COHYST–NEBRASKA'S COOPERATIVE HYDROLOGY STUDY FOR GROUNDWATER-SURFACE WATER MANAGEMENT

Colorado, Nebraska, Wyoming, and the U.S. Department of Interior have entered into an agreement related to four endangered species in the Platte River Basin. Part of the agreement is for the states to disallow or mitigate any new depletions of streamflow in the Platte River caused by development of groundwater supplies after July 1997. Twelve state and local agencies in Nebraska formed the Cooperative Hydrology Study (COHYST) to develop detailed databases and regional groundwater models that would assist Nebraska in meeting its obligations under the agreement and allow the agencies to manage groundwater and surface water in a scientifically defensible manner. COHYST is supported by eight other partners, including municipalities, environmental groups, and water-use organizations.

The COHYST model area covers 29,300 square miles of aquifers in the Platte River Basin in Nebraska, an area larger than Massachusetts, New Hampshire, and Vermont. The main product of COHYST is groundwater-flow models with sufficient detail to analyze the hydrologic effects of activities proposed under the agreement. Models now have 1-square-mile cells but eventually will be refined to 0.25-square-mile cells. The COHYST team has generated detailed data sets needed for the modeling, including crop and land-use maps for 1997; crop demand, recharge, and runoff using a sophisticated soil-water balance model; reinterpretation of lithology from more than 1,700 existing test holes; detailed mapping of up to ten hydrologic layers; and measurements of streambed hydraulic conductivity at a number of sites using four different techniques. The data are being prepared in greater detail than required for the current models in anticipation of future needs. As data sets are finalized, they are posted on an Internet site: http://cohyst.nrc.state.ne.us/

Future work planned in a recent 3-year extension of COHYST includes updating crop and land-use data for 2001, estimating historical crop and land-use, drilling new test holes, using geophysical techniques to fill in lithology between test holes, and measuring riparian forest evapotranspiration rates at two tower sites. COHYST intends to report on a number of these activities at the 2002 GSA meeting and will invite others involved in the three-state cooperative agreement to do likewise.