Hydrologic Analysis of Watersheds West of Zacatecas,Zacatecas, Mexico

Jaron Brown, Joshua Draper, Derek Lounsbury

April 18, 2007

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Abstract

The City of Zacatecas Mexico is experiencing rapid growth that is expected to continue forthe foreseeable future. This change from agricultural or natural lands to urbanized areaswill affect the runoff coming out from the areas. The change in runoff needs to be known inorder to appropriately design and build civil structures such as culverts and bridges. Thisstudy analyzes three watersheds in the western zone of Zacatecas to model current and futurerunoff conditions. HEC-1 and HEC-HMS were each employed to analyze the runoff change;both predicted an increase in runoff as a result of development changing land use. Runoffcalculations for current and future conditions by HEC-1 were 33 and 36 m3/s respectively;current and future runoff calculated by HEC-HMS was 56 and 63 m3/s respectively. Knowingthat the runoff will change significantly over the next few decades, civil engineers can buildstructures now that will be capable of safely and effectively passing runoff from storm eventsfor years to come.

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Contents

List of Tables i

List of Figures i

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Methods and Data 22.1 Watershed Delineation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Soil and land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.3 Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4 Model Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Results 10

4 Discussion of Results 12

5 Conclusions 13

6 References 14

Appendix 14

A Data 15

List of Tables

1 Basin Composite Curve Numbers. . . . . . . . . . . . . . . . . . . . . . . . . 5

List of Figures

1 The DEM from INEGI had an error that caused an unnatural corner in thedelineated watershed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 The watershed delineated from the USGS DEM is much more reasonable.Basins (from top to bottom): La Pimienta, La Escondida, Southern. . . . . . 4

3 The land use map does not account for the city extents which can be seen inlight gray. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Plot of IDF curves received from UAZ. . . . . . . . . . . . . . . . . . . . . . 75 Temporal rainfall distribution for Zacatecas. . . . . . . . . . . . . . . . . . . 7

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6 Hydrographs for present conditions. The higher peak came from HEC-HMSand the lower peak came from HEC-1. . . . . . . . . . . . . . . . . . . . . . 10

7 Hydrographs for future conditions. The higher peak came from HEC-HMSand the lower peak came from HEC-1. . . . . . . . . . . . . . . . . . . . . . 11

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1 Introduction

Zacatecas is a rapidly growing city in north-central Mexico that is home to the UniversidadAutonoma de Zacatecas (UAZ) which has participated for the last three years in a collab-orative program with BYU to develop hydrologic and hydraulic modeling experience usingthe Watershed Modeling System (WMS). This year, one of the objectives was to analyzethe effect of planned urbanization in order to perform flood runoff analysis for the city. Theresults can then be used to decide what flood prevention measures may be necessary.

1.1 Background

The city of Zacatecas is the capital of the state of Zacatecas, Mexico and is home to approx-imately 120,000 inhabitants. [1] Recently, the city has experienced rapid growth as it hasbecome a political, economic, and social center of great importance due to its geographiclocation. The city is also a world renowned tourist area and contains the state’s principaleducational institutions such as UAZ and others. All of these factors have attracted peoplefrom the surrounding cities and other areas to immigrate to Zacatecas. The large influxof people has resulted in an increase in urban and agricultural land uses that has changedthe characteristics of the surrounding watersheds including reduced infiltration, increasederosion rates, and climatic changes. [2]

1.2 Objectives

In order to quantify the effect that the current and future expansion will have on the hydro-logic resources of Zacatecas, accurate hydrologic models are needed in order to assess andpredict changes in runoff in the various watersheds. The objective of this project is to useWMS to delineate the watersheds and generate initial models in HEC-1 and HMS whichproduce reasonable results for the area. In later years, these models can be further refined,calibrated, and validated so they can then be used as predictive models.

