Quantifying the Downstream Impact of Implementing Irrigation in a Semi-Arid Mediterranean Basin in NE of Spain

Irrigation in arid and semi-arid regions is key to maintain the productivities and the well-being of farmers. However, irrigation is an important source of pollution to rivers due to the impact of agricultural drainage [1], which may contain high levels of salts, nutrients and other pollutants. In the present study we quantified the impact of implementing irrigation in a 10,000 ha semi-arid basin of the Noguera Ribagorçana river (Ebro Basin, NE Spain). Water quality data obtained during 20 years (2002-2022) in four different sampling points in the river (three before, and one after the main agricultural drainage returns of the basin, which drains 4,366 ha) were analyzed, focusing on nitrate concentration (NO₃^-, ppm), phosphate concentration (PO₄^3-, ppm), and electrical conductivity of the water (EC, dS/m). In 2002, less than 4,000 ha were under irrigation and during the studied period, a total of 5,571 hectares were brought under irrigation progressively over time, with the implementation of a new irrigation district in the area. Results showed a significative difference in the concentration of NO₃^- in the river water before and after the main agricultural drainage return of the new irrigation district. However, phosphorous concentration and electrical conductivity showed no significative differences between the sampling points before and after the main agricultural drainage returns. On the other hand, NO₃^- values at the sampling point after the main agricultural drainage return have increased over time as it did the irrigated area. Thus, along the 18 km of the Noguera Ribagorçana river observed in this study, NO₃^- levels have increased on average from 1.7 ppm at the first sampling point to 10.9 ppm at the last sampling point, after the returns of agricultural drainage in the basin. Therefore, we could state that implementing irrigation in 5,571 ha represents an increase of 9.2 ppm of NO₃^- in the water of the Noguera Ribagorçana river in the studied area.

[1] Blann, K. L.; Anderson, J. L.; Sands, G. R.; Vondracek, B. Effects of Agricultural Drainage on Aquatic Ecosystems: A Review. Crit. Rev. Environ. Sci. Technol. 2009, 39 (11), 909–1001. https://doi.org/10.1080/10643380801977966.