Field Data and Model Predictions for a Monolithic Alternative Cover

Water balance data from a test section simulating a monolithic alternative cover were compared to predictions made with two numerical models: UNSAT-H and Vadose/W. On-site data were used as model input to the greatest extent practical. More accurate predictions were obtained with Vadose/W than UNSAT-H. Surface runoff was overpredicted appreciably by UNSAT-H, which affected all subsurface hydraulic processes. In contrast, Vadose/W accurately predicted surface runoff, evapotranspiration, and the temporal variations in soil water storage. However, neither model predicted percolation accurately. Both models also failed to capture a key change in the transpiration pattern during the last winter-summer period of the study. Differences in the method used to simulate precipitation intensity appear partly responsible for the difference in accuracy by which the two models predict surface runoff. Simulations were conducted with the lower boundary condition as a unit gradient or a seepage face to evaluate how this boundary condition affects predictions of the water balance. Essentially the same predictions were obtained regardless of the lower boundary that was used.