 1 to December 31, 1999

1. PROJECT: W-188 CHARACTERIZATION OF FLOW AND TRANSPORT PROCESSES IN SOILS AT DIFFERRENT SCALES

2. ACTIVE COOPERATING AGENCIES AND PRINCIPAL LEADERS:

Arizona A.W. Warrick, Department of Soil, Water and Environmental Sciences, University of Arizona, Tucson, AZ 85721

P.J. Wierenga, Department of Soil, Water and Environmental Sciences, University of Arizona, Tucson, AZ 85721

W. Rasmussen, Department of Soil, Water and Environmental Sciences, University of Arizona, Tucson, AZ 85721

California M. Ghodrati, Dept. of Env. Sci. Pol. Mang., University of California, Berkeley, CA 94720-3110

J.W. Hopmans, Dept. of LAWR, Hydrologic Science, University of California Davis, CA 95616

W.A. Jury, Dept. of Envir. Sciences, University of California, Riverside, CA 92521

F. Leij, U.S. Salinity Lab - USDA-ARS, Riverside, CA 92507-4617

B. Mohanty, U.S. Salinity Lab - USDA-ARS, Riverside, CA 92507-4617

D.R. Nielsen, Dept. of LAWR, Hydrologic Science, University of California Davis, CA 95616

D.E. Rolston, Dept. of LAWR, Soil and BioGeochemistry, University of California Davis, CA 95616

P.J. Shouse, U.S. Salinity Lab - USDA-ARS, Riverside, CA 92507-4617

J. Simunek, U.S. Salinity Lab - USDA-ARS, Riverside, CA 92507-4617

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T. Skaggs, U.S. Salinity Lab - USDA-ARS, Riverside, CA 92507-4617

M.Th. van Genuchten, U.S. Salinity Lab - USDA-ARS, Riverside, CA 92507-4617

L.Wu, Dept. of Envir. Sciences, University of California, Riverside, CA 92521

Colorado L.R. Ahuja, USDA-ARS, Great Plains System Research Unit Fort Collins, CO 80522

T. Green, USDA-ARS, Great Plains System Research Unit Fort Collins, CO 80522

G. Butters, Dept. of Agronomy, Colorado State University, Ft Collins, CO 80523

Delaware Y. Jin, Dept. of Plant and Soil Sciences, University of Delaware, Newark, DE 10717-1303

Idaho J.B. Sisson, EG&G, Idaho National Engin. Lab., Idaho Falls, ID 83415-2107

J. Hubbel, EG&G, Idaho National Engin. Lab., Idaho Falls, ID 83415-2107

Illinois T.R. Ellsworth, University of Illinois, Urbana, IL 61801

Indiana J. Cushman, Mathematics Dept., Purdue University, W. Lafayette, IN 47905

P.S.C. Rao, School of Civil Engineering, Purdue University, W. Lafayette, IN 47905

Iowa R. Horton, Dept. of Agronomy, Iowa State University, Ames, IA 50011

D. Jaynes, National Soil Tilth Lab, USDA-ARS, Ames, IA 50011

Kansas G. Kluitenberg, Dept. of Agronomy, Kansas State University, Manhattan, KS 66506

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Kentucky E. Perfect, Dep. of Agronomy, University of Kentucky, Lexington, KY 40546

Montana J. M. Wraith, Land Resources and Environ. Sciences, Montana State University, Bozeman, MT 59717-3120

Nevada S.W. Tyler, Hydrologic Sciences Graduate Program, University of Nevada, Reno, NV 89532

G. Wilson, Desert Research Institute, University of Nevada, Reno, NV 89512

New Mexico J.H.M. Hendrickx, New Mexico Tech, Dept. of Geoscience, Soccoro, NM 87801

North Dakota R. Knighton, Dept. of Soil Science, North Dakota State University, Fargo, ND 58105-5638 (until June 1999)

Utah D. Or, Dept. of Plants, Soils & Biomet., Utah State University, Logan, UT 84322

WashingtonM. Flury, Dept. of Crop & Soil Sciences, Washington State University, Pullman, WA 99164

J. Wu, Dept. of Biological System Engineering, Washington State University, Pullman, WA 99164

Wyoming R. Zhang, Dept. of Renewable Resources, University of Wyoming, Laramie, WY 82071

CSREES R. Knighton, USAD-CSREES, Washington, DC 20250-2200

Adm. Adv. G.A. Mitchell, Palmer Research Center, 533 E. Fireweed, Palmer, AK 99645

3. PROGRESS OF WORK AND MAIN ACCOMPLISHMENTS: OBJECTIVE 1: To study relationships between flow and transport properties or processes and the spatial and temporal scales at which these are observed

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At the University of California, Davis, a study was conducted to develop a numerical model, which can more accurately simulate water transport during evaporation. The developed model will be incorporated into a pesticide fate and transport model. To improve the accuracy of the model, the water vapor diffusion was included in the general equation of water transport. The developed model was verified by the data obtained in column experiments. The experiments were designed to measure water evaporation and diazinon [O,O diethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate] volatilization at the same time. Diazinon was incorporated into the surface soil in the column. Two levels of initial water content were used. Wet air and dry N2 was alternately passed across the soil surface. Water evaporation was measured by weighing the column. Dynamic change of soil water content and relative humidity at various depths were measured by a Time Domain Reflectometry (TDR) system and a hygrometer, respectively. The modeling and measured results agree well.

The University of California, Riverside, in collaboration with Markus Flury at Washington State University, completed three solute transport studies that are relevant to spatial or temporal scaling. In the first, a theoretical examination was conducted of the relationship between transport observed in field lysimeters and that in the natural environment. It was shown that the artificial lower boundary condition imposed by the lysimeter drainage affects transport compared to the free drainage condition in undisturbed soil. The second study was a bromide transport experiment conducted in 16 undisturbed lysimeters at two flow rates. Significant results were: i) the demonstration that the 1.1 m2 area of the lysimeters was too small to encompass all of the variability of the soil type; 2) the flow was significantly different at the two flow rates, so that it did not scale as a function of net applied water; and 3) a flow and transport model was unsuccessful at reproducing the transport observed. The third study was a theoretical characterization of residence-time-dependent reactions using a transfer function formulation, which avoids the need for cumbersome numerical calculations. This method provides a convenient framework for analyzing problems in which the transport or reaction characteristics depend not on elapsed time but rather time spent in the system.

At the University of Wyoming, geostatistical methods, kriging and cokriging, were applied to estimate sodium adsorption ratio (SAR) in a 3375 ha agricultural field. In cokriging, more easily measured data of electrical conductivity (EC) were incorporated to improve the estimation of SAR. The estimated spatial distributions of SAR using the geostatistical methods with various reduced data sets were compared with the extensive salinity measurements in the large field. The results suggest that sampling cost can be dramatically reduced and estimation can be significantly improved using cokriging. Compared with the kriging results using total SAR data, cokriging with reduced data sets of SAR improves the estimations greatly by reducing mean squared error and kriging variance up to 70% and increasing correlation of estimates and measurements about 60%. The sampling costs for SAR estimation can be reduced approximately by 80% using extensive EC data together with a small portion of SAR data in cokriging.

At Washington State University, the relationship between flow and transport properties and their spatial and temporal scales has been studied in several related projects. Experimental, theoretical and numerical analyses were carried out to examine (1) residence-time dependency of degradation coefficients, (2) pesticide leaching in heterogeneous fields, (3) effects of flow rates and water contents on longitudinal and lateral dispersion, (4) particle-size distributions in soils

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and their mathematical representation, (5) consequences of land retirement strategies on water flow dynamics and salinization in the San Joaquin Valley, and (6) water erosion in forest and agricultural watersheds.

Washington State University, in collaboration with the University of California, Riverside, investigated the effects of travel or residence-time dependent reaction rate coefficients on solute transport. A theoretical approach was developed to describe transport with residence-time-dependent sink/source reaction coefficients. The solution to the transport problem with an arbitrary functional form for the reaction term is expressed in terms of the solution to the non-reactive transport problem. The solution is therefore independent of the nature of the transport process, and independent of any specific representation of the reaction coefficients.