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2 Methods and Data

In order to analyze the flood runoff, two HEC-1 models of the same area were built. The firstmodel simulated the current land use in Zacatecas. The second model simulated the land useafter the planned urbanization had taken place. HEC-1 was chosen because it is designed tosimulate single storm events and is the current industry standard for flood runoff analysis.The three kinds of data WMS needs in order to complete the models are geometric data fordelineation, basin hydrologic characteristics, and meteorological data. A digital elevationmodel (DEM) was used to delineate the basins. Both models were developed using the SCScurve number loss method and the Clark unit hydrograph method. A rainfall temporaldistribution was generated for the area and the precipitation input as basin averages. Afterthe HEC-1 analyses were finished, pre- and post-development conditions were also modeled inHMS using the ModClark method to investigate the difference between the lumped parametermodel (HEC-1) and the distributed model (HMS).

2.1 Watershed Delineation

The initial delineation process was accomplished using a DEM obtained from a Mexicangovernment body called INEGI (National Institute of Statistics, Geography and Informatics).[3] Unfortunately, the DEM had an error that caused WMS to delineate the watershed withan unnatural corner as can be seen in Figure 1 on page 3. Later, a DEM obtained from theUSGS yielded a much more reasonable delineation that was then used for all the modelingand can be seen in Figure 2 on page 4. The advantage of using a DEM for delineation is thatthe basin geometry can be automatically determined from the elevation information. Also,important parameters such as slope, time of concentration, and elevation-storage curves canbe automatically calculated. The disadvantage is that DEMs do not usually capture linearor man-made features, so the delineated watershed may not represent the real watershedaccurately. Even after delineating the watershed with the good DEM, some streams had tobe manually adjusted so that they lined up with the rivers on the topographic map of thearea.

Three outlets were specified for this study. One was downstream from the town of LaPimienta and resulted in the north (top) watershed in Figure 2. The second outlet was justdownstream of the town of La Escondida. La Escondida is the middle basin. The third,southernmost outlet split the La Escondida watershed into two sub-basins to account fordifferent topography in each area.

2.2 Soil and land use

Once a reasonable delineation was obtained, a composite curve number was needed to runHEC-1. The Watershed Modeling System calculates a composite curve number using landuse and soil type coverages or shapefiles. On each coverage or shapefile, polygons are definedwhich are given attributes that describe the land uses and soil types. Each soil type polygonis assigned to one of four hydrologic groups: A, B, C, and D. Each land use has four curve

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Figure 1: The DEM from INEGI had an error that caused an unnatural corner in thedelineated watershed.

3

Figure 2: The watershed delineated from the USGS DEM is much more reasonable. Basins(from top to bottom): La Pimienta, La Escondida, Southern.

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Table 1: Basin Composite Curve Numbers.

Basin Curve NumberLa Pimienta 71.8La Escondida 70.9Southern 57.1

numbers associated with it, one for each hydrologic group. WMS then determines the curvenumber for each land use polygon by overlaying the two coverages and determining whichsoil type is under each land use polygon. The composite curve number for the basin is thenfound by taking an average of the individual curve numbers weighted by area. For muchof the United States, this process can be automated using shapefiles which contain all thenecessary information, but such shapefiles did not exist for Zacatecas.

The only land use and soil type information available came in the form of printed mapsthat were scanned, imported into WMS, and digitized into polygons. After generating thesoil type and land use polygons, a CAD drawing of the city was superimposed over the mapsand it was discovered that the land use map did not account for the city; therefore, many ofthe land use polygons had to be redrawn or adjusted. Figure 3 on page 6 shows the CADfile and land use polygons superimposed on the land use map.

After correcting all the land use polygons to reflect the current extents of the city, thecomposite curve numbers for each basin were calculated and are listed in Table 1. Thenorthern basins’ curve numbers of about 70 initially seemed too high, but were deemedreasonable after a site visit. The southernmost basin’s curve number of 57.1 also agreedwith observed conditions.

2.3 Precipitation

UAZ sent intensity–duration–frequency (IDF) precipitation data for Zacatecas which ischarted in Figure 4 on page 7. Since HEC-1 only models single storm events, the IDFcurves were transformed into a normalized temporal rainfall distribution using the SCS de-sign storm method. The resulting curve is shown in Figure 5 on page 7. The distributionwas derived using data for a 10–year recurrence interval, but should apply to any storm inthe area. The models were then run with storm precipitation depths of 75 mm (3 inches)because that represents an average 10 year storm event.

