APPLICATION OF MODEL ABSTRACTION TECHNIQUES TO SIMULATE TRANSPORT IN SOILS
Title: Uncertainty in Climatology-Based Estimates of Shallow Ground Water Recharge
Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: September 24, 2007
Publication Date: December 14, 2007
Citation: Guber, A.K., Pachepsky, Y.A., Gish, T.J., Nicholson, T.J., Cady, R.E. 2007. Uncertainty in Climatology-Based Estimates of Shallow Ground Water Recharge. Fall Meeting of the American Geophyscial Union, December 10-14, 2007, San Francisco, CA. Abstract H23B-1304.
The groundwater recharge (GR) estimates for flow and transport projections are often evaluated as a fixed percentage of average annual precipitation. The chemical transport in variably saturated heterogeneous porous media is not linearly related to the average velocity. The objective of this study was to estimate the variability in annual, seasonal, and event-based GR at the field scale and to quantify sources of the uncertainty in such estimates.
Research was done at a 3.6 ha field, which is part of a 21-ha agricultural research site located at the USDA, Beltsville Agricultural Research Center, Maryland. The soils are sandy with buried clay lenses. Eight soil moisture multi-sensor capacitance probes (MCP) monitor water contents in soil layer 0-180 cm at 10 minute intervals. Surface runoff water is measured at the outlet of the field with a flow meter installed on a 45.7 cm H-flume. An energy balance weather station inside the field is the source of detailed meteorological data. A standard weather station is located in 4 miles. The water budget method was used to compute the GWR from precipitations, runoff, ET data and soil water content measurements at depths of 10, 30, 50, and 80 cm. ET and precipitation were assumed uniform across the field. The event-based estimates of GWR were defined as the infiltration water losses below depth of 80 cm at time interval between two consecutive local minima on soil water storage series.
The average estimated percentage of rainfall used for recharge for a single recharge event was 56%, 40%, 28%, 11% and 11% for rainfalls 0 to 10 mm, 10 to 20 mm, 20 to 30 mm, 30 to 60 mm, and larger than 60 mm, respectively. Estimated average recharge per single event differed by seasons and constituted 21%, 43%, 37% and 44% of precipitation in spring, summer, fall, and winter respectively. Total annual amount of rainfall, estimated ET and runoff over the 365 days of the observation period was 909 mm, 386 mm, and 276 mm, respectively. The recharge amount estimated from the water budget for this period was from 175 mm to 220 mm and constituted from 20% to 25% of precipitation. No statistically significant dependence of the estimated recharge on the soil water content in the beginning of the recharge event was found. We present data on and discuss the sources of uncertainty of the above estimates, i.e. plant water status, biomass, and yield; runoff contributing area; areas represented by individual MCP; errors in Pennman-Monteith daily ET estimates; lateral subsurface water pathways and subsurface run-on and run-off in the vicinity of sensors.