SENSITIVITY OF RESERVOIR-YIELD ESTIMATES TO MODEL TIME-STEP SURFACE-MOISTURE FLUXES

Determining the yield from a storage reservoir typically requires estimating the contribution to storage by precipitation and the loss of water by free-surface evaporation over time, among other factors. Due to the difficulty in estimating reservoir evaporation by accepted energy budget methods, it is useful to determine the sensitivity of estimated reservoir yield to both factors. First, an experiment comparing storage-yield curves generated by a daily, monthly, and annual time-step mass-balance reservoir model is performed. The monthly time-step model's estimated yield is slightly larger than that estimated using a daily time-step model, and the error becomes smaller as the storage capacity gets larger. In a second experiment, comparisons are made for yield estimates using different combinations of precipitation and potential evaporation time series and/or mean monthly values. Assuming that the best reservoir yield is determined using a year-round, open-water, potential-evaporation time series, fixed average monthly values of potential evaporation and monthly precipitation time series are shown to accurately estimate reservoir yield in Massachusetts regardless of the storage capacity. The sensitivity of yield estimates to reservoir ice and snow cover is shown to be negligible.