Irrivol: A method to estimate the yearly and monthly water applied in an irrigation district

Management of irrigation water requires knowledge of the volume of water that will be needed in each irrigated district. We present Irrivol, a method for estimation of irrigation water volumes that combines (1) the crop hectarages obtained either by area frame sampling or by a regression estimator with satellite data, and (2) the net crop water requirements (m(3) ha(-1)) calculated by Food and Agriculture Organization (FAO) methodology. We also assess the sensitivity of the yearly and monthly estimates obtained by the use of either actual or of average meteorological data. The net crop water requirements (hm(3)) in the irrigation district of Flumen (Aragon, Spain) in 1991, 1993, and 1994 were estimated. The annual and monthly estimates were compared to the corresponding irrigation water volumes invoiced to farmers by the water authority. The most reliable irrigation water estimates were obtained by combining the crop hectarages calculated by the regression estimator with the weather of the actual year. The ratio of the invoiced water volumes and our estimates using the above method ranged from 1.08 to 1.21 for annual estimates and from 1.05 to 1.35 for most of the monthly estimates.