Parameters of the irrigation module.

In semi-arid areas, irrigation represents the largest human footprint on the water cycle. Representing irrigation in land surface and hydrological models is a challenging task as irrigation decisions depend on environmental, economic and traditional knowledge factors. The objective of this work is twofold: (1) to evaluate the performance of the new irrigation module integrated into the Interactions between Soil, Biosphere, and Atmosphere (ISBA) model and (2) to assess the future trajectories of agricultural water use considering both climate and land use change. The evaluation of the new irrigation module in the ISBA model is done by: (1) comparing the observed and predicted fluxes by the ISBA model, with and without the activation of the irrigation module, and (2) comparing the irrigation water inputs at the level of irrigated perimeters. The evaluation shows that the integration of the new irrigation scheme in ISBA significantly improves the latent heat flux (LE) predictions for the period 2004-2014, compared to the model without this scheme. Considering several flux stations, the LE flux bias was reduced from -60 W/m2 for the model without irrigation to −15 W/m2. The evaluation at the irrigated perimeter scale highlights the ability of the irrigation model to reproduce the overall magnitude and seasonality of observed irrigation water quantities despite a positive bias. The agricultural water use was then projected considering two climate scenarios and two scenarios of land use change based on the actual trend of conversion to tree crops in response to the large subsidy attributed for the conversion to drip irrigation since 2008. It is shown that (1) irrigation water use could almost double for the more extreme scenarios and (2) that most of this drastic increase is attributed to land use change, including irrigation intensification and expansion. Within this context, the results presented in this study highlight the side effect of conversion to drip irrigation largely documented in the literature and open perspectives for making informed decisions for sustainable water management at the watershed level.