Assessing deficit irrigation strategies at the level of an irrigation district

Abstract Water scarcity limits the supply to irrigation schemes worldwide. In many areas, chronic water deficits force irrigation districts to allocate water at levels well below the crop water requirements for maximum yield. We have developed a model that simulates the water balance and the irrigation performance at the plot and scheme levels, to carry out scenario analyses in an irrigation scheme in Southern Spain (Genil–Cabra Irrigation Scheme) that is frequently subjected to water limitation. The major crops in the scheme are winter cereals, sunflower, garlic, and cotton. The model simulated the scheme performance for different allocation levels of 500, 1500, 2500 m 3 /ha, and full irrigation supply in terms of gross and net income, irrigation water productivity (IWP) and labour needs, using a series of 48 years of climatic data. For each level of water allocation, three strategies were considered. The first strategy allocated the water supply equally to all users, while a second one was aimed at maximising IWP, both assuming the same crop distribution that existed in 2000. In a third strategy, the cropping pattern was allowed to change relative to that existing in 2000. The behaviour of individual farmers in relation to irrigation performance was incorporated into the model, based on the characterization of their performance during four seasons. Results showed that at supply levels of 1500 m 3 /ha, the best strategy in terms of net income was the one that allocated the water to crops with high water productivity. However, when water supply is very low (500 m 3 /ha) scheme net income is maximised by adjusting the cropping pattern. As supply increased to 2500 m 3 /ha, there were no differences in scheme net income among the three strategies. The various levels of water allocation influenced the value of IWP for the scheme, leading to average values that varied between 1.12 €/m 3 for the 500 m 3 /ha supply to 0.50 €/m 3 under unlimited water supply. The best strategy varied depending on the level of water allocation and on the maximisation objective (income, IWP, labour) in a complex way. Nevertheless, strategies based on allocating water to crops of high water productivity, combined with a shift in the cropping pattern are recommended when water supply is constrained. The analysis emphasized the need to develop simulation tools for optimising water allocation under scarcity at the irrigation scheme level.