Estimation of Evapotranspiration by use of Remote Sensing techniques

The increasing concentration of CO2 and other radiative active trace gases in the atmosphere is causing tropospheric temperatures to rise. Changes in other climatic features such as precipitation, cloudiness, humidity, and windiness are likely to follow changes in temperature. Such changes could have deep implications for hydrologic processes in general and for water availability to sustain both rain-fed and irrigated agriculture, in particular. Evapotranspiration (ET) is the compound term describing the physical processes of water transfer into the atmosphere by evaporation from soil and transpiration through vegetation; ET constitutes an important component of hydrological balance particularly in semi-arid and sub-humid climates. Evapotranspiration is determined by various mathematical models using climatic factors such as temperature, radiation, humidity, and wind speed, while its direct measurement is seldom available in experimental fields. A relatively new technology using on remote sensing to retrieve land surface parameters such as surface temperature, albedo and vegetation indices - which are indispensable to remotely sensed ET models - is adopted here to map regional, meso- and macro-scale patterns of ET at the earth's surface in a globally consistent and economically feasible manner. The aim of our research is to assess a technique to retrieve ET maps from remote sensing images by the combination of two different procedures. The first one, known as triangle method, computes the evapotranspiration fraction (EF) de- fined as the ratio LE/Rn, where Rn is the net radiation at the surface and LE is the latent heat flux. LE is directly proportional to ET. In order to retrieve LE, and consequently the surface evapotranspiration, a second procedure computes the net radiation of the investigated area. Therefore, by inversion, it is possible to obtain an estimate of the ET. The validation of these variables, the net radiation and of the evapotranspiration derived from MODIS data, has been undertaken on the Capitanata area, Southern Italy, by comparison of model results with in-situ measurements provided by the Consorzio per la Bonifica della Capitanata di Foggia.