EL-NINO EFFECTS HURT NAMOI IRRIGATED COTTON GROWERS, BUT THEY CAN DO LITTLE TO EASE THE PAIN

Although the El Nino Southern Oscillation has a dominant effect on climate in a number of the world's large-scale irrigation areas, its use as a predictor of irrigation water demands and supplies in water resource systems modeling has received very limited attention to date. Herein existing models, using computer simulation and dynamic programming, are modified to include a Southern Oscillation Index (SOI) as a state variable when modeling a large irrigation system in a highly variable, summer precipitation environment. Irrigated cotton returns are considerably below average in 'dry' SOI seasons and above average in 'wet' SOI seasons. Optimal water-use decisions change markedly with the introduction of SOI as a state variable. However, the use of the new decisions has quite limited impacts on both the long term average and variability of returns to cotton growers when water is not transferable. Water trading would allow growers to enhance the advantages of 'wet' SOI seasons and reduce the disadvantages of 'dry' SOI seasons in some cases.