Evaluation of Evapotranspiration by Using Numerical Model and Remotely Sensed Surface Temperature Data

Abstract

Evapotranspiration (ET) involves a complex set of processes which are influenced by the local conditions. These conditions are characterized by meteorological and ground surface condition. ET for large area with different land covers were estimated by evaluating the energy balance components for each land use, using a numerical model of the energy balance which includes differential equations of wind velocity, specific air humidity, potential air temperature, soil temperature and soil moisture. Remotely observed surface temperatures were used to modify the parameters used within the model. The inputs for this numerical model are direct and diffused solar radiation, solar elevation, long wave radiation, surface temperature, ground moisture, crop transpiration resistance, leaf area index, crop canopy architecture, wind velocity, air temperature and relative humidity. The extinction of solar radiation intensity as it passes the land cover canopy is estimated by treating the plant canopy as a single ‘big leaf’. As for the upper boundary of this system, constant values of wind velocity, potential air temperature, and specific humidity were used at the height of 100 m above ground. The total daily latent heat of every land use for paddy field, bare soil and forest are 12.5 MJ.m-2d-1, 11.2 MJ.m-2d-1 and 15 MJ.m-2d-1. These values are equal to 5, 4.5 and 6 mm of water.