Numerical simulations were used to compare solute transport in field soils and in lysimeters. Simulations were carried out in homogeneous sandy and loamy soils under steady-state, unsaturated water flow conditions. Water flow was described by the Richards' equation and solute transport by the advection-dispersion equation. The effect of linear and nonlinear, and instantaneous and kinetic sorption was investigated. The results showed that for a conservative solute the differences between field soil and lysimeter increase as the coarseness of the soil increases. Decreasing water flux increases the difference between field soil and lysimeter. For solutes subject to linear equilibrium sorption, the sorption mechanism compensates the effects of the lower boundary condition.

A small-scale field experiment, carried out between 1996 and 1998, was analyzed in regard of longitudinal and lateral dispersion. Two-dimensional concentration distributions were used to derive the effects of water contents and flow rates on longitudinal and lateral dispersion. The modeling results indicate that longitudinal and lateral dispersion increase with decreasing water content of the soil. Dispersivities showed a dependency on flow rates and amount of cumulative infiltration, but this dependency appeared to be related to the degree of irregularities of observed flow patterns. Large dispersivities were associated with higher degree of irregularities in the flow patterns, and vice versa. Layer boundaries played a significant role for redirecting flow when flow rates were high and cumulative infiltration was large.

Particle-size distributions (PSDs) of soils are often used to estimate other soil properties, such as soil moisture characteristics and hydraulic conductivities. Prediction of hydraulic properties from soil texture requires an accurate characterization of PSDs. The objective of this study was to test the validity of a mass-based fragmentation model to describe PSDs in soils. It was found that a single power-law exponent could not characterize the PSD over the whole range of the measurements. Three main power-law domains were identified. The boundaries between the three domains were located at particle diameters of 0.51+-0.15 and 85.3+-25.3 micrometer.

Utah State University plans to collaborate with Dr. Wraith (MSU) to further investigated temperature effects on TDR measurements and the potential for using this Thermodielectric effect for estimation of soil surface area. The physically-based model for the phenomenon will be further refined to better describe the observed temperature response. A theoretical study on using the Thermodielectric model with radar backscattering models towards developing correction factors for remotely sensed water content information, and for possible remote delineation of different soil textural covers, will be conducted.

Iowa State University and the National Soil Tilth Laboratory investigated solute

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transport by applying a sequence of non-interacting, conservative tracers (Br and 3 benzoates) and the herbicide atrazine during a low intensity irrigation (4 mm/hr) of a 24 x 34 m plot in a Nicollet loam under continuous corn, no-till management. A different tracer was added before and every 2 hr. after the start of irrigation. Solute leaching was monitored by measuring the outflow quantity and quality from a 120-cm deep subsurface drain and by taking soil cores after the irrigation. Mass recoveries of tracers ranged from 82% for atrazine to 104% for Br. Mass leaching losses ranged from less than 1% for atrazine to over 6% for the conservative tracers. Arrival times for tracers to tile drainage followed a distinct trend starting at 112 min. for atrazine and Br that were both applied before irrigation started and decreasing to only 15 min. for the last conservative tracer applied. These times corresponded to water application depths of 8 and 1 mm, respectively. Two-dimensional modeling of the system confirmed that the observed patterns of tracer movement could not be explained by Richard’s equation/CDE approaches. Thus, a small fraction of the tracers traveled to the tile via preferential pathways and these pathways were not stable over the duration of the irrigation. Bimodal pore distribution models, MIM models, and other approaches will be tested against this data set.

At the University of Kentucky, steady-state transport of water and chloride is being investigated at three different spatial scales (1.3x10-4, 5.9x10-2, and 1.0x101 m3). The purpose is to establish an empirical relationship between the dispersivity of a saturated porous medium and its pore space geometry, as quantified by fractal parameters determined from the water retention curve. Experimental and computer modeling approaches are being employed towards this end. In the experimental approach, paired solute breakthrough and water retention curves are obtained for undisturbed porous media under both laboratory and field conditions. The resulting data sets are parameterized and related to each other using regression analysis. Preliminary results at the 1.3x10-4 m3 scale indicate that dispersivity increases exponentially as the mass fractal dimension of the medium approaches three. The computer simulations are being conducted on virtual 2-d and 3-d random porous media, generated using a fractal algorithm. Variation in the percolation thresholds of such media is being investigated using the Hoshen-Kopelman algorithm. It is also planned to obtain simulated solute breakthrough and water retention curves using a lattice-gas cellular automaton coupled with particle tracking. These data sets will be analyzed in the same way as the experimental results. This research should result in an increased capability to predict the transport of solutes at different spatial scales under steady state, saturated flow conditions. This improved predictive capability might then be used to develop improved guidelines for locating agricultural and industrial waste disposal/storage facilities. If implemented, such guidelines could lead to significant improvements in ground water quality.

The Colorado State University and USDA-ARS, Fort Collins have focused on measurements to assess the soil water dynamics in a dryland (rainfed) field in northeastern Colorado. Previous analyses of yield monitoring in this field under winter wheat in 1997 showed that the specific catchment or contributing area explained 60% of the variance in yield. Crop yield over a quarter section varied substantially with a variance of 320 bu2/ac2. The self-affine ("simple scaling") fractal described the spatial variability fairly well with a fractal dimension of 1.79 (two other fields had dimensions of 1.80 and 1.82). This indicates a degree of antipersistence in space, meaning that yield is likely to rise or fall rapidly over a short distance, and that new sources of variability contribute to the variation in yield at different, nested scales.

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Knowledge of this fractal geometry makes it possible to scale the yield variation within the range of analysis (102 to 104 m2). The common fractal dimension of approximately 1.8 for these three field shows an apparent regionality of the result, but temporal stability remains to be shown. The present field experiments are designed to investigate the causes of variability in soil water and associated crop yield (foxtail millett in 1999), while numerical simulations may help us gain a more quantitative, process-based understanding of the relative contributions of overland and subsurface lateral flow.

At the US Salinity Laboratory, the windows-based HYDRUS-1D and HYDRUS-2D software packages, were made more useful by (1) including the effects of immobile water to enable the simulation of preferential flow through the unsaturated zone, (2) incorporating pedotransfer functions to enable users to more rapidly estimate the hydraulic input parameters for specific applications, (3) coupling the codes with parameter estimation subroutines to enable users to calibrate the codes to site-specific conditions, and (4) developing more sophisticated 32-bit graphical user interfaces (GUIs) for the codes, as well as including more extensive help files. The HYDRUS codes have been applied to a large number of agricultural problems (infiltration, tile drainage design, crop production,fate and transport of agricultural chemicals in the subsurface), as well as to many problems in the general area of soil and groundwater pollution involving non-agricultural chemicals (such as radionuclides and contaminants released from industrial and municipal waste disposal sites). The coupling of the HYDRUS codes with parameter estimation subroutines enables the inverse estimation of a variety of hydraulic and solute transport parameters from laboratory and/or field experiments. The parameter estimation options provide much more effective methods for estimating the unsaturated soil hydraulic properties from relatively standard infiltration, multistep outflow and evaporation experiments.

Reliable estimates the unsaturated soil hydraulic properties (water retention, hydraulic conductivity) are needed in many hydrologic, subsurface pollution and crop production studies; unfortunately, these properties are very difficult to measure rapidly and accurately in the field. As an alternative, they can be estimated indirectly using pedotransfer functions (PTFs) from more easily measured soil survey type data such as soil texture and bulk density. USSL developed a hierarchical neural network-based PTF approach that is both flexibility (by providing better estimates when additional information is available) and accurate (neural networks provide the best possible models). The approach also leads to uncertainty estimates, and hence gives insight into the reliability of the predictions. This research resulted in a computer software package, called Rosetta, that may be used to estimate the unsaturated soil hydraulic properties from more easily measured or more readily available soil survey type data. Users are able to download the software from the home page (www.ussl.ars.usda.gov) of the U.S. Salinity Laboratory in Riverside, CA. The information should help users to obtain better estimates for infiltration, drainage, leaching, and surface runoff or related flow processes, for specific soil types.

