Impacts of large-scale irrigation and climate change on groundwater quality and the hydrological cycle: A case study of the Alqueva irrigation scheme and the Gabros de Beja aquifer system.

This research aims to analyze the impacts of the large-scale Alqueva Irrigation System (AIS) on the water cycle in selected sub-basins and the underlying Gabros de Beja aquifer system (GBAS) in Southern Portugal. The Alqueva reservoir and irrigation project is one of the largest strategic water reservoirs in Western Europe and the AIS's primary source. The closure of the dam in 2002 resulted in significant changes to the region's land use and agricultural practices, shifting from predominantly rainfed dry cereals to intensively irrigated olive and almond orchards. Therefore, this study used SWAT+ to simulate water flows from 1934 to 2021 and examined the evolution of groundwater quality and its correlation with irrigation, using data from about 50 wells from 2002 to 2021. Kriging spatial interpolation, Mann-Kendal and Sen's trend tests and the correlation technique were used. The findings revealed several noteworthy trends. First, there was a significant historical decrease in precipitation, which can be attributed to climate change. The simulation indicated a decrease in runoff and recharge, along with an increase in actual evapotranspiration due to irrigation. Furthermore, the hydrogeostatiscal analysis showed that the aquifer experienced significant salinization after the AIS implementation. In contrast, a preponderant decreasing trend in nitrate concentration was observed, which may be attributed to (a) a decrease in fertilizer use, (b) dilution effects and (c) alteration in nitrates pathways due to changes in crop types. Finally, the correlation analysis suggested that nitrates and chlorides were highly correlated to actual evapotranspiration and precipitation evolution, which may be associated to irrigation. In conclusion, the large-scale irrigation implementation and climate change significantly altered the water cycle of the study region. Overall, these findings addressed existing knowledge gaps and provided valuable insights that can be extrapolated to draw conclusions and generalize climate change and irrigation's effects on fluvial ecosystems.