Using satellite and other data to estimate the annual water demand of an irrigation district

Locating and forecasting water needs can assist the location of water in dr y regions, and improve the management of reservoirs and the canal network. Satellite, ground data, and agrometeorological data were combined to foreca st the volume of irrigation water needed during 1993 and 1994 in an irrigat ion district of 327 km(2) located in the Ebro basin, Spain. The main crops were rice, alfalfa plus forage, winter cereals (barley and wheat), sunflowe r and maize. Their extent was estimated every year by frame area sampling a nd a regression estimator with satellite data. Initial crop area statistics were obtained by expansion of the sample areas to the entire study area an d then a regression estimator with the multitemporal supervised classificat ion of two Landsat-5 TM images was applied. This procedure improved the pre cision of the estimates by expansion. Net water requirements (m(3) ha(-1)) of the above mentioned crops were computed from reference evapotranspiratio n estimates, crop coefficients and effective precipitation. These computati ons were performed for an average year, i.e. by using long-term averaged me teorological data. Crop hectarage and net crop water requirements were mult iplied to obtain, for the entire study area, the volume (hm(3) = 10(6) m(3) ) of the net crop water requirements. After subtraction of water taken dire ctly from the rivers and non-productive sunflower, the irrigation water vol umes were estimated. The comparison of these forecasts with the volumes of water invoiced by the Ebro Basin Water Authority confirmed the feasibility of forecasting the volume of water applied to an individual irrigation dist rict. This is an objective and practical method for estimating the irrigati on water volume applied in an irrigated area.