Direct measurement of the soil-water retention curve is difficult and time consuming. Because the shape of the retention curve is similar to that of the soil particle-size distribution function, methods have been developed for predicting the retention curve from more easily measured particle-size data. Often times, however, it is necessary to estimate retention properties when only soil texture data (percent sand, silt, and clay) is available, rather than the full particle-size distribution. W188 members at USSL conducted a study to determine if it is possible to

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predict the particle-size distribution from soil texture data. Procedures were developed for relating texture data to parameter values in several candidate models of the particle-size distribution, including a power law distribution and a logistic distribution. The estimation procedures were tested on approximately 100 soils from the SGP97 and UNSODA databases. The results were mixed, with some of the predicted particle-size distributions being very accurate, while others were somewhat poor. Ongoing efforts are aimed at refining the estimation procedures and making explicit the conditions under which accurate predictions can be expected. Further research is required to determine if the predicted particle-size distributions are useful for predicting retention properties.

Network models of randomly sized capillary tubes are commonly used as surrogate media in theoretical investigations of the transport properties of soils and rocks. The conductivity of network models can be calculated by critical path analysis, a method based on the connectivity of highly conducting pathways and the statistics of percolation theory. USSL used percolation cluster statistics and critical path analysis to derive an analytical expression for the expected value of the hydraulic conductivity as a function of system size, and have numerically verified the theory. They also derived a relationship between the expected values of the hydraulic and electrical conductivities.

Soil moisture is an important state variable in the hydrologic cycle, and its spatio-temporal distribution depends on many geophysical processes operating at different spatial and temporal scales. To achieve a better accounting of the water and energy budgets at the land-atmosphere boundary, it is necessary to understand the spatio-temporal variability of soil moisture under different hydrologic and climatic conditions, and at a hierarchy of space and time scales. During the Southern Great Plains 1997 (SGP97) Hydrology Experiment, W188 members at USSL and UC-Riverside measured the 0 to 6 cm soil water content on consecutive afternoons at four hundred locations in a small, gently sloping range field (LW07). The soil moisture measurements were made using portable impedance probes. Spatio-temporal data analyses of the two sampling events showed a significant change in the field variance but a constant field mean, suggesting moisture was redistributed by (differential) base flow, evapotranspiration, and condensation. Among the different relative landscape positions (hill-top, slope, valley), the slope was the largest contributor to the temporal variability of the soil moisture content. Using a sequential aggregation scheme it was observed that the relative position influencing the field mean and variance changed between the two sampling events, indicating time-instability in the spatial soil moisture data. Furthermore, high resolution (impedance probe) sampling and limited (gravimetric) sampling gave different field means and variances.

At the University of Nevada, Reno, emphasis has been put on quantifying the effects of spatial variability in natural watersheds and engineered materials on pollutant fluxes. In natural watersheds, the nature of nitrogen fluxes (both mineral and organic) in forested catchments in the Lake Tahoe, CA/NV area was investigated. Integrated ground water discharge measurements were made to determine the sources and averaging of surface spatial heterogeneity. In addition, focus was put on the role of dissolved organic nutrients in the overall nutrient budgets of forested catchments.

In engineered materials, Nevada, in collaboration with the U.S. Salinity Laboratory in Riverside, has investigated the transport of dissolved metals and oxyanions in mine waste material from precious metal mining. Significant public lands have been impacted by the

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disposal of mine wastes containing environmental pollutants. Transport, via infiltration through these wastes is extremely complex, due both to the poorly sorted characteristics of the "soil-like" waste and by the highly heterogeneous chemical reactivity of the waste materials. Specifically, Nevada and USSL have modified the numerical code UNSATCHEM to include arsenic geochemistry in the reactive transport of water and solutes through unsaturated media. Arsenic is a common contaminant found at mine sites and its fate from mine waste material is critical to the environment.

OBJECTIVE 2: To develop and evaluate instrumentation and methods of analysis for characterization of flow and transport at different scales

At the University of California, Davis, a combined penetrometer-moisture probe was developed to study the influence of water content on soil strength. The coiled TDR moisture probe consists of a 2 parallel copper wires, each 0.8 mm diameter and 30 cm long, coiled around a 5 cm long PVC core with a 3 mm separation between wires. The performance of the combined probe was compared with a conventional 2 parallel TDR probe for a Columbia fine sand loam, a Yolo silt clay loam and washed sand. Calibration curves relating the soil bulk dielectric constant measured by the coiled probe with water content were obtained in the laboratory and data were analyzed based on mixing model approaches. Bulk dielectric values for at least two soil materials were affected by soil compaction around the probe. Field experiments were carried out to measure simultaneously the penetration resistance and water content along the soil profile. Results showed a detailed water content profile with excellent correlation with the gravimetric method.

X-ray Computed Tomography (CT) is a non-invasive technique that allows for three- dimensional, nondestructive imaging of heterogeneous materials. It has become evident that the methods we use to measure the hydraulic properties can have a significant impact on the obtained results. In particular, a dependence on drainage outflow rate has been observed, suggesting interference from dynamic phenomena such as air and water entrapment. The main objective of this study is to use the microtomography capabilities of beamline 13-BM of the Advanced Photon Source (APS, cyclotron) at Argonne National Laboratory (Chicago), to observe spatial and temporal distribution of air and water phases during drainage experiments. X-ray computed tomography (CT) offers an advantageous possibility to non-invasively investigate these dynamic processes on the pore scale. We are using the APS scanning capabilities to investigate the spatial distribution of air and water in a soil sample when exposed to various outflow scenarios. We aim to explain differences in the obtained hydraulic characteristics by variation in air distribution (and potential discontinuity of the water phase) with varying outflow conditions. By using soil cores of 0.5-2 cm diameter, we obtained three-dimensional images of the soil core at spatial resolutions of 5-50 micrometer

The University of California, Riverside, is in the process of developing a field study of preferential flow that will involve an analysis of the relation between flow rate, water content,

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and finger formation and propagation. An important part of the analysis will be the observation of the finger formation process during and after infiltration ceases and redistribution begins.

Typical methods of irrigation scheduling are either based on soil/plant monitoring, or ET estimation. However, soil/plant measurements reflect the status of the soil/plant condition at the time of measurement, and can not accurately predict when irrigation is needed. Additionally, irrigation scheduling based on ET estimates is not soil- or site-specific. From an agricultural production standpoint, the dissolved salts or salinity of soil water, commonly measured by electrical conductivity (ECw) is the critical parameter. The bulk electrical conductivity (ECb) measured by TDR in the field did not have a direct effect on plant growth. A capillary model assuming that liquid phase and solid-surface conductivity (ECs) via exchangeable cations behave as resistors in parallel developed by Rhoades et al. (1976) was used to relate ECb and ECw.

State-space models based on site-specific measurements were used to forecast the soil water and salinity regime, which provide a flexible irrigation scheduling for irrigation managers. Periodic measurement of soil water content and ECb can be used to forecast the future field soil-water and salinity regime.

At the University of Wyoming, a hybrid method of Laplace transform and finite element method is developed to solve one- and two-dimensional convection-dispersion equations. The method is semi-analytical in time through Laplace transform. Then the transformed partial differential equations are solved numerically in the Laplace domain using the finite element method. Finally the nodal concentration values are obtained through a numerical inversion of the finite element solution, using a highly accurate inversion algorithm. The proposed method eliminates time steps in the computation and allows using relatively large grid sizes, which increases computation efficiency dramatically. Numerical results of several examples show that the hybrid method is of high efficiency and accuracy, and capable of eliminating numerical diffusion and oscillation effectively.

