Open water evaporation estimation for a small shallow reservoir in winter using surface renewal

摘要

Evaporation is one of the main components of the energy and water balance of reservoir water behind dams and is a major component of water loss. Measurements in winter of sensible heat (H) for the small and shallow reservoir of Midmar Dam, KwaZulu-Natal, South Africa were made using the surface renewal (SR) method, a renewal model method that uses the average cubic air temperature structure function and eddy covariance (EC). The small magnitude of H during winter makes it difficult to test the reliability of the H estimates obtained using instruments and sensors mounted above the water surface using the SR, renewal model and EC methods. Latent energy flux was estimated as a residual of the energy balance using additional measurements of net irradiance Rn above the water surface and the water-stored heat flux. The SR sensible heat flux (HSR) was estimated for heights of 1.0, 1.3, 1.9 and 2.5 m above the water surface using two air temperature time lags r of 0.4 and 0.8 s of the 10-Hz measurements. The SR method depends on a weighting factor a which represents the capability of the atmospheric turbulence to mix the scalar, within the air parcel to be renewed. The factor a was determined for each measurement height and time lag from the slope of a linear regression relationship forced through the origin of measured EC sensible heat flux (HEC) values on the y-axis vs. HSR or renewal model H on the x-axis. All a values obtained using the renewal model method were not statistically different from that obtained using the SR method for z = 1.0 m for both time lags. Using a calibration dataset, an average a value for the 1.0- and 1.3-m heights of 0.198 for r = 0.4 s and 0.245 for r = 0.8 s for the SR and renewal model methods was obtained. The 30-min HSR, renewal model and HEC estimates were often the smallest component of the energy balance (generally 40 to 40Wm2) and compared reasonably well for the validation dataset. The heat storage flux G was larger in magnitude (0–200Wm2) compared to the sensible heat flux. The SR, renewal model and EC latent energy fluxes, each calculated as residuals of the energy balance, were almost the same in magnitude as the available energy flux Rn G due to the relatively small magnitude of the sensible heat flux during the winter measurement period. The daily evaporation ranged between 1.0 and 3.9 mm.