Nitrate Exported in Drainage Waters of Two Sprinkler-Irrigated Watersheds
2003
Cavero, J. | Beltrán, A. | Aragües, R.
Nitrate contamination of surface waters has been linked to irrigated agriculture across the world. We determined the NO₃–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 NO₃–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 NO₃–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 NO₃–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.
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