Kansas State University has continued to develop and evaluate the dual-probe heat-pulse (DPHP) method for measuring soil volumetric water content (). An experiment was performed during the past year to evaluate the DPHP method in a controlled laboratory setting. Eight different soil materials were used with a broad range of physical properties: sand contents from 11 to 97%, clay contents from 0.4 to 38%, organic matter contents from 0.9 to 4.8%, and bulk densities from 1.10 to 1.45 Mg m3. Each of 8 Tempe pressure cells (8.55-cm diameter) was fitted with 2 DPHP sensors. Sensors were connected to a data acquisition and control system for unattended data acquisition, processing, and storage. DPHP sensors were used to measure during desorption sequences from saturation to 100kPa. Measurements at potentials below 100kPa were obtained by packing soil to target water contents and densities in individual pressure cells. Independent measurements of were obtained from mass changes of the pressure cells. The measurement procedure was repeated 2 times, so that data was collected with 4 DPHP sensors for each soil material. The DPHP sensors used in this experiment were constructed with thermistors instead of thermocouples for measuring the temperature rise in response to heating.

The combined results for all soil materials indicate good agreement between the DPHP and pressure-cell methods for measuring . A general calibration relationship can be used to account for the bias towards overestimation of at lower water contents. Thus, soil-specific calibration appears to be unnecessary. The relatively small root mean square error (RMSE) indicates that excellent precision can be achieved with the DPHP method in a laboratory setting.

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A manuscript summarizing this work is in draft.An experiment was performed during the past year to evaluate the DPHP method for

measuring in a field setting. Six monitoring stations were distributed in a sub-watershed of the Black Vermillion River (northeast Kansas) at locations with a variety of soil, crop, and tillage conditions. Each monitoring station was equipped with a data acquisition and control system for unattended data acquisition, processing, and storage. At each monitoring station, in situ measurements of were obtained at 3-h intervals with 5 DPHP sensors buried horizontally, 10 cm below the soil surface. Precipitation and were monitored continuously for 80-100 days at each station. Independent measurements of were obtained at all stations by periodically collecting soil samples with a sampling ring (3-cm height, 5-cm diameter) centered at the 10-cm depth. Water content comparisons were made by pairing the average from the 10 soil samples with the average from the 5 DPHP sensors for each monitoring station and sampling date. The DPHP sensors used in this experiment were constructed using thermocouples for measuring the temperature rise in response to heating. Results indicate that DPHP sensors can be used to measure with high precision (RMSE = 0.027 m3 m3) in a field setting.

Collaboration with Iowa has resulted in the development of a new method for measuring soil water flux density. In this method a heat tracer is used to quantify the magnitude of convective heat transfer caused by soil water movement. The magnitude of the soil water flux density is then estimated from the magnitude of the convective heat transfer. A preliminary report on this work was given at our last meeting. The work was completed during the past year, and a manuscript is in press (Ren et al., 2000).

Additional work conducted during the past year (collaboration with Arizona) has focused on simplifying evaluation of the analytical solutions in the method presented by Ren et al. (2000). We have shown that all of the integrals in their solutions can be reduced to the integral

which is known as the well function for leaky aquifers. This function has been extensively tabulated and thus provides a convenient method for evaluating the solutions in Ren et al. (2000). It is also known that W(u,) can be approximated by the exponential integral for small values of . This provides another useful approach for evaluating the analytical expressions in Ren et al. (2000).

Collaboration with K. L. Bristow (CSIRO, Townsville, Australia) has resulted in the development of a four-probe heat-pulse sensor that permits measurements of , electrical conductivity (EC), and temperature. Two of probes (center two) are employed in the usual way to determine with the DPHP method. One of these probes contains a heater and the other a thermocouple (or thermistor) for measuring the temperature rise in response to heating. The thermocouple also can be used to measure absolute temperature prior to heating. The four parallel probes also are used as electrodes that form a classical Wenner array for measuring EC. A brief report on the development and evaluation of the four-probe heat-pulse sensor is included in a review article (Bristow et al., 2000) that has been submitted for publication.

Washington State University is developing an experimental and theoretical methodology to determine the moisture characteristics from freezing experiments. Soils and porous media of different composition are tested and measurement results compared with results

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obtained with classical methods. When a water-containing porous medium freezes, a fraction of the water in the medium remains in the liquid state. As the temperature of the medium changes, the unfrozen water fraction changes. The relationship between unfrozen water content of a medium and its temperature is called the freezing characteristic. Thermodynamic theory provides a link between temperature of frozen materials and water potential or activity of unfrozen materials; and may therefore provide a means to obtain the soil moisture characteristic from a freezing characteristic.

At Utah State University a study focusing on the roles of adsorption and capillary condensation in variably-saturated porous media was initiated. Utah State plans to use new models for pore space geometry comprising an angular pore cross section connected to slit-shaped spaces for a more realistic representation of natural pore spaces. The pore scale model will then be upscaled to represent a sample scale retention properties. Work is underway to extend the calculations to estimation of unsaturated hydraulic conductivity.

Iowa State University and the National Soil Tilth Laboratory have been evaluating a new method for simultaneous, nondestructive measurement of soil water content and bulk density. The method is based on the fact that soil heat capacity is determined primarily by soil water content and bulk density. Therefore, with measures of soil water content and heat capacity, we can determine bulk density. A thermo-TDR probe utilizes a heat pulse technique to estimate heat capacity and time-domain reflectometry to estimate water content. W-188 members evaluated the method on packed columns of sandy loam soil over a range of bulk densities and water contents. Estimates of water content and bulk density obtained from the probe were compared to gravimetric measures. Thermo-TDR measured soil water contents were within 0.03 m3 m-3 of gravimetric values on 88% of the samples. Thermo-TDR measured bulk density was within 0.15 Mg m-3 on 94% of the samples and within 0.10 Mg m-3 on 59% of the samples. Using the thermo-TDR measured water content and bulk density, and assuming a particle density, W-188 members were also able to estimate porosity, air-filled porosity, and percent saturation. 94% of the thermo-TDR measured porosities and air-filled porosities were within 0.05 m3 m-3 of the gravimetrically determined values. 65% of the thermo-TDR determined percent saturations were within 5 percentage points of the gravimetrically determined values. W-188 members are further testing the method on packed columns of clay loam soil. This new use of the thermo-TDR is one demonstration of the potential of combined thermal and electrical pulses for nondestructive measurement of soil properties and for monitoring soil processes. This method is fully adaptable to field settings.

Simple field applicable methods to measure solute transport properties using time domain reflectometry (TDR) were developed and evaluated by laboratory experiments. One method using a series of fluorobenzoate tracers based on resident concentrations for estimating two domain solute transport model parameters (immobile water content, im, mass exchange coefficient, , and dispersion coefficient, Dm) has been successfully used to characterize solute transport in soil. However, the method is time consuming and expensive. We developed and evaluated simple TDR methods for obtaining im, , and Dm. The TDR methods use a 20-cm vertically installed probe, or a 8-cm probe installed diagonally into 2-cm surface soil. We compared the estimates of im, and Dm from the new method versus those obtained from effluent breakthrough curve data and the fluorobenzoate tracer method. The TDR methods can be applied to determine spatial and temporal distributions of solute transport properties as well

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as water content simultaneously at multiple field locations in a period of time.A field method was developed in order to study field water and chemical transport

properties. The experimental setup used a set of dripper lines with pressure-compensated drippers as point sources. With this setup, desired discharge rates, of either water or chemical solutions, could be applied to the soil surface at multiple locations at the same time. Each flow (or discharge) rate was applied until steady-state conditions occurred; i.e., when the ponded area beneath each dripper reached a constant size. The steady state radius, of each location, was then measured and recorded. Plotting the infiltration rates per unit area (q) versus the reciprocal of the ponded radii (1/rs) usually produced a straight line (a linear relationship). Wooding's solution (1968) was used to determine Ks and , while the solution of Jaynes et al. (1995) was the basis for determining m/ and . The proposed method was evaluated on a greenhouse soil-pit. Six locations were tested on the soil pit. Four different flow rates; 2, 4, 8, and 12 L/hr, were consecutively applied to the soil surface at each location. At the 12 L/hr rate, four flourobenzoic acids were applied sequentially at each location to evaluate chemical transport properties. In addition to the greenhouse study, field measurements were obtained. Five transects, 15 m each, were located in a corn field. Dripper lines were used on each transect. A total of fifty sites were included for hydraulic and chemical transport properties. Hydraulic properties were evaluated on compacted and non-compacted rows by applying three different flow rates; 2, 4, and 8 L/hr. Chemical transport properties were determined by sequential application of flourobenzoic tracers and by using a TDR technique. TDR probes were installed diagonally from the surface to about the 2 cm depth. A background and step solutions of CaCl2 were used to evaluate m/ and from the TDR measurements. Analysis of the field data is ongoing.

