Evaluation of the applicability and validity of hydrologic simulation models for various cropping systems in different hydrogeologic and soil conditions is needed for a range of spatial scales. We calibrated and tested the ADAPT model for simulating streamflow from 552 to 1,985 km2 watersheds in central Illinois, where more than 79 percent of the land is used for maize‐soybean production and tile drainage is common. Model calibration was performed with a seven year period (1987 through1993) of measured streamflow from one of the watersheds, and model testing was done using independent weather and measured streamflow data from the two neighboring watersheds for the same seven year period. Simulations of annual streamflow were accurate with a coefficient of determination and Willmott's index of agreement of 0.98 and 0.99, respectively. For simulation of monthly streamflow, Willmott's index of agreement ranged from 0.93 to 0.95. For simulation of daily streamflow, Willmott's index of agreement ranged from 0.84 to 0.85. The daily simulations challenged the temporal and spatial resolution of our measured precipitation data. Discrepancies between simulated and measured data may result from the model's inability to effectively address frozen soils and snowmelt runoff processes and in accurately representing evapotranspiration.

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Long term data on surface water quality can sometimes be assembled by combining data collected by different agencies at different times and assuming that between agency differences in data quality are insignificant. The objective of this paper was to assess the quality of riverine nitrate (NO3) concentrations in Illinois measured and reported by four agencies from 1967 to 1974 by comparing median values for similar sampling locations and periods. A total of 17 river reaches were identified for which two agencies reported NO3 concentrations during similar periods. Nonparametric comparison of median values and analysis of covariance with discharge as a covariant produced similar results. Nitrate concentrations reported by the U.S. Geological Survey (USGS) from 1967 to 1971 were not statistically (P > 0.05) different from values reported by the Illinois State Water Survey (ISWS) for two of three river reaches. Additionally, NO3 concentrations reported by USGS from 1972 to 1974 were not statistically different than concentrations reported by the Illinois Environmental Protection Agency (IEPA) for four of five river reaches. From 1969 to 1971, NO3 concentrations reported by the Illinois Department of Public Heath and the Illinois Environmental Protection Agency (IDPH/IEPA) were less than one‐fourth the magnitude of values reported by ISWS. The median NO3 concentrations measured by the Central Illinois Public Service (CIPS) were significantly greater than those measured by USGS and IDPH/IEPA in the three comparable sampling locations. The use of NO3 concentrations measured by CIPS and IDPH/IEPA prior to 1972 is not recommended.

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