Assessment of the impact of drought and restrictions on irrigation district water allocations on yield using a remote sensing for evapotranspiration approach
Authors: Bellvert Joaquim , Casadesus Jaume , Pamies-Sans Magí , Girona Joan 
Published: March 2024

The Mediterranean region is warming 20% faster than the global average. In addition, climate change is expected to exacerbate this situation in the next decades by increasing potential evapotranspiration, decreasing rainfall and increasing the frequency and intensity of droughts. During the last two years, many irrigated areas of Spain have already suffered from water shortages due to lack of water in reservoirs. In some cases, this has led to impose severe restrictions on water allocations of irrigation districts (ID). Since yield is inextricably linked to the amount of water used by plants, the primary effects of water shortage often appear on crop production. However, water restrictions may vary among irrigation districts, among others, depending on the total available water in the reservoir, land uses, or level of modernization. Thus, any imposition of water restriction has a different impact on crop productivity depending on these parameters. From a decision-making point of view, it would be very useful for watershed policy makers to have a tool capable to simulate the impact of decreases in rainfall and/or water restrictions on crop productivity at irrigation district and/or catchment level. Therefore, this study introduces a novel approach to assess the impact of different climate scenarios and restrictions of irrigation water allocations on crop productivity. The study was conducted in a total of eight irrigation districts located in the north-east Ebro basin (Catalonia, Spain), with different water allocations, which corresponded with a total irrigated area of 150,028 hectares. The following six scenarios were simulated: Control, without water restrictions; Pr25 and Pr50, a reduction in rainfall of 25 and 50%, respectively; Irri25, Irri50, Irri75, a reduction of irrigation water allocation of 25, 50 and 75%, respectively. The crop water productivity functions defined in the literature for multiple crops were used. In addition, actual crop evapotranspiration (ETa) was estimated daily at 20 m resolution using a remote sensing two-source energy balance model with Copernicus-based inputs. Overall, results showed that averaged ETa of all irrigation districts decreased by 14, 19, 29, 50 and 66% respectively for Pr25, Pr50, Irri25, Irri50, Irri75 in comparison to Control. On the other hand, yield losses varied among irrigation districts. Those IDs with higher water allocations showed a significant decrease in yield of around 27% in comparison to Control for scenarios Pr25, Pr50, Irri25 and Irri50, without significant differences among them. On the other hand, yield decreased by 72% in the Irri75. Instead, other irrigation districts with very low water allocations observed an averaged significant decrease in yield of 62% in comparison to Control in all the scenarios. A detailed analysis of the impact of the six simulated scenarios on crop productivity of each irrigation district and crop type is also conducted in this study.