Water and Energy Balance Components for Measurement and Estimation

This chapter describes evapotranspiration (ET) measurement approaches. The challenge with these approaches is in extrapolating point or localized measurements to large spatial scales and in extrapolating measurements to other time periods. Energy balance methods based on satellite imagery can be applied successfully to estimate actual ET over large areas. Mass balance involves measuring the change in mass caused by evaporation (E) or ET and determining the E or ET rate or volume as a residual of a balance. The Bowen ratio energy balance method rearranges the energy balance equation to cancel aerodynamic transport terms. ET can also be measured directly using eddy covariance sampling of the boundary layer according to the relationship described in the chapter. The chapter also describes the combined use of aerodynamic expressions for sensible heat transfer and energy balance methods to estimate ET as a residual of the surface energy balance.