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Title: EVALUATING UNCERTAINTIES IN GROUND-WATER RECHARGE ESTIMATES THROUGH ADVANCED MONITORING

Author
item Starr, James
item Timlin, Dennis
item CADY, RALPH - US NRC
item NICHOLSON, THOMAS - US NRC

Submitted to: American Geophysical Union
Publication Type: Proceedings
Publication Acceptance Date: 6/20/2002
Publication Date: 5/28/2002
Citation: Starr, J.L., Timlin, D.J., Cady, R., Nicholson, T. 2002. Evaluating uncertainties in ground-water recharge estimates through advanced monitoring. Am. Geophy. Union 2002 Spring Meeting Abstracts. 83 (19):S161.

Interpretive Summary: Environmental risk, as estimated by many environmental models, is highly sensitive to infiltration and ground-water recharge. This field study used high-frequency monitoring of the soil water content and water-table levels to build confidence in modeling infiltration and ground-water recharge. At this site, the shallow water table is highly responsive to frequent episodic rainfall events. Both conceptual-model uncertainty and parameter uncertainty were evaluated using near-continuous water content, water-table elevation, and meteorological data. The conceptual models included variations on four fundamental approaches to estimate recharge. Highly-detailed monitoring data were evaluated using time-series analysis to capture individual recharge-event characteristics. For each conceptual modeling approach, uncertainty distributions were developed for the input parameters and resulting groundwater recharge estimates. Advanced monitoring instruments provided realistic site data for evaluating these conceptual models. Part of the evaluation was the examination of realistic assumptions such as no lateral flow and no offsite contribution to recharge. A comparison of the independent conceptual models suggested that the soil-water content and water-table models gave more realistic estimates than the meteorological model, but neither water-contents nor water-table measurements alone were consistently superior. For example, during relatively dry periods, high-frequency water-content measurements may provide a more realistic estimate of ground-water recharge.

Technical Abstract: Risk, as estimated by many multimedia environmental models, is highly sensitive to infiltration and ground-water recharge. This field study used high-frequency monitoring of vadose-zone water content and piezometric levels to build confidence in modeling of infiltration and ground-water recharge. At this site, the shallow water table is highly responsive to frequent episodic rainfall events. Both conceptual-model uncertainty and parameter uncertainty were evaluated using near-continuous water content, water-table elevation, and meteorological data. The conceptual models included variations on four fundamental approaches to estimate recharge. Highly-detailed monitoring data were evaluated using time-series analysis to capture individual recharge-event characteristics. For each conceptual modeling approach, uncertainty distributions were developed for the input parameters and resulting recharge estimates. Advanced monitoring instruments provided realistic site data for evaluating these conceptual models. Part of the evaluation was the examination of realistic assumptions such as no lateral flow and no offsite contribution to recharge. A comparison of the independent conceptual models suggested that the water content and piezometer models gave more realistic estimates than the meteorological model, but neither water-contents nor piezometer measurements alone were consistently superior. For example, during relatively dry periods, high-frequency water-content measurements may provide a more realistic estimate of ground-water recharge.