ESTIMATING GROUND-WATER DISCHARGE TO STREAMS FOR USE IN THE NEBRASKA COOPERATIVE HYDROLOGY STUDY (COHYST) MODEL CALIBRATION

COHYST is producing ground-water flow models to analyze the effects of water-management activities proposed under the Three-State (CO, NE, WY) Cooperative Agreement. Ground-water discharge to streams is an important calibration data set for these models. This discharge was estimated for 16 tributaries to and 7 reaches of the N. Platte River, 1 tributary to and 2 reaches of the S. Platte River, 3 tributaries to and 5 reaches of the Platte River, 12 tributaries to and 5 reaches of Frenchman Creek/Republican River, 3 reaches of the Loup River, and 3 tributaries to the Big Blue River. Ground-water discharge to streams was estimated using period-of-record fall (October-November) daily flows from streamflow gaging stations operated by the U.S. Geological Survey and Nebraska Department of Natural Resources. Spring and summer flows were not used because of diversions, returns of diversions, runoff from irrigation and precipitation, and evapotranspiration along the streams. Winter flows were not used because ice conditions can affect the accuracy of streamflow data. The lowest fall streamflows represent ground-water discharge to streams. For tributaries, the fall 7-day low flow with a recurrence interval of 5 years was used as the minimum estimate of ground-water discharge at the station and the 14-day, 2-year low flow was used as the maximum. For main-stem reaches, total fall downstream outflow minus total fall upstream inflow was computed for each year of record. The minimum estimate of ground-water gain or loss to the reach was the fall gain or loss with a recurrence interval of 5 years; the maximum estimate was the gain or loss with a 2-year recurrence interval. Long-term ground-water discharge to tributaries of the N. Platte, S. Platte, and Platte Rivers upstream of the Loup River ranged from zero for Wood River to 146 cubic feet per second (cfs) for Birdwood Creek. Ground-water discharge to tributaries of the Frenchman Creek/Republican River ranged from zero for several tributaries to 33 cfs for Medicine Creek (pre-dam). Ground-water discharge to main-stem reaches ranged from a gain of 150 cfs on the N. Platte River from Mitchell to Minatare (18 mi) to a loss of 150 cfs on the Platte River from Odessa to Grand Island (56 mi).