Measurement of evaporation from bare soil and its estimation using surface resistance

Evaporation from soil, E(s), is important to land surface energy balance and has been estimated in many studies using a surface resistance approach. We investigate the accuracy of this approach using detailed measurement and simulation. Hourly evaporation rates were measured using microlysimeters and load cells at two semiarid sites with bare soil. A numerical model of water (liquid and vapor) and heat fluxes in a soil profile (the soil water, energy, and transpiration (SWEAT) model) provided an accurate simulation of measured evaporation rates. Using output from SWEAT, relationships between soil resistance r(s) and soil surface water content theta(s) (0-20 and 0-50 mm) are determined and are then used to estimate E(s). These r(s)-based models performed well over a period of several days but provided poor estimates of E(s) on an hourly or even a daily basis. A characteristic divergence between measured 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 underestimated E(s) by about 60%. Our work suggests that r(s) calculated from both evaporative demand and near-surface soil water content theta(s) is likely to be more accurate.