At the University of Arizona, field work on the large field project at the Maricopa Agricultural Center (MAC) has been stopped for now. This research project had three goals: (1) an objective assessment of state-of-the-art monitoring systems that are currently being used, or proposed for use at low-level radioactive waste (LLW) disposal facilities and decommissioned facilities designated under the Site Decommissioning Monitoring Plant (SDMP), to detect early releases of radio nuclides to the environment; (2) how best to set up these monitoring systems into an overall strategy for LLW and SDMP sites; and (3) an evaluation of relevant strategies for monitoring flow and transport in the vadose zone. We also seek to understand how, or if, the monitoring systems compromise the integrity of the soil material they are monitoring. Present efforts are directed toward a more detailed analysis of the flow and transport data, and some model testing.

At the Colorado State University and USDA-ARS, Fort Collins, an automated (time domain reflectometry) TDR system has been developed, which is mounted on an ATV with hydraulics for rapid probe insertion and allows on-the-go measurement of near-surface soil water contents. This field-based system has taken up to 350 TDR measurements per day along predetermined (radial) geometries suitable for geostatistical and fractal analyses of the spatial variability. The researchers from Colorado procured and designed the field installation of a system of automated capacitance probes (EnviroSCAN system by Sentek) fully buried for multi-season, automated measurement of soil water contents along a transect with significant topographic undulation, including both north and south aspects. A finite-element numerical process model for water movement in variable landscapes was modified. It simulates both overland (kinematic wave) and subsurface (Richards’ equation) water movement, initially along

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the instrumented transect. This work is investigating the dominant processes responsible for observed space-time variability in soil water content related to crop growth.

W188 members at US Salinity Laboratory in collaboration with the UC-Riverside developed a new methodology to directly measure the porosity and its microscopic characteristics. The methodology is based on the analysis of binary images collected with a backscattered electron detector from thin sections of soils. Pore surface area, perimeter, roughness, circularity, and maximum and average diameter were quantified in 36 thin sections prepared from undisturbed soils. Saturated hydraulic conductivity (Ks), particle size distribution, particle density, bulk density and chemical properties were determined on the same cores. We used the Kozeny-Carman equation and a combined neural network and bootstrap analysis to predict the formation factor from microscopic, macroscopic, and chemical data. The predicted Ks was in excellent agreement with the measured value (R2=0.91) when a hydraulic radius defined as rH =pore area/pore perimeter and the formation factor were included in the Kozeny-Carman equation.

OBJECTIVE 3: To apply scale-appropriate methodologies for the management of soil and water resources

The University of California, Davis, developed and tested a two-dimensional root water uptake model, which can be incorporated in numerical multidimensional flow models. The two-dimensional uptake model is based on the model by Raats (1974), but is extended with a radial component. Subsequently, the root water uptake model was incorporated in a two-dimensional flow model, and root water uptake parameters were optimized, minimizing the residuals between measured and simulated water content data. Water content was measured around a sprinkler-irrigated almond tree for a 16-day period at 25 locations, following irrigation. To calibrate the flow and root water uptake model, a genetic algorithm was used to find the approximate global minimum of the optimized parameter space. The final fitting parameters were determined using the Simplex optimization algorithm. With the optimized root water uptake parameters, simulated and measured water contents values during the 16-day period were in excellent agreement, with R2 values ranging between 0.94 and 0.99 and a root mean squared error of 0.013 m3 m-3. The developed root water uptake model is extremely flexible and allows spatial variations of water uptake as influenced by non-uniform (drip irrigation) and uniform water application patterns.

One-dimensional vertical convective solute models and solutions for vadose zone transport with and without root water uptake are presented, assuming negligable mechanical dispersion and molecular diffusion. Although potentially simplistic, the type of solutions presented can be extremely useful for longer-term and watershed-scale characterization of salinity and/or contaminant movement. Possible solutions for transient and gravity flow are discussed, whereas analytical solutions for steady flow are presented using the method of characteristics for three different root water uptake distribution functions. It is illustrated how these analytical solutions can describe transient solute transport in the root zone underlain by a shallow water table. For cases of both upward and downward steady state flow, advantages of the presented solutions are computational speed and absence of numerical dispersion and

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oscillation. Finally, it is discussed how the convective transport models presented here can be used for field-scale solute transport modeling by incorporating soil heterogeneity in a stochastic-convective approach.

At the University of California, Riverside, substantial effort has been spent in the last year studying the reactions and transport of selenium species in the laboratory and relating the findings to the behavior of selenium in a large-scale reclamation project underway at Kesterson, CA to methylate and volatilize selenium residues from contaminated soil. Significant findings include: (i) the discovery that under normal conditions in soil Se demethylates soon after being formed, but that with the addition of an alternative carbon source the demethylation step can be blocked; and (2) Se transport by leaching is limited by the rapid transformation of the more mobile forms to the immobile elemental Se form, which is a stable end state.

A new project was initiated to implement agricultural best management plan (BMP) within the Newport Bay Watershed, CA. This project deals with agricultural operations (includes nursery operations) in the Newport/San Diego Creek. The main objective of the project is to evaluate the effectiveness of implementing Management Measures (MMs) with the long-term goal of meeting nutrient total maximum daily loads (TMDLs) established by the Regional Water Quality Control Board. MMs will be implemented through workshops and individual consultations with growers (Phase II) and for monitoring and analyzing the effectiveness of MM implementation on improving surface runoff quality from representative sites (Phase III). The Phase I baseline monitoring of agricultural operations and an Agricultural Nutrient Management Plan (ANMP) will be completed prior to the initiation of this project. Five sites (1 nursery, 1 orchard, 2 strawberry fields, and 1 celery field) were selected to monitor the nutrient runoff. Each site has a control and a treated plot. Flow from each plot will be measured by directing surface runoff through a 4' long 8" PVC pipe with an area-velocity flow meter allowing for continuous monitoring of flow. Water samples will be collected using auto-samplers during an irrigation or rainfall event, if runoff is present. Composite water samples collected during irrigation events will be analyzed for pH, ECw, NO3-N, NH4-N, TKN, P-Total and P-Ortho.

Washington State University investigated the potential of chitosan, a natural beta-1-4-linked glucosamine polymer, to reduce plant transpiration. Chitosan was applied foliarly to pepper plants and water use was monitored. Peppers were grown in pots in a growth-chamber and a green-house, where transpiration was measured by weighing pots. In an accompanying field study, water use was determined by soil moisture measurement with TDR and an automated irrigation system. Foliar application of chitosan reduced water use of pepper plants by 13, 26 and 43% for green-house, growth-chamber and field conditions, respectively. Yields were only marginally affected by chitosan treatment. Scanning Electron Microscopy and histochemical analysis confirmed that chitosan induced partial closure of stomata.

In an effort to adapt the process-based, watershed scale hydrology and erosion model, WEPP (Water Erosion Prediction Project), to forest watershed applications, Washington State University identified problems in the original WEPP codes (version 98.4). Modifications were made to the subroutines for simulating hydraulics of various hydraulic structures associated with road systems in forest watersheds. The modified WEPP model was rigorously tested. An application study using the refined WEPP model to simulate water erosion from insloping forest roads was completed.

