Water resources and nitrate discharges in relation to agricultural land uses in an intensively irrigated watershed.

Application of integrated hydrological models to manage water resources and non-point agricultural pollutants are increasingly used in decision-making processes. In this study SWAT (Soil and Water Assessment Tool) was used to simulate the water balance and nitrate pollution in an intensively irrigated agricultural catchment (Flumen River in Monegros, Aragon, NE Spain). Rainfall comprised only 45% of the inputs of water in the Flumen watershed and the rest is contributed through irrigation canals from two other rivers outside the Flumen watershed. Green water storage and green water flow are the dominant components of the water balance in the watershed, which is related to the important contribution of water for irrigation. In general, green water storage and green water flow are quite similar in the subwatersheds dominated by irrigation agriculture that are located in the central part of the watershed. A similar pattern was observed for blue water, with high amounts in the central irrigated subwatersheds compared to the non-irrigated subwatersheds. Consequently, nitrate infiltration in the aquifer was higher in the inner irrigated subwatersheds (100-250 kg N ha year) but much lower than the lateral flow rates estimated in the non-irrigated subwatersheds (1400-2000 kg N ha year). Two scenarios simulating the effects of expected climate change factors in this zone were performed. A reduction in the availability of water for irrigation will transform the area from irrigated crops to cereal. In this case the water flow of River Flumen at the outlet of the watershed is reduced by 15%. If a reduction of 40% nitrate fertilization is applied, the nitrate exported to Flumen River would decreased by 28%. These results suggest that dosing irrigation water and fertilizers in accordance with crop requirements would contribute to buffer peaks of water and nitrate discharges and to a more efficient agricultural use of the resources.