A Simple Model for Estimating Water Balance and Salinity of Reservoirs and Outflow

Reservoir storage reduces fluctuation in streamflow salinity, yet increases outflow salinity because of water evaporation. These processes are highly relevant to developing water management strategy, yet the method to predict outflow salinity has not been adequately examined. The study reported here examined the water and salt balance in a reservoir using a two-layer model. This model assumes that inflow blends with the storage, but the water evaporation takes place from the surface layer, and the percolation losses from the subsurface. The thickness of the first layer where salinity increases with evaporation was estimated through calibration against the measured outflow salinity. The changes in salinity were computed using a moving average method on a monthly time step. This model was applied first to Red Bluff Reservoir of the Middle Pecos River, then to Elephant Butte, Amistad, and Falcon along the Rio Grande. The outflow salinity projected by the model was in good agreement with the measured, except under a few circumstances where mixing of inflow and reservoir storage was suspected to be incomplete. The accuracy of prediction can be improved by improving the estimate of initial salinity of reservoir storage, which is currently taken as being equal to outflow salinity at the onset of the simulation.