Storage analyses for ephemeral streams in semiarid regions

A model has been developed for determining the probability of a reservoir being unable to provide a specified downstream water supply. By applying the model with a number of assumed storage capacities, the long-term water supply potential of a stream below a reservoir can be evaluated. Previous methods for determining available water supply from streamflow records using a reservoir storage analysis have met with limited success in semiarid regions. The shortcomings are due to the failure of the methods to account for zero-flow periods and the high day-to-day variability of discharge of many streams. The reservoir storage model presented in this report is designed to account for these streamflow characteristics. Reservoir inflow, outflow, and evaporation are modeled as varying daily, and values of storage probability are adjusted for zero-flow periods. (USGS)