Jiftah Ben Asher, Ben Gurion university Israel, Beer Sheva, Beer Sheva, 894900, Israel and Guo Yu Qiu, Beijing Normal University, China, Beijing, beijing, China.
Evapotranspiratin during the initial stage for annual crops is predominately in the form of evaporation. The time evolution of evaporation from a bare soil, following irrigation, affect its surface temperature that alternatively has been used to estimate the evaporation rate. The objective of this study was to quantify latent heat flux (LHF) of three soils textures throughout their drying process. Sand, loam, and clay were used. The soils were placed in a growth chamber with constant wind velocity (0.5 ms-1), air temperature (25°C), relative humidity (50%), and light intensity (80,000 lux, 300 -400W/m2). LHF was measured with microlysimeters. The relationships between the LHF and the surface temperature was determined by infrared thermometer on an hourly basis. After 540 hours through which soil moisture changed from wet to dry the cycle was completed. Evaporation was dominated by two different stages of drying. The first stage was controlled by available energy at the surface. In the second stage, evaporation was controlled by the rate of water vapor supply from the soil layers below. Evaporation in the first stage varied between 0.4 and 0.45 mm/hr (equivalent to 280-300 W/m2). This stage lasted 84 hrs , 96 hrs and 120 hrs in the sand loam and clay soil respectively, accompanied by a constant temperature of 31.7±1.2 ºC ,30.1±1.0 ºC and 29.0±1.0 ºC.. In the second stage evaporation reduced to about 0.05 mm/hr (30-35 W.m2) and the elapsed time was 200, 250 and 300 hrs for the respective soils with temperatures of about 43ºC in all soils texture. The minor changes of dry soil temperature with time were not affected by relative humidity, strengthening the assumption of no LHF.