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Abstract:
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"Amongst the most crucial information in water balance assessment, is the evapotranspiration (ET) of the plants, especially field crops. A large literature is available on potential and reference ET, and it is a modeling exercise to assess the crop areas, the level of effective development and the water stress found under the presupposed field conditions. This becomes increasingly difficult when variables are varying in time, space and crop types. Models to calculate ET have very early faced the most difficult parameter, the crop itself. Knowing about the crop was the necessity for calculating anything about it. When facing a water basin of very large area, agro-climatically transient in its various parts, treating the ET calculation by the energy-balance becomes interesting. Information about the vegetation cover is indeed minimal and often very well provided by satellite information. Some earlier experience of water depletion for crops is briefly overviewed for PRC, The Philippines and Uzbekistan, with some water depletion results. Some various satellites are used, Landsat 7 ETM+, NOAA AVHRR, TERRA MODIS and TERRA ASTER. Only satellites able to provide temperature measurements are fulfilling the requirements of such analysis. Some meteorological satellites are also used for calculating ET in Global Climatological Models, but are of too coarse pixel resolution for the application to crop ET per se. Issues of cloud cover have also been raised, and addressed most of the time positively. Validations of such monitoring algorithms have been widely performed and are always found acceptable. It is concluded that even if research is still actively tackling the calculation of ET by means of remote sensing measurements, the numerous applications of the different energy balance models available show that it has long passed the initial development and can be used fairly confidently."
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