Soil Water Drainage Measurement and Soil Water Sampling Doug Cobos, Ph.D. Decagon Devices and Washington State University
About the presenter Ph.D. in Soil Physics, University of Minnesota Research Scientist/Director of R&D, Decagon Devices Adjunct Faculty, Dept. Crop and Soil Science, WSU Lead Engineer on Thermal and Electrical Conductivity Probe (TECP) for NASA s Phoenix Mission to Mars (2008)
Soil water: why do we care? Soil water sampling Measure solute concentrations in soil solution Plant nutrition Ecotoxicology Plume assessment Soil water drainage Water balance Aquifer recharge Landfill cover efficacy Wastewater disposal Regulatory compliance
Why do we care? Drainage x solute concentration = solute flux Nutrient leaching from agricultural operations Pesticide fate Contaminant or pathogen transport Radioactive waste e-coli Waste containment Aquifer recharge/contamination Regulatory compliance
Outline Soil water sampling Techniques Sampler material considerations Drainage measurement Water balance residual Zero tension pan lysimeters Wick lysimeters Controlled tension lysimeters Large scale/weighing lysimeters Practical considerations Installation Spatial variability
Soil water sampling Porous cup or plate installed in soil Suction applied to the cup through a tube extending to surface Accumulated soil water pulled to surface through suction tube or through companion tube
Soil water tension, suction, potential Terminology and units Soil water tension and suction used interchangeably Soil water potential is negative of suction Common units: 100 kpa = 1 bar Field capacity 33 kpa suction in US Wetter than field capacity, water drains through soil
Vacuum considerations for soil water sampling Vacuum applied must be greater than soil water tension Air entry potential of porous material 10 kpa only measures drainage (leachate) water 100 kpa can measure resident soil water, but slow
Temporal considerations Spot measurements with portable pumps Only gives brief snapshot Static vacuum Measurement weighted heavily toward wet time periods
Temporal considerations Controlled vacuum Pumping system controlled by measurement of soil tension Representative sample collected over time
Porous cup/plate chemistry Porous cup must not affect solution chemistry Sorption of ions, organics, metals No universal material although new Silicon Carbide cups from UMS are a big step forward Have to check the literature to determine suitability (Weihermuller review a good place to start). Weihermuller et al., 2007. In Situ Soil Water Extraction: A Review. J. Environ. Qual. 36: 1735-1748
Outline Soil water sampling Techniques Sampler material considerations Drainage measurement Water balance residual Zero tension pan lysimeters Wick lysimeters Controlled tension lysimeters Large scale/weighing lysimeters Practical considerations Installation Spatial variability
Solution sampling vs. drainage measurement Soil water sampling Drainage measurement Solute concentration Yes Yes* Close water balance No Yes Soil water flux (drainage) No Yes Solute flux No Yes* Often wish to measure the solute flux Solute flux = solute concentration x drainage Accurate quantification of drainage is very difficult in vadose zone *Water balance residual method does not yield solute concentration or solute flux
Water balance residual Precipitation + Irrigation = Runoff + Storage + Evapotranspiration + Drainage Measure precipitation & irrigation Measure or estimate runoff Measure or estimate ET Measure soil water storage (volumetric water content) Drainage is calculated as whatever is left over
Soil Hydrologic Cycle (150 cm precip. with vegetative cover) 10% error in ET = 17% error in Drainage transpiration evaporation 60 cm 150 cm Precipitation, dewfall (or irrigation) 15 cm runoff 30 cm Storage (soil moisture) Drainage 45 cm Groundwater
Soil Hydrologic Cycle (20 cm precip. with little vegetative cover) 10% error in ET = 300% error in Drainage transpiration 4 cm evaporation 20 cm Precipitation, dewfall (or irrigation) 11 cm runoff 4.5 cm Storage (soil moisture) Drainage 0.5 cm Groundwater
