MEASUREMENT OF EVAPORATION FROM BARE SOIL AND ITS ESTIMATION USING SURFACE-RESISTANCE

Evaporation from soil, E(s), is important to land surface energy balan ce and has been estimated in many studies using a surface resistance a pproach. We investigate the accuracy of this approach using detailed m easurement and simulation. Hourly evaporation rates were measured usin g microlysimeters and load cells at two semiarid sites with bare soil. A numerical model of water (liquid and vapor) and heat fluxes in a so il profile (the soil water, energy, and transpiration (SWEAT) model) p rovided an accurate simulation of measured evaporation rates. Using ou tput from SWEAT, relationships between soil resistance r(s) and soil s urface water content theta(s) (0-20 and 0-50 mm) are determined and ar e then used to estimate E(s). These r(s)-based models performed well o ver a period of several days but provided poor estimates of E(s) on an hourly or even a daily basis. A characteristic divergence between mea sured E(s) rates and potential evaporation rates at a time during the early daylight hours was not well simulated by r(s)-based models. An r (s)(theta(s)) function for a similar soil at a different location unde restimated E(s) by about 60%. Our work suggests that r(s) calculated f rom both evaporative demand and near-surface soil water content theta( s) is likely to be more accurate.