|Nicholson, Thomas - US NRC|
|Cady, Ralph - US NRC|
|Schwartzman, Adam - US NRC|
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: April 30, 2008
Publication Date: July 28, 2008
Citation: Guber, A.K., Gish, T.J., Pachepsky, Y.A., Nicholson, T.J., Cady, R.E., Schwartzman, A. 2008. Hydropedologic Analysis of Ground-Water Recharge at the Field Scale [abstract]. International Hydropedology Conference, Penn State, University Park, PA, July 28-31, 2008. Technical Abstract: Estimating ground-water recharge is an important element in water resources characterization, vulnerability assessment, and utilization. Contaminant sources often occur in the unsaturated zone where ground-water recharge may mobilize it to migrate into a water table aquifer. Cumulative soil water fluxes at a depth below the root zone can give a reasonable estimate of this recharge. These fluxes can be calculated using soil water mass balance computations of the rainfall minus (a) the runoff, (b) the evapotranspiration, and (c) the increment changes in soil water storage in the mass balance layer above the flux computation depth. For such mass balance computations, one has to define: (a) the sequence of time intervals to compute the mass balances; and (b) the thickness of the mass balance layer. For the mass balance method to be applied correctly, each mass balance time interval should only include a single event of soil water storage increase followed by its decrease. For ground-water recharge to be estimated reliably, the depth of water flux estimation should be such that a further increase in the thickness of the mass balance layer does not change the flux estimate. A field study was conducted at the ARS Beltsville site to evaluate a series of techniques for estimating ground-water recharge using near-continuous measurements of soil water contents at several depths in the soil profile using multisensor capacitance probes (MCP). These measurements were combined with meteorological and hydrologic data measured at the USDA-ARS OPE3 experimental site, to estimate ground-water fluxes. Two 2-hectare area field sites were instrumented with 24 MCPs. The MCP sensors were placed at 30, 50, 80, 120, 150, and 180 cm depths. At these sites, sandy alluvial soils are underlain by low-permeable sediments ranging in depths of 1 to 3 meters, creating a shallow perched ground-water system most of the time. Rainfall and total runoff from the fields were measured. Evapotranspiration was estimated using standard weather data. Soil water contents were recorded every 10 minutes for an 18-month period. Graphs of the computed soil water storage time series had distinct sawtooth shapes indicating distinct recharge events, which helped to define reliable water balance time intervals. The estimated soil water fluxes at the depths of 80, 120, and 150 cm had excellent correlations (R2>0.99) which indicated that the depth of water balance computations should be at 80 cm depth. The spatial variability of the estimated fluxes was substantial, and can be interpreted in part using data fusion techniques that combined remote sensing imagery, crop production yield maps, and dense ground penetration radar (GPR) survey data. The average recharge estimates compared favorably with the hydrograph separation results for the perennial stream located approximately 50 m from the edge of the experimental field site. The high-frequency MCP soil water measurements provide an excellent means for improving ground-water recharge estimation.