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TECHNICAL REPORTS

Surface Water Quality

Environmental Impact of Irrigation in La Violada District (Spain)

II. Nitrogen Fertilization and Nitrate Export Patterns in Drainage Water

Unidad de Suelos y Riegos, Centro de Investigación y Tecnología Agroalimentaria (CITA-DGA) and Laboratorio de Agronomía y Medio Ambiente (DGA-CSIC), Apdo. 727, 50080-Zaragoza, Spain

^* Corresponding author (disidoro{at}aragon.es)

Received for publication February 23, 2005. Fertilizer leaching affects farm profitability and contributes to nonpoint-source pollution of receiving waters. This work aimed to establish nitrate nitrogen export from La Violada Gully in relation to nitrogen fertilization practices in its basin (La Violada Gully watershed, VGW, 19 637 ha) and especially in La Violada Irrigation District (VID, 5282 ha). Nitrogen (N) fertilization in VID (and VGW) was determined through interviews with local farmers for the hydrologic years 1995 and 1996 and NO₃-N load in the gully was monitored from 1995 to 1998. The N fertilizer applied in VGW was 2175 Mg in 1995 and 2795 Mg in 1996. About 43% was applied in VID (945 Mg in 1995 and 1161 Mg in 1996). The most fertilized crop was corn: 398 kg N ha^–1 (665 Mg) in 1995 and 453 kg N ha^–1 (911 Mg) in 1996. Nitrogen fertilization was higher than N uptake for irrigated crops, especially for corn and rice. Nitrate N load in La Violada Gully averaged 427.4 Mg yr^–1. Seventy-five percent of the exports took place during the irrigation season (321.8 Mg). During the non-irrigation season maximum NO₃-N loads (3.1 Mg NO₃-N d^–1) were found after heavy rains following the N side-dressing of wheat in the rain-fed area of VGW (February). During the irrigation season NO₃-N load was determined by outflow from the district (caused by irrigation) and to a lesser extent by changes in NO₃ concentration (caused by fertilization), showing peaks in April (pre-sowing corn N fertilization and first irrigations) and June to August (highest irrigation months and corn side-dress N applications, maximum 6.3 Mg NO₃-N d^–1 in July). Adjusting N fertilization to crops' needs, improving irrigation efficiencies, and better scheduling N fertilization and irrigation in corn could reduce N export from VID.

Abbreviations: N_CR, nitrogen in canal releases • N_DN, denitrification • N_F, nitrogen fertilization • N_GI and N_GO, nitrogen in ground water inflows and outflows • N_I, nitrogen in irrigation water • N_L, nitrogen load in La Violada Gully • N_MW, nitrogen in municipal wastewaters • N_P, nitrogen in precipitation • N_SF, symbiotic fixation • N_SR, nitrogen in surface runoff • N_U, nitrogen uptake by crops • N_V, volatilization • VGW, La Violada Gully watershed • VID, La Violada Irrigation District