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Direct Measurement Instruments for Estimation of Vertical Water Movement Pan Lysimeter

Direct Measurement Instruments for Estimation of Vertical Water Movement Pan Lysimeter Passive Capillary Sampler Tension Lysimeter. Joel Hubbell AgE 558, Spring 2001. State Variables for Characterizing the Vadose Zone.  Soil Water Potential Hydraulic gradient  Water Content

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Direct Measurement Instruments for Estimation of Vertical Water Movement Pan Lysimeter

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  1. Direct Measurement Instruments for Estimation of Vertical Water Movement Pan Lysimeter Passive Capillary Sampler Tension Lysimeter Joel Hubbell AgE 558, Spring 2001

  2. State Variables for Characterizing the Vadose Zone Soil Water Potential Hydraulic gradient  Water Content Mass of water-changes in time and space  Flux Rate  Concentration of Solutes

  3. Pan LysimeterPassive Capillary Sampler (PCAPS)Tension Lysimeter  Theory of operation  Construction, installation and operation  Advantages and limitations  Comparison of devices

  4. Pan Lysimeter Intersect flow and route to sampling location Impermeable layer beneath site Numerous configurations Pan, plastic, glass block, funnel

  5. Pan Lysimeter

  6. Pan Lysimeter

  7. Pan Lysimeter

  8. Pan Lysimeter

  9. Pan Lysimeters Advantages: “Simple” design Lower costs Available materials Limitations: Divergent flow (under sampling) Collect 0 to 36-45% of flow (optimum) Difficult to design to predict flux Samples macropores-”wet” conditions

  10. Passive Capillary Sampler Intercepts flow Collects with a constant pressure Wick fluid with fiberglass rope

  11. PCAPS

  12. Passive Capillary Sampler

  13. Passive Capillary Sampler Assets: Passive design Macro/micro pore flow Best in sands More representative flux 103, 80, 64 % of WB Well published-12+ articles Limitations: Soil water potential <-150 cm “wet” Clay and silts lower recoveries Design/materials more complex Installation more complex-undisturbed

  14. Tension Lysimeter

  15. Tension Lysimeter

  16. Tension Lysimeter Advantages: Able to sample tensiometeric range Good for sands, silts and clays Matches SWP in device with existing conditions Could be automated Should produce best estimate of flux Limitations: Complex design Sophisticated instrument High cost of operation Few papers

  17. Comparison of Instruments Flux representiveness Pan/Gravity Lysimeter PCAPS Tension Lysimeter Overall Low Medium-High High For Low water potential(dry) Low Low High High water potential (wet) Best Good Good Chemical representiveness of sample Highest Good Fair

  18. Comparison of Instruments Pan/Gravity Lysimeter PCAPS Tension Lysimeter Primary 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 Sample Sand Sand Clay-course sand

  19. Comparison of Instruments Pan/Gravity Lysimeter PCAPS Tension Lysimeter Cost to install and construct tool Medium-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 operation Low Low High

  20. Conclusions Pan Lysimeter Least accuracy/complexity PCAPS More accurate/complex Tension Lysimeter Best/most complex  Problem is standard for comparison Also impedance match  Spatial variability in recharge Preferential flow-Finger, funnel flow, fast flow paths

  21. Caution Although the technique (flux meters) has worked well for those researchers that have perfected it subtleties, it does not appear to be pragmatic on a large-area basis and should be used only for very localized and specialized studies. (Wagenet, MOSA 1986)

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