Toward a Robust Phenomenological Expression of Evaporation Efficiency for Unsaturated Soil Surfaces

The evaporation rates of water from several soil types were measured under controlled conditions. When the layer of soil is sufficiently thin, the evaporation efficiency β, the ratio of the evaporation rate from the soil surface relative to that from the watered surface, is described well by a function of the mean volume fraction θ of water. The function has a common form for several types of soil and can be parameterized by a single parameter, the characteristic volume fraction θc of water, as β = 1 − exp(−θ/θc). The value of θc depends on the soil type and also on the wind speed. The latter dependence can be robustly described in terms of the evaporation resistance for the watered surface. These findings are expected to become a basis for calculating the evaporation efficiency for the soil with an arbitrary depth in a systematic way.