EVAPORATIVE FLUX FROM IRRIGATED COTTON AS RELATED TO LEAF AREA INDEX, SOIL-WATER, AND EVAPORATIVE DEMAND

A study was made to formulate and test empirical relationships for estimating soil evaporation and transpiration based on evaporative demand and crop development with limiting and non-limiting soil water regimes. The relationships were compared with evaporation and transpiration data for irrigated cotton (Gossypium hirsutum L.). Evaporation from the soil surface was determined by measuring diurnal changes in the surface soil-water content and weight changes in embedded containers. Evapotranspiration [ET] was calculated from the quantity of irrigation water applied, rainfall, drainage and changes in soil-water content measured by the neutron scattering technique. Evaporation from the soil surface on the 1st day after irrigation (ES0) was related to potential evaporation (Eo) and leaf area index (LAI) as: ESo = Eo exp(-0.623 LAI). On successive days after irrigation, surface evaporation (ES) was computed as: ES = 5.8t0.6 -5.8(t - 1)0.6, where t is time in days. For transpiration on the day after irrigation (EPo), the following relationship was found: EPo = Eo(1 - exp(-0.623 LAI)). For limiting soil-water conditions, transpiration (EP) was computed with: EP = EPo/ (1 + 6.2 exp - 15.2 FAW), where FAW is the fraction of available water in soil. The results obtained with these equations agreed well with values for ES and EP measured in this study and with data presented by Ritchie (1972).