Estimation of Evapotranspiration at Alfalfa Field on Irrigated Land in Arid District

Abstract

For the purpose of estimating the evapotranspiration on an irrigated land in arid district, the budget of radiation, the budget of energy and meteorological parameters were measured at an alfalfa field in an irrigated farm. In the daytime, the latent heat flux increased to 450 W/m2 which was about 4.5 times greater than the sensible heat flux. The bulk transfer coefficient of sensible heat flux was estimated to be 0.0088. The canopy resistance (rc) on the alfalfa field was equal to zero before 10 o'clock because of the evaporation of dew. The rc values were about in the range of 40∼60 sec/m from 12 till 17 o'clock then increased rapidly. A model for rc which contains solar radiation as variable was adopted. On the basis of these results, a model for estimating the evapotranspiration was developed. Then sensible heat flux, latent heat flux, reflectivity, amount of dew and transpiration were estimated from air temperature, vapor pressure of air, wind speed, solar radiation, long wave radiation, net radiation and soil heat flux which were measured for 17 days at the alfalfa field. Computed diurnal change of reflectivity was about the same as measured one. There was a positive correlation between estimated daily amount of transpiration and measured daily soil water decrease.