The U.S. Army Corps of Engineers is responsible for prudently managing the water stored in the large reservoirs on the main stem of the Missouri River--reservoirs such as Oahe, Garrison, and Fort Peck. Evaporation, especially during the height of summer, is a significant term in the water budget of these reservoirs; but the Corps' method for estimating this evaporation is based on labor-intensive, 50-year-old technology: Once a day an observer measures the change in water level in an evaporation pan of surface area 1 square meter, applies a calibration coefficient, and extrapolates this evaporation estimate to the entire reservoir--which, in the case of the Oahe Reservoir, has a surface area, typically, of about 556 square miles and stretches 215 miles between Pierre, S.D., and Bismarck, N.D. Clearly, this method is fraught with uncertainty.

As an alternative, we developed an automated, internet-based method that produces hourly estimates of reservoir evaporation by using data from National Weather Service (NWS) surface-observing sites near the reservoir. We demonstrate this method for the Oahe Reservoir using data from the NWS sites in Pierre and Mobridge, S.D., and in Bismarck, N.D. In particular, we adapt state-of-the-art bulk turbulent flux algorithms that have been developed to estimate fluxes over the ocean and over sea ice to this problem of estimating reservoir evaporation. One catch is that the NWS data stream does not include surface water temperature, a key variable in the bulk-flux algorithms. We therefore developed a method to bootstrap this quantity from the available NWS data by using a semi-empirical model of the Bowen ratio. As a result, we have been estimating evaporation, sensible heat flux, surface stress, and surface temperature at the Oahe Reservoir for about nine months and are archiving these estimates on a website that we have created for in-house Corps use. The evaporation estimates seem reasonable; we provide them hourly rather than daily (as with the pan-based estimates); and we even provide evaporation estimates through the winter, when the evaporation pans are not useful.

In our presentation, we will discuss the algorithm that we have developed, the available internet data sources, our attempts to extrapolate the individual evaporation estimates for Pierre, Mobridge, and Bismarck over the entire Oahe Reservoir, and a comparison of the predictions from our surface temperature algorithm with in situ measurements of surface temperature near the Oahe Dam.