Through the conversion of forests and grasslands to croplands and pastures, humans have altered the movement of water along the land surface and the transport of water between the land surface and the atmosphere. Land surface process models, such as the IBIS model (Foley et al., 1996; Kucharik et al., 2000), may be used to evaluate the effects of land cover change on the water balance and hydrology of large drainage basins.

We have performed a series of simulations to determine the effects of land cover change on the water balance and hydrology of the Mississippi River Basin. Two models are used—the Integrated BIosphere Simulator (IBIS) and the HYDrologic Routing Algorithm (HYDRA). The IBIS model describes physical, physiological, and ecological processes occurring in vegetative canopies and soils. IBIS uses input climate data and vegetation and soil properties to simulate energy, water, and biogeochemical cycles at small time-steps (30-60 minutes) and at 0.5-degree resolution. The HYDRA model (Coe, 2000) then routes surface runoff and subsurface drainage to streams and lakes at a 5-minute resolution and at hourly time-steps. Lenters et al. (2000) have validated the IBIS-modeled water budget over the Mississippi River Basin at several scales. Similarly, the HYDRA-modeled stream discharge compares favorably to United States Geological Survey stream gauge data (Donner et al., 2001). This work extends those studies through the introduction of an improved version of IBIS for use over the continental United States. The regional version of IBIS includes algorithms that simulate the phenology of corn, soybean, and wheat. The effects of a conversion of forests and grasslands to croplands within the Mississippi River Basin are evaluated through a comparison of a control simulation (containing only natural ecosystems) with an experimental simulation (containing current land cover of natural vegetation and crops).

Results show that a conversion of forests and grasslands to croplands decreases the evapotranspiration and increases the surface runoff, drainage, and stream discharge in proportion to the fractional crop cover. Model results may be compared with results from an analysis of stream gauge measurements of discharge over various types of land cover in order to evaluate how land cover contributes to the magnitude and variability of streamflow within the basin.