direct measurement instruments for estimation of vertical water movement pan lysimeter passive...
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Direct Measurement Instruments for Estimationof Vertical Water Movement
Pan LysimeterPassive Capillary Sampler
Tension Lysimeter
Joel HubbellAgE 558, Spring 2001
State Variables for Characterizing the Vadose Zone
Soil Water PotentialHydraulic gradient
Water ContentMass of water-changes in time and space
Flux Rate Concentration of Solutes
Pan LysimeterPassive Capillary Sampler (PCAPS)
Tension Lysimeter
Theory of operation Construction, installation and operation Advantages and limitations
Comparison of devices
Pan Lysimeter
Intersect flow and route to sampling locationImpermeable layer beneath siteNumerous configurations
Pan, plastic, glass block, funnel
Pan Lysimeters
Advantages: “Simple” designLower costs Available materials
Limitations: Divergent flow (under sampling)Collect 0 to 36-45% of flow (optimum)Difficult to design to predict fluxSamples macropores-”wet” conditions
Passive Capillary Sampler
Intercepts flowCollects with a constant pressure
Wick fluid with fiberglass rope
Passive Capillary SamplerAssets:Passive design
Macro/micro pore flowBest in sandsMore representative flux
103, 80, 64 % of WBWell published-12+ articles
Limitations:Soil water potential <-150 cm “wet”Clay and silts lower recoveriesDesign/materials more complexInstallation more complex-undisturbed
Tension Lysimeter
Advantages: Able to sample tensiometeric rangeGood for sands, silts and claysMatches SWP in device with existing conditions
Could be automatedShould produce best estimate of flux
Limitations:Complex designSophisticated instrumentHigh cost of operationFew papers
Comparison of InstrumentsFlux representiveness
Pan/Gravity Lysimeter PCAPS Tension LysimeterOverall
Low Medium-High High
For Low water potential(dry) Low Low High
High water potential (wet)Best Good Good
Chemical representiveness of sampleHighest Good Fair
Comparison of InstrumentsPan/Gravity Lysimeter PCAPS Tension LysimeterPrimary pore size sampled
Macro pore Macro pore Macro and micro
Water Pressure range of operation 0-unk (Wet range only) 0 to about –80 cm 0 to -700cm
(up to –150 cm)
Vary tension range for sampling? (flexibility)No Limited 0 to -80 cm Yes, 0 to –700 cm
Best to SampleSand Sand Clay-course sand
Comparison of Instruments
Cost to install and construct toolMedium-high Medium-high High
Complexity of design
Low High Very high Ease of operation
Easy Medium Difficult Mode of operation
Passive Passive Active
Cost of operationLow Low High
Pan/Gravity Lysimeter PCAPS Tension Lysimeter
Conclusions
Pan LysimeterLeast accuracy/complexity
PCAPSMore accurate/complex
Tension LysimeterBest/most complex
Problem is standard for comparisonAlso impedance match
Spatial variability in recharge Preferential flow-Finger, funnel flow, fast flow paths