Distribution of limited irrigation water based on optimized regulated deficit irrigation and typical metheorological year concepts

Under limited availability of irrigation water during the growing season, farmers must avoid a premature depletion of this resource that may lead to a sizeable decrease in yield. Four levels of net irrigation availability were considered for an onion (Allium cepa L) crop cultivated in Castilla-La Mancha region (Spain): 1.0 I-N (ref) (typical net irrigation requirements in the area: 743.3 mm); 0.9 I-N (ref); 0.8 I-N (ref).; and 0.7 I-N ref. The available amount of irrigation water was distributed during the growing period by combining the modified optimized regulated deficit irrigation (ORDI) [which determines the actual evapotranspiration (ETa) and maximum evapotranspiration (ETm) ratios (ETa/ETm) per growing stage that maximize yield for a certain amount of irrigation water] and the typical meteorological year (TMY) methodologies. So, optimized irrigation schedules that reach the optimized ETa/ETm ratios per stage and yields were calculated for three TMYs (dry, intermediate, and wet) obtained from a 54-year climatic series (1951-2004). The six months before sowing of each year were analysed to determine if that period was dry, intermediate, or wet. The yield obtained for each level of irrigation availability and the corresponding TMY was considered as the forecasted yield for the current year. Optimized irrigation schedules were corrected during the growing period by replacing the daily climatic data of the TMY by those of the current year. At the end of each growing stage, optimized irrigation schedules were recalculated taking into account the real amount of irrigation water available for the following periods and the forecasted climatic conditions. The effect of this methodology on final yield was simulated by MOPECO (economic optimization model for irrigation water management) during 8 seasons (2005-2012). As a result, onion yields at harvest were approximately 5.1% lower than the forecasted yield at the beginning of the growing period for the different availabilities of irrigation water. As a consequence of premature depletion of irrigation water, only 3 of 24 analysed scenarios reached excessive deficit during the last stage. (C) 2014 Elsevier B.V. All rights reserved.