Evaluating the Impacts of Agricultural Transformation from Rainfed to Irrigation on Streamflow and Nitrates in a Mediterranean Agricultural Watershed in Spain

Agriculture intensification, such as irrigation, creates a lot of pressure on the available water resources and the environment. This paper explored the water quality dynamics, specifically nitrates, before and after transformation from rainfed to irrigation agriculture within the Cidacos River Watershed in Navarra, Spain. The watershed occupies 477 km², of which approximately 260 km² have been traditionally rainfed cultivated, whereas 77 km² were transformed from rainfed to pressurized irrigation between 2009 and 2011. The newly irrigated area is located in the watershed's lower region, close to the river's mouth. Water quality data sampling has been taken at several points along the watershed from 2000 to date, up to the outlet of the Cidacos River in Traibuenas, where it joins with the Aragón River. Streamflow and nitrate concentration data have been measured at the watershed's outlet in Traibuenas since 2017. A previous baseline study by Merchán et al. (2020) showed an increase in the electrical conductivity and nitrate concentration in the river's lower reaches affected by irrigation. However, no information about the effect on streamflow, nitrate loads, and yields resulting from the irrigation was explored. The aim of this study was therefore to fill this research gap by using hydrological models such as the Soil Water Assessment Tool (SWAT) model, to recreate, simulate and understand the behaviour of the Cidacos River in the irrigated area from 2017 to the present, if the transformation from rainfed to irrigation had not occurred; and then compare those simulated variables with the measured ones since 2017. Simulation of streamflow and nitrate loads were done using the SWAT model until Olite from 2000 to 2020 under rainfed conditions. This was later extended for the entire watershed up to the mouth of the Cidacos River from 2017 to the present when the transformation to irrigation was fully completed and the Traibuenas outlet gauging station started operating. The results were then compared to the measured data for the irrigated region before and after the transformation to irrigation. For the simulations, the model was calibrated from 2000 to 2010 and validated from 2011 to 2020, and its sensitivity and uncertainties were analyzed for streamflow and nitrates at the Olite gauging station. The model evaluation results were satisfactory for both streamflow and nitrate loads, with streamflow having values of NSE = 0.82/0.83 and R² = 0.83/0.84 during calibration and validation periods, respectively. Similarly, the statistical evaluation values for nitrate loads were NSE = 0.71/0.68 and R² = 0.72/0.79 during calibration and validation periods, respectively. Subsequently, the calibrated parameters were used to simulate the entire watershed, considering all the territorial variables specific to each zone. Comparative analysis between the periods before and after the implementation of irrigation indicated a 6.6% and 43.2% increase in streamflow and nitrate loads, respectively, which subsequently increased the nitrate concentrations. The results from this study could provide useful information and guidance on nitrate pollution control, thus contributing to the management of the effects of agriculture on water quality and enhancing sustainable agricultural practices.