Rainfall Losses
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“Evaporation data is usually required for reservoir studies, particularly for low-flow analysis. Reservoir evaporation is typically estimated by measuring pan evaporation or computing potential evaporation”.[1]
“Evapotranspiration (ET) is difficult to estimate because it is a complex process. It is determined by the atmospheric demand for water vapor (potential ET) and the availability of water to be evaporated. ET is a sum of pure evaporation from free water surfaces, such as wet vegetation, puddles, and lakes, and the transfer of soil moisture through plants and out their leaves (transpiration). The former process depends only on the atmospheric conditions (temperature, humidity, wind), whereas the latter also depends on plant characteristics (stomatal resistance) and on soil moisture availability”.[2]
“Many models are available for estimating potential evapotranspiration from meteorological data (Jensen, Burman, and Allen 1990; ASCE 1996). They vary in their assumptions, the processes described, the input data required, and the temporal scale for which they are appropriate. Potential ET can also be estimated from pan evaporation data if suitable pan coefficients are available” (National Engineering Handbook 210 Part 630 Hydrology: Chapter 20 Watershed Yield, NRCS, 2009).[2]
“Even if potential ET is adequately estimated, the actual ET is less than or equal to this amount and depends primarily on soil moisture availability. Because of this interplay between the atmospheric demand and the soil moisture, determining the actual ET is problematic without a detailed hydrologic model operated at a short time step (i.e., a day or less). If adequate assumptions can be made, however, reasonable estimates of actual ET as a fraction of potential ET are possible”.[2]
Best Practices Resources
Flood Hydrology Manual (Bureau of Reclamation)
Citations:
Revision ID: 3648
Revision Date: 09/17/2022