The drainage related ground-water contamination problem in the western San Joaquin

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Valley, CA, was thoroughly evaluated using a ground-water modeling approach. A transect in the general direction of regional ground-water flow and extending horizontally from the west valley boundary to the valley axis and vertically from the ground surface to the bottom of the confined aquifer was selected as the model domain. The effects of various management alternatives, including land retirement, ground-water management, and source control, on ground-water flow and water balance were determined through the runs of MODFLOW, a three-dimensional, finite-difference groundwater model. Major conclusions include: all the three tested management plans affect drainage the most, while the flow out of the semi-confined aquifer towards the valley axis is the second important water balance component highly sensitive to management conditions. Increasing pumping from within the ground-water flow system may help control the water table by reducing the needs for importing water, yet this management option may not be economically feasible. Source control has proved to be the most pragmatic solution.

Water erosion is a major agricultural and environmental problem in the Northwestern Wheat and Range Region (NWRR). The high erosion rate is a result of a combination of winter precipitation, intermittent freezing and thawing of soils, steep land slopes, and improper management practices. Models of different levels of sophistication have been developed for water erosion prediction. These models, however, are highly limited in their applicability for long-term, continuous simulation under the unique climatic conditions of the NWRR. On one hand, empirical models lack event-based or off-site erosion prediction capability and may require time-consuming, costly model calibration before use, and on the other, process-based models, such as the USDA's WEPP model, cannot properly account for the NWRR's winter hydrology and erosion processes. Lack of models as prediction tools has largely hampered the process of determining and adopting agricultural Best Management Practices (BMPs) by farmers in the NWRR. A study has been initiated to elucidate the mechanisms by which soil freezing and thawing affect runoff and erosion through laboratory experimentation, and to adequately incorporate the identified effects into the WEPP model for use in determining long-term erosion and agricultural BMPs.

The University of Delaware (UD) has continued its work on studying the fate and transport of viruses in both saturated and unsaturated porous media, mainly through conducting laboratory column experiments. Specifically the following problems have been addressed.

Interactive Effect of Water Content and Solution Chemistry on Virus Retention and Transport: Previous transport studies conducted under unsaturated water flow conditions have indicated that soil water content is an important factor controlling the transport of microorganisms through the subsurface. However, most of the experiments of microbial transport in unsaturated porous media have been conducted either under unsteady flow conditions (e.g., infiltration experiments) or in columns having a significant water content distribution (ranging from unsaturated at the top to groundwater table conditions at the base). Furthermore, the mechanisms responsible for the observed effect of water content are not well understood although several have been hypothesized. The objectives of our study were to: (1) evaluate and quantify the effect of water content on virus sorption and removal during transport through columns packed with inert (oxides-removed) and reactive (water-washed) sand, (2) evaluate the interactive effect of solution chemistry and water content on virus transport and removal, and (3) examine the mechanisms involved in virus removal and transport under

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unsaturated flow conditions.Column experiments were conducted under different water contents (saturated and

unsaturated), following procedure described in Jin et al. (in press). Two bacteriophages, MS-2 and X174, were used as model viruses in this study and their properties are given in Jin et al. (1997). Accusand (Unimin Corporation, Le Sueur, MN) of various sizes was mixed. All grades of Accusand consisted essentially of quartz with tracer levels of metals and metal oxides and low organic matter contents. The mixed sand was treated in two ways to create different surface properties. The first treatment was to simply wash the sand in tap and deionized water until rinsing water was free of suspended impurities. In this manner, the “reactive” components were preserved. The amount of free iron oxides (not part of the mineral structure) in the water-washed sand was determined to be 32.5 g Fe g-1. The second treatment was to remove metal and/or metal oxides following a procedure by Mehra and Jackson (1960) so that the sand would be “inert” to viruses. The presence of metal and metal oxides on sand surfaces have been found responsible for significant viral removal during transport in our previous saturated experiments (Chu et al., in review). Three buffer solutions were used: (1) phosphate buffered saline solution (containing Na2HPO4, NaCl, and KCl) with ionic strength of 0.163 M (PBS); (2) PBS with ionic strength of 0.002 M (PBS2); and (3) artificial ground water (containing CaCl2, MgCl2, NaHCO3, and KCl) with ionic strength of 0.002 M (AGW). By using the three buffers, the effects of both ionic strength and ionic composition were evaluated.

We observed the following results: (1) the effect of water content was significant on both MS-2 and X174 in columns packed with the reactive (water-washed) sand in the high ionic strength PBS buffer but not in the low ionic strength buffers PBS2 and AGW, (2) in columns packed with the inert (oxides-removed) sand in the PBS buffer, the effect of water content on MS-2 was observable but much less significant than that in the water-washed sand, the effect on X174 was minimal, and (3) under the conditions examined in this study, the effect of solution chemistry was due to changes in ionic strength rather than ionic composition. These results indicate that the effect of water content was both virus- and ionic strength-dependent. The difference observed between reactive and inert sand seem to suggest that the increased virus removal (sorption and/or inactivation) under unsaturated flow conditions was mainly due to increased virus interactions at the liquid-solid interface rather than at the air-water interface.

Effect of Iron Oxides on Virus Sorption, Inactivation and Transport: Our previous studies have shown that metal oxides play an important role in virus sorption and inactivation during transport. This study was designed to examine the effect of two iron oxides, goethite and hematite that are commonly found in natural soils, on the retention and transport behavior of MS-2 in saturated columns. Experiments were run using the following sand: (1) cleaned with 0.2 M citrate buffer and sodium dithionite (to remove metal and metal oxides), (2) coated with goethite (containing 127.1 g g-1 Fe), and (3) coated with hematite (containing 112.2 g g-1 Fe). PBS and AGW were used as buffer solutions. To recover viruses sorbed by sand during transport, beef extract 3% solution (pH 9.5) was used to flush the columns and mass balance was calculated.

The following results were observed: (1) large fractions of the input viruses were removed in the presence of iron oxides and the removal was mainly due to inactivation (mass recovery values were between 10 and 20%), and (2) more MS-2 were removed by the hematite-coated sand in the PBS buffer than in AGW whereas more MS-2 were removed in AGW than in

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PBS by the goethite-coated sand. The different effects of the buffer solutions on MS-2 behavior suggest that the relative importance of ionic strength and composition of the buffers depends on the properties of the solid phase. In the hematite-coated sand ionic strength of the buffer solution played a dominant role while in the goethite-coated sand phosphate might have competed with the viruses for sorption sites. We are in the process of characterizing (measurements of surface area and point zero of charge) the two iron oxides so that the mechanisms involved in their interactions with MS-2 can be further examined.

At Utah State University a collaborative project with Drs. Leij (USSL), Snyder (UPR) and Hadas (Volcani, Israel) is underway to shed light on aggregate coalescence in soils subjected to wetting-drying cycles. An analytical model describing the rate of deformation of a pair of soil aggregates as a function of capillary forces will be extended to represent the behavior of an aggregate bed. Measurements using rheometers will provide inputs to the model that will be compared with experimental measurements. Model results provide the necessary input required for modeling post-tillage soil pore size evolution for which Dr. Leij (USSL) will be developing an analytical solution.

At Iowa State University and the National Soil Tilth Laboratory, a 4-yr study has been initiated where W-188 members manage the nitrogen fertilizer for 8 farmers within a 1000 ac subbasin. The late spring nitrate test, LSNT, as developed for Iowa is being used in conjunction with starter and a side-dress application of liquid N to manage N within the corn/soybean rotations. Anhydrous ammonia is not being used due the inherent variability in its application. A paired watershed approach is being used to gauge the effect of an “optimum” N management strategy on surface water quality. In addition to surface water quality, yield, soil N, chlorophyll meter values, crop growth is being collected. LSNT recommended N-fertilizer rates were slightly higher in 1997 and about 50 kg/ha less in 1998, and slightly lower in 1999 than farmer program rates. Since the project was initiated in 1997, there has been a 3.5 mg/L reduction in the nitrate-N concentration leaving the LSNT watershed compared to the control watershed, while corn yields have been comparable in two out of three years. Overall, we have documented a 20% reduction in nitrate concentrations with essentially no change in N-fertilizer use. This is the first study to document the impact of an LSNT program on water quality at a spatial scale that is environmentally meaningful.

