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# Field Studies to Confirm Uncertainty Estimates of Ground-Water Recharge (NUREG/CR-6946)

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## Publication Information

**Manuscript Completed:**July 2007**Date Published:**August 2008

**Prepared by:** A. Chinkuyu 1, A. Guber 2, T. Gish1, D. Timlin1, J. Starr11, T. Nicholson 3, R. Cady 3, A. Schwartzman3

1U.S. Department of Agriculture Agricultural Research Service Beltsville Agricultural Research Center Beltsville, MD 20705-2350

2University of California Riverside Department of Environmental Sciences Riverside CA 92521

3U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research Washington, DC 20555-0001

T.J. Nicholson, NRC Project Manager

**Prepared for:** Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission  Washington, DC 20555-0001

NRC Job Codes Y6363 and Y6724

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## Abstract

Little is known regarding data requirements and model uncertainties for evaluating surface and subsurface fluxes at the small watershed scale, common to nuclear facility sites. This field study was conducted to evaluate data and monitoring approaches for determining both short- and long-term ground-water recharge estimates on different spatial and temporal scales. Four methods for estimating within-field ground-water recharge were evaluated: (1) simple mass budget method of precipitation less evapotranspiration, surface runoff, and soil water storage measured with multiple capacitance probes (MCP) (***water budget method***); (2) negative changes in soil water storage less evapotranspiration (***MCP**ET method***); (3) ***stream flow*** hydrograph***separation method***; and (4) 35% of the observed precipitation (***35% P***). Estimates of recharge were determined and compared for the 12-month period of observations of soil moisture, precipitation, meteorological variables, surface runoff, and stream flow which were recorded every 10 minutes at an intensively-monitored, agricultural-production site in Beltsville, Maryland, where all of the surface- and subsurface-water discharge into a first-order stream. The recharge amount estimated from the *water-budget method* for the whole observation period was from 20% to 25% of precipitation which is close to recent ground-water recharge estimates for Maryland and Pennsylvania obtained with different methods. Overall average percentage of rainfall used for recharge from a single recharge event was 35.3%. The *MCP-ET method* gave about twice as much recharge from a single event as compared with the *water-budget method*, most probably because of lateral movement of water with focused flow. The *stream flow-separation method* provided base flow estimates that on average were close to the values derived from spatially averaged soil water budget estimates. The other two methods, *MCP-ET* and *35% P*, gave much larger recharge estimates than the *stream flow separation method*. Overall, using MCP data provided real-time, near-continuous estimates of soil water storage and high accuracy in delineating the recharge events. The high-spatial variability of estimated ground-water recharge over the watershed area is an indication of soil and hydrologic variability and suggests that simple averaging techniques may generate unreliable watershed-wide ground-water recharge estimates. 

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