Eleven state and local agencies and eight partner organizations created the Cooperative Hydrology Study (COHYST) to develop detailed databases and regional groundwater models that would provide a scientifically defensible decision support system (DSS) to assist Nebraska in water management planning and in meeting its obligations under a 1997 Cooperative Agreement (CA) with Wyoming, Colorado and the U.S. Department of Interior. The CA calls for each state to implement a plan that would mitigate new depletions to the U.S. Fish and Wildlife Service (USFWS) target flows for critical habitat in the Platte River from Lexington to Chapman, NE. The COHYST model area covers 29,300 square miles of aquifers in the Platte River Basin from Columbus to 6 mi inside Wyoming and Colorado. The main product of COHYST is a set of three overlapping regional groundwater-flow models with sufficient detail to analyze the stream depletion effects of surface and groundwater management options. This paper describes the models and the detailed data sets developed or being developed for the modeling, including crop and land-use maps for1982, 1997, and 2001; changes in land use over time; crop water demand, net recharge, and runoff using a complex soil-water balance model; mapping of ten hydrostratigraphic layers from 1,700 existing and 300 new test holes; virtual measurements of riparian woodland evapotranspiration by eddy covariance methods, measurements of streambed hydraulic conductivity, temperature tracing of discharge to ground water, direct measurements of canal seepage, geophysical techniques to obtain lithology beneath canals, and measuring pumpage to confirm model parameters. Calibrated models exist with 160-ac cells but eventually could be refined to sub-regional models with 10-ac cells. Calibration of the models is described, as well as how the models are being used to delineate zones of hydrologically connected groundwater, to analyze mitigation plans, and to support local agencies in developing water management plans.