The primary products of COHYST are groundwater-flow models that can be used to analyze the effects of water-management activities proposed under the Three-State Cooperative Agreement between Nebraska, Wyoming and Colorado. A fundamental step in this process is the calibration of groundwater-flow models of the area.

The 29,300 square-mile study area in central and western Nebraska was divided into three model units, which overlap adjoining units by at least 12 linear miles along the river (COHYST Flow Modeling Strategy, at http://cohyst.nrc.state.ne.us/cohyst_data.html). Ideally, initial models should simulate conditions prior to any surface-water or groundwater development, when the flow system was in dynamic equilibrium (steady state). However, surface water irrigation occurred in the study area as early as the 1890s. The lack of data makes formal calibration for that time period impractical. The flow system is assumed to have been in equilibrium with the effects of early canal operations by the time major groundwater development for irrigation began in the 1950s. Therefore, the pre-groundwater development period was defined as prior to 1946. Sufficient calibration data exist for 1946 and later over much of the area. Pre-groundwater development models were calibrated against observed water levels and estimated groundwater discharge to streams.

Initially, single-layer groundwater-flow models were constructed with 4 square- mile cells. The final goal is to construct and calibrate transient groundwater-flow models for the period 1949-1997, with ¼ square-mile cells and multiple hydrostratigraphic layers. Several intermediate models were constructed to incorporate increasing complexity as refined geologic or net recharge data became available. All models were calibrated against observed water levels and groundwater discharge to streams, and transient models also were calibrated against water-level change maps.