MANAGING NEBRASKA'S GROUNDWATER RESOURCES IN THE PLATTE AND REPUBLICAN RIVER BASINS USING REGIONAL GROUNDWATER MODELS

Nebraska is a party to Interstate agreements limiting water use in the Platte and Republican River Basins. Management of Nebraska's water resources are split between 23 Natural Resource Districts (NRDs) and the Nebraska Department of Natural Resources (NDNR). The NRDs are local entities responsible for managing groundwater while surface water is managed by NDNR. The hydraulic connection between the two resources requires the NRDs and the NDNR to integrate their management objectives for long-term sustainability.

Tightening water supplies in the Platte River Basin and Nebraska's entry into the Cooperative Agreement prompted the formation of the Cooperative Hydrology Study (COHYST). COHYST has developed a set of three regional groundwater models of the Platte River Basin in Nebraska. These models are being used to determine stream depletions from post-1997 development and to determine sustainable levels of water use in the Platte River Basin.

Every year, the Republican River Compact Administration (RRCA) determines the allowable stream depletions to the Republican River for Nebraska based on annual water supplies. Recently, the RRCA established the means to account for stream depletions due to groundwater pumping through the use of the RRCA groundwater model. The RRCA model is also used to account for an imported water supply, the result of diverted surface water from the Platte River that reaches the Republican River due to a groundwater mound.

The subdivision of the Platte and Republican Basins into numerous NRDs presents additional challenges to the management of the groundwater resources both within and between basins. Stream depletions due to groundwater use in a particular NRD are dependent on development levels in adjacent NRDs. These interdependencies make it difficult to allocate stream impacts for each individual NRD. The sum of depletions independently determined for each NRD does not yield the total stream depletions determined for each basin as a whole. Management of water supplies, including any imported water, within each NRD depends on the ability to parse out the relative impacts due to development within each NRD. Through a large number of model simulations, the NRD-specific stream depletions and the relative balance between supply and demand for each NRD is being assessed within these two basins.