Nitrate Exported in Drainage Waters of Two Sprinkler‐Irrigated Watersheds

Nitrate contamination of surface waters has been linked to irrigated agriculture across the world. We determined the NO 3 –N loads in the drainage waters of two sprinkler‐irrigated watersheds located in the Ebro River basin (Spain) and their relationship to irrigation and N management. Crop water requirements, irrigation, N fertilization, and the volume and NO 3 –N concentration of drainage waters were measured or estimated during two‐year (Watershed A; 494 irrigated ha) and one‐year (Watershed B; 470 irrigated ha) study periods. Maize ( Zea mays L.) and alfalfa ( Medicago sativa L.) were grown in 40 to 60% and 15 to 33% of the irrigated areas, respectively. The seasonal irrigation performance index (IPI) ranged from 92 to 100%, indicating high‐quality management of irrigation. However, the IPI varied among fields and overirrigation occurred in 17 to 44% of the area. Soil and maize stalk nitrate contents measured at harvest indicated that N fertilizer rates could be decreased. Drainage flows were 68 mm yr −1 in Watershed A and 194 mm yr −1 in Watershed B. Drainage NO 3 –N concentrations were independent of drainage flows and similar in the irrigated and nonirrigated periods (average: 23–29 mg L −1 ). Drainage flows determined the exported mass of NO 3 –N, which varied from 18 (Watershed A) to 49 (Watershed B) kg ha −1 yr −1 , representing 8 (Watershed A) and 22% (Watershed B) of the applied fertilizer plus manure N. High‐quality irrigation management coupled to the split application of N through the sprinkler systems allowed a reasonable compromise between profitability and reduced N pollution in irrigation return flows.