Drainage water quality and quantity from subsurface drain tubes and crop yield have been measured since 1996 for a 22 ha field in central Iowa containing soils in the Ottosen - Kossuth association. The field was in a corn - soybean rotation with N-fertilizer applied in the spring to corn only. Even at the low (L) N-fertilizer rate (57-67 kg ha -1), nitrate-N concentrations in the drainage water exceeded the maximum contaminant level (MCL) established by the USEPA for drinking water during the years that corn was grown and in all years for the medium (M) (114-135 kg ha-1) and high (H) (172-202 kg ha-1) N-fertilizer treatments. Optimum corn yield N-fertilizer rates were between the L and M treatment in 1996 and the M and H treatment in 1998. Thus, it appears that optimum corn production cannot be sustained within this field under the current management scheme without producing drainage waters that exceed the MCL for nitrate-N. If water quality is included in the concept of sustainable agricultural production systems, then dramatic changes will need to be made in the production of corn in these artificially drained soils before it is sustainable. Data from this field is being used to assess the accuracy of RZWQM for predicting N-leaching as a function of N-

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fertilizer application rate.At the University of Arizona, the effect of gravity on flow from spheroids was

examined. This came about from efforts with Or et al. (see submitted article to WRR) when looking at relationships useful for describing subsurface, tension permeameter. Known analytical relationships include: a. Steady-state absorption for all spheroids and all hydraulic properties; b. Steady-state solution with gravity included for Gardner function (K exponential with h); and c. Time-dependent case for absorption from spheres with and without gravity but only for small times.

The University of California, Berkeley, continued the development and application of fiber optic systems to measure solute transport. Small scale variations in transport parameters may have a profound influence on larger scale flow processes. As reported last year, the multiplexed Fiber optic mini-probe (FOMP) system provides the opportunity for measurement of solute resident concentration in small soil volumes on a continuous basis. The 20 channel FOMP system was used in repeated miscible displacements in a repacked clay loam soil column (20 cm long and 10 cm diameter) to examine small-scale, point to point variability in convective-dispersive transport processes. Transport parameters, measured 10 cm below the surface, were compared at two drip irrigation point densities and two fluxes. Irrigation densities of one irrigation drip point per 4 cm2 and 11 cm2 of column surface area produced similar results. The breakthrough curves measured at 0.10 cm hr-1 had a larger immobile phase than at a flux of 1.07 cm hr-1. In the clay loam soil the mobile immobile model fit the breakthrough curves better than the convective-dispersive equation with r2 values of 99.6 and 97.1, respectively. This analysis demonstrated that dispersion and mass recovery were much more variable than pore water velocity in this repacked clay loam soil. However, even in the most variable transport conditions encountered, only 17 sampling points were necessary to describe the column average transport parameters within 20% of the mean.

There is a necessity for improved physical understanding of solute transport processes in heterogeneous soil systems. In situ nondestructive techniques like time domain reflectometry (TDR) and fiber optic mini-probes (FOMPs) allow to collect unique measurements of solute transport processes in soils for the purposes of model development and validation. This year the University of California, Berkeley, examined the application of TDR and FOMPs to measure solute transport at various points laterally and at two depths in a heterogeneous clay-loam soil. A miscible displacement experiment was performed at a constant irrigation flux to examine the applicability of these probes to field soils. In their first application to a field soil, the FOMPs were successfully calibrated and performed well in measuring solute breakthrough curves. Two flow regimes were identified in the soil profile, the first where lateral spreading of the solute occurred in the surface horizon, followed by convergence into preferential flow pathways in the second transport zone. The measured transport response was heterogeneous with at least two identifiable vertical flow phases. It was demonstrated using transfer function modeling and data from a corresponding laboratory study that the FOMPs were measuring the slower phase, while the TDR probes captured a composite of the fast and slow phases. The combination of these two techniques may be a means to separate solute transport phases in heterogeneous media and relate laboratory column results to field studies.

The Colorado State University and USDA-ARS, Fort Collins continued studies of water management effects on chemical leaching, atrazine and bromide (a surrogate for nitrate)

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movement under alternative irrigation approaches was evaluated. A two-year field study monitored chemical movement under irrigated corn with three contrasting irrigation strategies; traditional every-furrow irrigation, alternate-furrow irrigation (that is, irrigation of every other furrow but at 2x the rate of every-furrow irrigation), and sprinkler irrigation. The rationale for alternate-furrow irrigation is creation of additional sinks (the dry furrow) for lateral water flow and hence reduced downward leaching of solutes. Results from both the field study and a 2-D solute transport model (CHAIN-2D with a 2-D root growth model) revealed essentially no difference in net downward movement of solutes at the plot scale when comparing every-furrow and alternate-furrow irrigation. Though the dry furrows did indeed accumulate solute, ridge positions were the predominant lateral sink due to plant water uptake. The addition of a dry furrow simply resulted in less solute accumulation in the ridge position, and the spatially average solute mass in the upper 0.3m did not increase or decrease relative to the every-furrow case. In both cases, >95% of the applied bromide remained in the root-zone or was accounted for by plant-uptake. Somewhat deeper chemical movement was observed in the sprinkler irrigated field plot but it was unclear if this was a treatment effect or a result of the spatial separation of the sprinkler plot from the furrow irrigated plots. The 2-D flow and transport model, calibrated with on-site measurements in the first year of the study, provided excellent predictions of bromide movement in the furrow irrigated treatments observed in the second year of the study.

At the US Salinity Laboratory, a partially modified subsurface drainage system was studied in a cotton field in the San Joaquin Valley, CA. USSL used time domain reflectometry (TDR) to measure soil water content and bulk EC of the soil to study the water and salinity response to water table management. Three pits representative of various depths of groundwater table were selected for installing TDR probes (at 10, 30, 50, 70, 90, and 110 cm). Daily water content and bulk EC were measured during the growing season 1997. A 2-dimensional water and solute transport model (SWMS_2D) was used to simulate and evaluate the groundwater contribution to total ET and the salinity dynamics in response to groundwater table management. Finally, the state-space method was employed to model and forecast soil water regime and salinity dynamics of the soil as affected by the depth of groundwater table.

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4. IMPACT STATEMENT:

Research conducted by the W-188 members in 1999 made significant contributions to our current understanding of water, heat and solute transfer in soils and porous media. The major impacts made by the W-188 group can be summarized as follows:

Instrument development and improvement: A variety of soil physical instruments have been, or are being, developed and improved. Among these instruments are sensors to measure water contents, bulk densities, soil moisture retention, heat and solute fluxes, and tracer concentrations. The development of such instruments and the corresponding measurement theories is an essential prerequisite for measurement of soil physical parameters in laboratory and field settings. Soil physical parameters, such as water content or soil moisture retention, are being used by many agricultural and environmental disciplines other than soil physics. New and improved instruments will therefore have a broad and important impact on any basic and applied research and extension dealing with soils and porous media in general.

Model development and improvement: Mathematical models are being used more and more frequently to predict and estimate water flow and solute transport. Many models are difficult to use and therefore often susceptible to operator errors. W-188 members have put emphasis on developing user-friendly interfaces to ease the use and application of complicated models. A variety of new models have been developed that will help to make more realistic predictions of kinetic reactions and of root water uptake and evaporation, thus ultimately leading to an improvement of environmental fate predictions and irrigation scheduling in agriculture. W-188 members have also successfully implemented new algorithms to estimate soil hydraulic properties from readily available data, such as soil texture and bulk densities. These algorithms will help to make better use of existing soil data and improve site-specific management practices through more realistic predictions of water flow and solute transport. Many of the models developed by W-188 members are readily available on-line.

Theoretical Advances of Water Flow and Solute Transport: Theoretical advances have been made in the microscopic understanding of water adsorption and condensation in pore scale models. Through upscaling procedures, it is possible to estimate unsaturated hydraulic conductivities and interfacial areas. Such quantifications are essential for many mass and energy transfer processes in soils, and provide an important explanation for many unresolved problems regarding solute and particle transport. W-188 members have developed better mathematical formulations of kinetic solid-liquid and decay/degradation processes, leading to an improvement of current transport and fate models for chemicals and pathogens.

