Evaluation of the ADAPT Model for Simulating Water Outflow from Agricultural Watersheds with Extensive Tile Drainage

In settings with relatively uniform soils and land use, field-scale hydrologic and water quality models may be useful simulation tools for analyzing the effects of climate change or policy alternatives. In this study, we evaluated the performance of the ADAPT model for simulating water outflow (tile drainage plus surface runoff) from 622 to 1500 km 2 watersheds in central Illinois, where a large portion of the land is used for maize-soybean production and tile drainage is common. Soil saturated hydraulic conductivity and porosity, rooting depth, leaf area index, drain spacing and depth, and drainage coefficient were used as calibration parameters to optimize the fit between measured and simulated water outflow from a 1500 km 2 watershed for 1987-1993. The applicability of the calibration parameters was tested by using these values for simulations of two neighboring watersheds for the 1987-1993 period and a second time period (1979-1986) for the three watersheds. The model effectively simulated annual water flow for all time periods and all watersheds (Willmott's index of agreement ranged from 0.95 to 0.99). For monthly and weekly water flows, Willmott's index decreased and ranged between 0.88 and 0.95 and between 0.88 and 0.93, respectively, and performance was markedly poorer for one of the three watersheds. For daily water flow, Willmott's index ranged from 0.77 to 0.89, and this is due partly to the temporal and spatial resolution of the input data and the streamflow data. The largest deviations between simulated and measured flow tended to occur during soil freezing and snowmelt periods. The results indicate that, with calibration, the model can provide good simulation of the annual water outflow and reasonable simulation of the monthly to weekly water outflow from tile-drained watersheds. The field-scale model performance was nearly as good as simulations that incorporated spatially explicit approaches to simulating watershed-scale hydrology in relatively uniform watersheds.