Assessment of crop water needs and its sustainability based on future climate scenarios: the Aude Department (South-West France)

Since 1970, South-western European regions (Iberian Peninsula and South France) have been subjected to an air temperature increase of almost 2 °C, while generally southern Europe assisted to a 20% drop in annual precipitation. Agriculture is by far the sector with the greatest freshwater withdrawals, and it is essential to perform an accurate assessment of water consumption for irrigation, in order to develop strategies to reduce water abstractions from the ecosystem. In this context, this work aims at modelling water consumption for agriculture in the Aude river basin (South-West France), in order to assess the amount of water needed during the growing season of each crop in the current conditions, and in the future scenarios of climate change, according to different climate models. This project relies on the application of SIMETAW# model (Simulation of Evapotranspiration of Applied Water), which, from a set of climatic and soil data, computes the daily reference, well-watered crop, and actual evapotranspiration (ET0, ETc, ETa), the evapotranspiration of applied water (ETaw), an irrigation schedule, and crop growth and yield for a specific site. For climate inputs, the work relies on the high-resolution data (0.11-degree resolution) supplied by Copernicus Cordex, which provides historical records and future estimations according to RCPs (Representative Concentration Pathways) 2.6, 4.5 and 8.5. In the calculation of the well-known Penman–Monteith ET0 formulation, SIMETAW# also considers the effect of the increase of atmospheric CO2 concentration on stomatal resistance, which plays as a counterbalance with the increase of temperature due to climate change, by reducing stomatal opening for transpiration in plants, determining lower water loss through stomata. The model calculates ETa in both irrigated and rainfed conditions, distinguishing the irrigation methods according to the most relevant crops of the region, namely wine grapes cultivations, forage crops, wheat, olives, vegetables and fruits. Results show that, in Aude basin, the variation of total irrigation demand between 1990 and 2050 is expected to be very low in scenario RCP 2.6 (< 1%), while in RCP 4.5 a 2.5% increase is foreseen. Differently, RCP 8.5 expects a substantial decrease of irrigation requirements (-23%), due to the large increase of CO2 concentration in the atmosphere. Low water-demanding crops, such as winter wheat and wine grapes, are less sensitive to climate variations, thus their irrigation demand is expected to remain rather stable in the future, however summer crops (fruits and vegetables) will require greater irrigation inputs. The study demonstrates that, in some climate scenarios, crop water requirements may decrease due to the reduction of stomatal conductance. Still SIMETAW#, as most of the crop water models currently applied, does not take into account other climate change effects that can be damaging for the vegetation (e.g., heat waves, floods, spread of pathogens, etc.), together with the reduced availability of water supply in the basin, which can also have a consequence on the irrigation scheduling.