Estimation of soil evaporation using the differential temperature method

Evaporation of soil water is a major water balance component during early growth stages of irrigated field crops, row crops with incomplete cover, and in soils with high water table. Quantification of soil evaporation can help in environmental and irrigation management. The objective is to develop a new method to estimate daily soil evaporation using differential measurements of temperature. A major advantage of this approach is that measurements of sensible heat flux can be replaced by those of surface temperature. An empirical coefficient was determined from integrated energy fluxes over daytime hours and mean daytime temperatures. It leads to a new soil evaporation transfer coefficient that can replace the aerodynamic resistance to calculate sensible heat flux. Experiments were conducted in a field with sandy soil that was irrigated with sprinklers and included a weighing lysimeter to measure actual evaporation. Measurements were of net radiation, soil heat flux over a wet and a reference air-dried soil, and air and surface temperatures of both soils. Regression between modeled and actual evaporation on a daily basis produced a slope of 1.05 and r(2) = 0.9. Thirty days of cumulative model evaporation exceeded the measurements by 5%. The proposed coefficient can theoretically vary from 0 for wet soil to 1 for dry soil and can thus provide limits between 0 and potential evaporation. Actually, the coefficient increased from 0.2 to 0.8 in wet soil and from 0.8 to 1.0 in dry soil. The soil evaporation transfer coefficient was easy to measure, and it was sufficiently stable to adequately estimate soil evaporation.