2.4 Model Comparison

After the results of the HEC-1 analysis were delivered to UAZ, a comparison with HEC-HMS using the ModClark unit hydrograph method with gridded SCS curve numbers wasperformed. For comparison purposes, the La Pimienta basin was not considered and thetwo southern basins were combined into a single watershed. This was done because WMS

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Figure 3: The land use map does not account for the city extents which can be seen in lightgray.

6

Figure 4: Plot of IDF curves received from UAZ.

Figure 5: Temporal rainfall distribution for Zacatecas.

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is currently only capable of setting up the input files for HEC-HMS for one basin. Thesame land use and soil type coverages were used with the same hyetograph and precipitationdepth. Developing the HEC-HMS models uncovered an important anomaly in version 8.0 ofWMS which was the version used in this project. For HEC-1, if the temporal distributionof rainfall does not total 1 unit and a precipitation depth is specified, WMS will normalizethe distribution and then multiply by the depth. However, for HEC-HMS, WMS does notnormalize the temporal distribution. It uses the total rainfall from the distribution andmultiplies by the given depth. The result is that a temporal distribution that describesa large amount of rainfall multiplied by a large average depth produces numbers that aretoo big to be written the precipitation file. For example, the Zacatecas distribution wasproduced using 120 mm of rain and input into WMS without normalizing it. The basinaverage depth of 75 mm was entered and the HMS file saved. When viewed in HMS, theprecipitation value was zero where the peak was suppposed to be because the numbers weretoo high to be written to the file. After normalizing the temporal distribution, everythingworked as expected.

To summarize the HEC-1 model parameters, the ADOT (desert/mountains) method wasused for calculating time of concentration. This equation is defined as:

Tc = 2.4A0.1L0.25Lca0.25S−0.2 (1)

where Tc = time of concentration, hrs

A = basin area, km2

L = length of the longest flow path, m

Lca = length along the main channel from the outlet to the point opposite the

centroid of the basin area, m

S = slope of the channel from outlet to furthest point in the watershed,m/m

This equation was used because the area around Zacatecas is very similar to that of theState of Arizona. Nopal (prickly pear) cactus, sagebrush, and sandy/rocky soil are prominentfeatures of the terrain in both locations, so it seemed to be a reasonable fit for our model ofZacatecas.

For a loss method, the SCS Curve Number method was selected. As has been previouslydescribed, composite curve numbers were used for each basin or sub-basin. Precipitation datawas defined as a basin average of 75 mm and a given temporal distribution. Muskingham-Cunge routing was chosen for its simplicity and ease of use. Manning’s n value was .03,and the channel shape was assumed to be roughly trapezoidal with a bottom width of 4meters and side slopes of 5:1. The HEC-HMS model parameters were as similar to theHEC-1 runs as possible. Again the Tc was calculated with the ADOT equation, and thesame precipitation data (same basin average and temporal distribution) were used. Theunit hydrograph method differed however. For HEC-HMS, the ModClark method was used.Also, the models were different in the way the CN values were used. While HEC-1 computes

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a composite curve number for the basin area, HEC-HMS uses a grid (in our case it was a40 X 40 grid), and a different curve number was computed for each grid cell individually.Then, the grid cells are used one by one for routing and runoff calculations. The difference inmethods for handling CN values is what makes the models different and where the differentresults come from.

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3 Results

The results of a HEC-1 model of the La Pemienta watershed indicated a peak flow of ap-proximately 14 cms. No urban growth is planned within the boundaries of the La Pimientawatershed; ergo, flow rates under future conditions of Zacatecas remained the same.

