Simulating regional impacts of irrigation on the atmospheric and terrestrial water cycle using Earth System Model physics

Abstract. This study presents regional simulations over the Iberian Peninsula between 2010 and 2022 with the atmospheric (ICOLMDZ) and land surface (ORCHIDEE) components of the IPSL climate model in a new limited area model configuration (25 km resolution). It uses a recently developed river routing and irrigation scheme based on a water-conservative supply-and-demand approach. Two simulations, with and without irrigation, are compared to isolate the impacts of simulated irrigation on land surface-atmosphere interactions and the water cycle. First, an evaluation of the simulations is conducted to characterize existing model biases in river discharge, precipitation and evapotranspiration (ET), and assess whether they can be improved by simulating irrigation. The simulated irrigation is too low in southern Spain because of a lack of available water in the reservoirs, and likely because of the absence of representation of river dams. In northern regions such as the Ebro Valley, the simulated irrigation is more realistic and reduces the biases of river discharge and ET in summer and autumn. Second, atmospheric changes induced by irrigation are studied in summer (JJA). Large atmospheric responses are found over intensely irrigated areas, mainly consisting of a shift in energy partitioning between the turbulent fluxes (increase in latent heat flux and decrease in sensible heat flux, up to 50 W m⁻²), and a lowering of the atmospheric boundary layer (−100 m) and of the lifting condensation level (−250 m). Increases in precipitation are statistically significant only over the mountainous areas surrounding the Ebro Valley, and are closely linked to increases in convective available potential energy. Finally, atmospheric moisture recycling over the Iberian Peninsula is identified by showing that the increase in ET in the presence of irrigation exceeds the amount of water added by irrigation. This is made possible by an increase in precipitation over land, although most of this increase is located in lightly irrigated areas rather than in intensively irrigated areas. These results point to remote atmospheric effects of irrigation and motivate further investigation into surface-atmosphere coupling processes in the presence of irrigation in the IPSL model.