Yield response and optimal allocation of irrigation water under actual and simulated climate change scenarios in a southern Italy district

The potential effect of climate change on the optimal allocation of irrigation water was investigated for a Southern Italy district. The study was carried out on 5 representative crops (grapevine, olive, sugar beet, processing tomato, asparagus), considering six simulated climate change conditions, corresponding to three 30-year periods (2011-2040; 2041-2070; 2071-2100) for two greenhouse gas emission schemes proposed by IPCC (A2 and B1), plus the current climatic condition. The framework adopted was based on: i) the modeling of crop yield response for increasing levels of water supply, under current and future climatic conditions, through a non-linear regression equation and ii) the definition of the best water allocation by means of a mathematical optimization model written in GAMS. Total irrigation water (TIW) volume was allowed to vary from a low total supply 10000 m3 to 7000000 m3, whilst a fixed surface, corresponding to that currently occupied in the studied district, was assigned to each crop. The economic return was studied in terms of Value of Production less the fixed and variable irrigation costs (VPlic). The TIW volume that maximized the VPlic of the whole district surface under the current climatic condition was 5697861 m3. The total volume was partitioned among the five crops as a function of the surface occupied: grapevine>olive>processing tomato>asparagus>sugar beet. Nevertheless, grapevine and olive received seasonal volumes corresponding only to 59 and 50% of total irrigation water requirements. On the contrary, processing tomato and asparagus received seasonal water volumes close to those fully satisfying irrigation water requirements (100% and 85% ETc). Future climatic conditions slightly differed from the current one for the expected optimal allocation. Under water shortage conditions (160000 m3) the whole irrigation water was allocated to the horticultural crops. Forecasted growing season features varied to a different extent in relation to crop and scenario considered with the more intense changes observed for A2 and olive.