Using the HEC-1 and HEC-HMS models to calculate the runoff rates of the La Escondidawatershed, hydrographs illustrating flow rates at the outlet of the La Escondida watershedunder current land use conditions and after projected urban growth were created as seenin Figure 6 on page 10 and Figure 7 on page 11, respectively. The peak flows determinedusing the HEC-1 model were approximately 33 cms for the present conditions and 36 cmsfor the expected future conditions. The peak flows for the present and future conditionsdetermined using HEC-HMS were approximately 56 cms and 63 cms, respectively. Bothmodeling methods calculated a 9% to 13% increase in runoff under future land use conditions.The value of the peak runoff rates determined using the HEC-HMS model for the presentand anticipated future conditions were approximately 70% to 75% greater than the peakvalues determined using the HEC-1 model.

Figure 6: Hydrographs for present conditions. The higher peak came from HEC-HMS andthe lower peak came from HEC-1.

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Figure 7: Hydrographs for future conditions. The higher peak came from HEC-HMS andthe lower peak came from HEC-1.

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4 Discussion of Results

HEC-HMS consistently gave much higher peak flows than HEC-1. The only difference inthe HEC-1 and HEC-HMS models that were used is the method of accounting for spatialvariability in the data, so it must account for the difference in flows. HEC-1 with the Clarkunit hydrograph method and SCS curve number loss method is a lumped parameter modelmeaning that spatial heterogeneity is treated by using weighted averages. HEC-HMS withthe ModClark method and gridded SCS curve numbers divides the watershed into a grid oftiny subbasins each with its own parameters and routes the results from each grid cell toproduce the final hydrograph.

These models have not yet been perfected. Due to the fact that the scanned maps didnot cover the entire watershed and educated guesses were made as to the true land use andsoil type for areas not covered, results are probably not truly representative of the runofffound in real life. Also, since the land use maps provided by colleagues in Mexico did notcoincide with the AutoCAD map of the city, placing large amounts of confidence in the mapin other areas would be a mistake. Furthermore, since there are no gauges on the streamsto give real-time estimates on the flowrates, it is impossible to say whether the results foundin this study are even in the ballpark with what actually happens during a storm event likethe one modeled.

Future researchers working on this project may wish to verify the land use map or look fora more current one as well as find land use and soil type maps for the rest of the watershed.The models could also be calibrated and validated so they could be deemed reliable. Thiswould require gauging the stream and searching for historical records of stream flows in LaEscondida and La Pimienta.

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5 Conclusions

Despite the model deficiencies mentioned above, it is necessary to remember that the objec-tive of this analysis is to quantify the relative change in runoff between present and futureconditions. Both HEC-1 and HEC-HMS predicted about a 13% increase in the runoff dueto urban growth over the next few decades. If planners really want to know if existing struc-tures downstream (culverts and bridges) are going to be able to pass the increased flows,they need only gauge the streams during storm events and predict a 13% or so increase inwhatever flow rate they find in the field. The data found here will be useful in that respect.

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6 References

References

[1] http://en.wikipedia.org/wiki/zacatecas. website, April 2007.

[2] Oscar Antonio Dzul Garcıa. Cooperacion universitaria internacional 2007 universidadautonoma de zacatecas–brigham young university. January 2007.

[3] Saul Gutierrez. Implementation of digital information to design hydrologic models inmexico. Master’s project, Brigham Young University, December 2006.

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Appendix

A Data

Attached to the hard copy of this report is a CD that contains all of the data necessary tocontinue work on this project. The README.txt file from the CD is reproduced below.

This CD contains the project and supporting files for the Zona Poniente project. Allrequired files to continue the work are included. Please see the file listing below for adescription of the contents.

In order to interpret the file listing below, an ending "/" denotes a

directory.

