Field Data from a Capillary Barrier and Model Predictions with UNSAT-H

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Water balance data are presented from a capillary barrier test section located on the final cover of a municipal solid waste landfill in a semiarid region (E. Wenatchee, Washington, U.S.). Water balance and meteorological data were collected from November 1992 to August 1995. Estimates of the water balance were made using the program UNSAT-H, with input consisting of meteorological data, soil properties, and vegetative information. Estimates of evapotranspiration and soil-water storage by UNSAT-H agree reasonably well with the field data. Peak soil-water storage was underestimated during the winter and evapotranspiration was overestimated in late winter. Water contents were estimated reasonably, although the changes in water content of the sand obtained from UNSAT-H were not as large as, and occurred less quickly than, that in the field. Percolation was generally overestimated, with the greatest overestimation occurring during Winter 1993, which had substantial snowfall. Surface runoff was underestimated; no runoff was obtained from UNSAT-H, whereas 7.4 cm of runoff was measured in the field. The overestimates in percolation appear to be closely related to underestimates in runoff and extra storage in the sand layer caused by the geocomposite drain used in the test section. Snowmelt, freezing of the soil surface, and hysteresis in soil hydraulic properties also appear to have had an effect on the differences between estimated and measured water balances.