Spatial Variability: A variety of research projects have focussed on the assessment and characterization of spatial variability of soil physical and agronomic parameters. For instance it has been found that slope is one of the most important contributor to the spatial variability of soil moisture. W-188 members have shown that the measurement of electrical conductivity is a good indicator for SAR (Sodium Adsorption Ratio), thus providing a much cheaper means to assess SAR on the field scale. Better characterizations of spatial variability and use of appropriate interpolation techniques will decrease costs in spatial sampling campaigns.

Experimental Findings: The W-188 group has made several important experimental observations. For instance, selenium is a major environmental contaminant that appears to be of natural origin. Agricultural leaching processes that will be difficult to reduce are dissolving

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selenium from subsurface porous formations and it is ending up in drainage ponds that are used for wetlands. Volatilization is a viable way of controlling Se at the endpoint provided that we develop an improved understanding of the processes that maximize it. Research conducted by W-188 members has the potential for developing an expert system for Se management under a variety of circumstances. Preferential flow research is critical to the determination of the causes of flow variability in the natural environment. If the causes are due to fluid instability, then soil properties are of minor importance and current models are inadequate for describing it. W-188 research strives to address this important problem. Important advances have been made in regard to the transport of pathogenic viruses through unsaturated soils and sediments. The better understanding of the factors controlling pathogen transport will lead to a better regulatory assessment of well-head protection and pollution prevention.

The W-188 group is very proliferant in publishing the scientific results in technical journals. The list of publications shows the productivity of the group and emphasizes its impact on the scientific literature.

5. ACTIVITIES PLANNED FOR 2000:

This is the first progress report for the W-188 5-year research project (1999-2004). Research in 2000 will continue on the objectives as described in Section 3. It is planned to initiate a discussion and possible collaboration with the W-82 committee during the next technical meeting of the W-118 group.

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6. PUBLICATIONS DURING 1999:

Abbaspour, K. C., R. Schulin, and M. Th. van Genuchten. 1999. SUFI: An inverse program for conditional parameter estimation. In: M. Th. van Genuchten, F. J. Leij and L. Wu (eds.), Proc. Int. Workshop, Characterization and Measurement of the Hydraulic Properties of Unsaturated Porous Media, Part 1, pp. 705-712, University of California, Riverside, CA.

Arya, L. M., F. J. Leij, M. Th. van Genuchten, and P. J. Shouse. 1999. Scaling parameter to predict the soil water characteristic from particle-size distribution data. Soil Sci. Soc. Am. J. 63(3): 510-519.

Arya, L. M., T. S Dierolf, A. Sofyan, A. Sofyan, I. P. G. Widjaja-Adhi, and M. Th. van Genuchten. 1999. Significance of macroporosity and hydrology for soil management and sustainability of agricultural production in a humid environment. Soil Sci. 164(8):586-601.

Arya, L. M., F. J. Leij and M. Th. van Genuchten. 1999. Relationship between particle-size distribution and soil water retention. In: M. Th. van Genuchten, F. J. Leij and L. Wu (eds.), Proc. Int. Workshop, Characterization and Measurement of the Hydraulic Properties of Unsaturated Porous Media, Part 2, pp. 931-946, University of California, Riverside, CA.

Arya, L. M., F. J. Leij, P. J. Shouse, and M. Th. van Genuchten. 1999. Relationship the hydraulic conductivity function and the particle-size distribution. Soil Sci. Soc. Am. J., 63(5): 1063-1070.

Bakhsh, A., Colvin, T.S., Jaynes, D.B., Kanwar, R.S., and Tim, U.S. 1999. Applying GIS for interpretation of spatial variability in yield: A case study. ASAE Paper No. MC99-125.

Bakhsh, A., Kanwar, R. S., Jaynes, D. B., Colvin, T. S., and Ahuja, L. R. Simulating the impacts of precision farming and variable n-fertilizer rates on No3 -N losses and crop yield using RZWQM. Paper No.99-2146 ASAE Paper No. 99-2146. 1999.

Bittelli, M., Campbell, G.S. and Flury, M., 1999. Characterization of particle-size distributions in soils with a fragmentation model. Soil Sci. Soc. Am. J., 63:782-788.

Bradford, S. A., L. M. Abriola, and F. J. Leij. 1999. Multi-fluid hydraulic properties for fractional wettability porous media. In: M. Th. van Genuchten, F. J. Leij and L. Wu (eds.), Proc. Int. Workshop, Characterization and Measurement of the Hydraulic Properties of Unsaturated Porous Media, Part 1, pp. 165-712, University of California, Riverside, CA.

Bristow, K. L., G. J. Kluitenberg, C. J. Goding, and T. S. Fitzgerald. 2000. A small multi-needle probe for measuring soil thermal properties, water content and electrical conductivity. Computers and Electronics in Agriculture (submitted).

Cambardella, C.A., T.B. Moorman, D.B. Jaynes, J.L. Hatfield, T.B. Parkin, W.W. Simpkins, and D.L. Karlen. 1999. Water quality in Walnut Creek watershed: Nitrate-Nitrogen in soils,

subsurface drainage water, and shallow groundwater. J. Environ. Qual. 28:25-34.

Campbell, C. G., M. Ghodrati and F. Garrido. 1999. Comparison of time domain reflectometry, fiber optic mini-probes, and solution samplers for real time measurement of solute transport processes in soil. Soil Sci. 164 (3) : 156-170.

Casey, F.X., D.B. Jaynes, R. Horton, and S.D. Logsdon. 1999. Comparing field methods that estimate mobile-immobile model parameters. Soil Sci. Soc. Am. J. 63:800-806.

Cassel-Sharmasarkar, F., S. Sharmasarkar, R. Zhang, G. F. Vance, and S.D. Miller, 1999. Micro-Spatial variability of soil nitrate following nitrogen fertilization and drip irrigation. Water, Air, and Soil Pollution 116:605-619.

Cassel-Sharmasarkar, F., S. Sharmasarkar, R. Zhang, G. F. Vance, S. D. Miller, and J. M. Reddy. 1999. Modeling nitrate movement under flood and drip irrigation. International Commission of Irrigated and Drainage Journal (in press).

Chen, D., D.E. Rolston, and T. Yamaguchi. 1999. Calculating partition coefficients of organic vapors in soil at small soil-water content. Soil Sci. (in press)

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Šimnek, J., R. Kodešová, M. M. Gribb, and M. Th. van Genuchten. 1999. Estimating hysteresis in the soil water retention function from modified cone penetrometer tests. Water Resour. Res. 35(5): 1329-1345.

Šimunek, J., K. Huang, M. Šejna, and M. Th. van Genuchten. 1999. The HYDRUS-2D Software Package for Simulating Two-Dimensional Movement of Water, Heat and Multiple

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Young, M.H., Z.Y. Zou, L.L. Hofmann and P.J. Wierenga. 2000. Neutron probe calibration using field infiltration data. Soil Science (submitted).

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Zhang, R., P. Shouse, and S. Yates, 1999. Estimates of soil nitrate distributions using cokriging with pseudo-crossvariograms. Journal of Environmental Quality 28:424-428.

Zhang, R., 1999. Reply to "Comments on 'Infiltration models for the disc infiltrometer'". Soil Science Society of America Journal 63:1034-1035.

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PATENTS

Horton, R., D. E. Ressler, T. C. Kaspar, and J. L. Baker. 1998. Method and tool to increase N-use efficiency and reduce leaching. Patent No. 5,797,459.

Horton, R., D. E. Ressler, T. C. Kaspar, and J. L. Baker. 1999. Method to increase N-use efficiency and reduce leaching. Patent No. 5,913,368.

7. SIGNATURES

_______________________________________________ ____________M. Flury, Chair DateTechnical Committee, W-188

________________________________________________ ___________G.A. Mitchell DateAdministrative Advisor, W-188