Water balance residual No measurement of soil water solute concentration Must be used with soil water samplers to estimate solute flux
Zero tension (pan) lysimeters Most basic measurement of drainage Simple collection pan buried in soil
Zero tension problems Water flows from low tension to high tension (high potential to low potential) Lower boundary of pan lysimeter = zero tension Water in unsaturated soil will flow around lysimeter: flow divergence Representative sample only collected under saturated conditions
Pan Lysimeter
Zero tension problems (cont) Flow divergence problems mitigated somewhat by: Large measurement footprint (several m 2 ) Vertical walls (all the way to surface best) Collection efficiencies of < 10% are still common
Static tension lysimeter Vacuum pump or wick used to create static tension Flow divergence greatly reduced Flow convergence possible but uncommon
Wick Lysimeters Wick (hanging water column) used to pull tension on soil water Static tension chosen to optimize water collection efficiency
Wick Lysimeters Divergence control tube minimizes divergence and convergence Jm/Ja (%) Jm/Ja (%) 120 100 80 60 1 mm/yr 10 m m /yr/yr 40 100 mm/yr/yr 1000 mm/yr 20 Sand 10000 mm/yr 0 0 20 40 60 80 100 Barrier Height (cm) 100 80 1 mm/yr 60 40 20 Silt Loam 10 mm/yr 100 mm/yr 1000 mm/yr 10000 mm/yr 0 0 20 40 60 80 100 Barrier Height (cm) 120 100 80 Jm/Ja (%) 60 40 20 Clay 1 mm/yr 10 mm/yr 100 mm/yr 1000 mm/yr 10000 mm/yr 0 0 20 40 60 80 100 Barrier Height (cm) Data from Gee et al., 2003
Controlled tension lysimeter Same construction as pan lysimeter Tension in lysimeter actively controlled based on soil water tension Soil water tension measured with tensiometer or other matric tension sensor Control system reads tensiometer and vacuum gauge and controls vacuum pump
Controlled tension lysimeters
Controlled tension lysimeter Most accurate drainage measurement method Drawbacks with traditional systems Expensive Complex, power intensive, maintenance intensive New turn-key systems easier to use
Weighing Lysimeters Large surface area Deep enough to encompass root zone Precision load cell weighs lysimeter continuously Lower boundary controlled with active suction system
Weighing Lysimeters Photos courtesy of UMS GmbH
Weighing Lysimeters Photos courtesy of UMS GmbH
Weighing Lysimeters Photos courtesy of UMS GmbH
Weighing Lysimeters Best possible quantification of hydrologic cycle Climate change Ecohydrology Contaminant transport Drawbacks Installation Maintenance intensive Expensive
Outline Soil water sampling Techniques Sampler material considerations Drainage measurement Water balance residual Zero tension pan lysimeters Wick lysimeters Controlled tension lysimeters Large scale/weighing lysimeters Practical considerations Installation Spatial variability
Installation Undisturbed soil always preferable Intact monolith in wick and weighing lysimeter Horizontal insertion of pan and controlled tension lysimeters
Installation Good hydraulic contact with soil column is critical Diatomaceous earth/silica flour Jack up pan and controlled tension lysimeters Below root zone Representative vegetation
Spatial variability How many measurements do you need to quantify drainage at a site? Spatially variable soils Topography Spatially variable vegetation Macropore flow
Technique Measures Accuracy Spatial scale Installation/ Maintenance Cost Comments Solute conc. Drainage Solute flux Soil water sampling Y N N Depends on static vs. controlled tension Small $ Sampler material must be compatible with solute Water balance residual N Y N Poor Field Scale $$ Indirect estimate. Errors depend on ET measurement accuracy Pan lysimeter Y Y Y Poor Medium - Large $ Flux divergence under unsaturated conditions Static tension (wick) lysimeter Y Y Y Fair Small - Medium $$ Mid range accuracy, mid range cost Controlled tension lysimeter Weighing lysimeter Y Y Y Excellent Medium $$$ Minimal flux divergence/convergence issues. New turn-key systems lower maintenance Y Y Y Best Medium $$$$$ Quantify and manipulate hydrology
Questions? doug@decagon.com
Soil water samplers