Coordenada outlet cuenca.txt Original outlet coordinates

coordinates.xls Updated outlet coordinates

Descripcion.pdf Original project description recieved from Mexico

HEC-1/ Contains the project used to run the HEC-1

simulations for UAZ

HMS/ Contains the projects used for comparing HEC-1 to

HEC-HMS

images/ Project images

Presentations/ Presentations given in Mexico and at BYU

project plan.rtf

README.txt

ZonaPonCNtable.txt Mapping table for WMS

Zona_Poniente_Hydrologic_Modeling_Report.pdf Final report

Zona-Poniente-IDF.xls Original IDF data and generate temporal

distribution

ZonaPoniente.wmv Google earth video of the watershed area

./HEC-1: Project used for analysis in Zacatecas

usgs-delineated.bsn

usgs-delineated.dwg

usgs-delineated.fac

usgs-delineated.fdr

usgs-delineated.gdm

usgs-delineated.ini

usgs-delineated.lsf

usgs-delineated.map

usgs-delineated.sto

usgs-delineated.tre

usgs-delineated.wpr

zonaponiente-10yr.xys This file is the temporal distribution used.

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./HMS: Projects used for BYU class CE En 531

basinStates/

cn_report.txt

Future/ Project representing future conditions

good-landuse.map This is a land use coverage with correct polygons.

hydrographs.xls Excel file that compares the pre and post

condition hydrographs

optimizer/

Present/ Project representing present conditions

PUNCH

Run_1.log

Run_3.log

southfut.access southfut is the future conditions of the

southern basin

southfut.basin

southfut.control

southfut.dss

southfut.dss.msg

southfut.gage

southfut.grid

southfut.hms

southfut.log

southfut.map

southfut.met

southfut.mod

southfut.out

southfut.run

southfuture.hc1 HEC-1 files for future conditions of southern basin

southfuture.out

southfuture.sol

southpre4.access southpre4 is the current conditions of the southern

basin

southpre4.basin

southpre4.control

southpre4.dss

southpre4.dss.msg

southpre4.gage

southpre4.grid

southpre4.hms

southpre4.log

southpre4.map

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southpre4.met

southpre4.mod

southpre4.out

southpre4.run

southpresent.hc1 HEC-1 files for present conditions on southern basin

southpresent.out

southpresent.sol

Thumbs.db

uso de suelo zac2.jpw

uso de suelo zac2.tiff

ZonaPonCNtable.txt

./HMS/basinStates:

./HMS/Future:

zonaponientefuturo.2dg

zonaponientefuturo.bsn

zonaponientefuturo.dwg

zonaponientefuturo.fac

zonaponientefuturo.fdr

zonaponientefuturo.gdat

zonaponientefuturo.gdm

zonaponientefuturo.ini

zonaponientefuturo.lsf

zonaponientefuturo.map

zonaponientefuturo.sto

zonaponientefuturo.tre

zonaponientefuturo.wpr

./HMS/optimizer:

./HMS/Present:

zonaponientepresente.2dg

zonaponientepresente.bsn

zonaponientepresente.dwg

zonaponientepresente.fac

zonaponientepresente.fdr

zonaponientepresente.gdat

zonaponientepresente.gdm

zonaponientepresente.ini

zonaponientepresente.lsf

zonaponientepresente.map

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zonaponientepresente.sto

zonaponientepresente.tre

zonaponientepresente.wpr

./images:

edafologia Zac2.tiff Southern area soil type map

Edafologica Zac.jgw

Edafologica Zac.jpg

Edafologica Zac.jpw

Edafologica Zac.tiff

Geologica Zac.tiff

Map words.rtf Description of mapping table entries

Simbologia edafologia2.tif Soil type map legend

Simbologia edafologia.tif Soil type map legend

Simbologia geologica.tif Geology map legend

Simbologia uso de suelo.tif Land use map legend

Topografia Zac.jpg Topologic map

Topografia Zac.jpw

topografico Zac2.tiff

uso de suelo zac2.jpg Southern land use map

uso de suelo zac2.jpw

uso de suelo zac2.tiff

Uso suelo Zac.jpg Northern land use map

Uso suelo Zac.jpw

ZonaPoniente.jpg Very high resolution image of watershed

area.

./Presentations:

gobernadora.ppt A presentation of the project geared toward

the governer of Zacatecas.

landusepoly.PNG

presentacionenespaol.ppt Preliminary presentation in spanish

Thumbs.db

zac-area of interest.jpg

Zacatecas-mexico.jpg

zona poniente english.ppt

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