Estimating Areal Evaporation Using Multispectral Satellite Observations

Abstract

A quantitative knowledge of evaporation is required in many studies of hydrologic, meteorologic, and ecologic processes (Brutsaert, 1982). Evaporation (E) directly couples land surface heat and water balance equations: $${{R}_{n}} = \lambda E + C + \nabla \sum$$ (1a) $$P = E + Q + \Delta S$$ (1b) where, in the heat balance Equation (1a), Rn is the net radiation, C is the sensible heat flux, ΔΣ is the change in heat storage within the canopy and soil, and λ is the latent heat of vaporization. In the water balance Equation (1b), P is precipitation, Q is runoff, and ΔS is the change in water storage (soil moisture and ground water). The storage terms are neglected in long-term studies to obtain: $${{R}_{n}} = \lambda E + C$$ (2a) $$P = E + Q$$ (2b) Budyko (1986) introduced the following empirical equation for climatologic annual evaporation, which has been recently evaluated theoretically by Milly (1994): $$E = P\left[ {Dtanh{{D}^{{ - 1}}}\left( {1 - \cosh D + sinhD} \right)} \right]0.5$$ (3) where D, called the dryness index, is defined as the ratio of net radiation (Rn) and latent heat equivalent of precipitation (λP), i. e., D = (Rn/λP).