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Agronomic management in Ciudad Real, a province in central Spain, is characteristic of semi-arid cropped areas whose water supplies have high nitrate (NO3-) content due to environmental degradation. This situation is aggravated by the existence of a restrictive subsurface layer of "caliche" or hardpan at a depth of 0.60 m. Under these circumstances, fertirrigation rates, including nitrogen (N) fertilizer schedules, must be carefully calibrated to optimize melon yields while minimizing the N pollution and water supply.

Such optimization was sought by fertilizing with different doses of N and irrigating at 100% of the ETc (crop evapotranspiration), adjusted for this crop and area. The N content in the four fertilizer doses used was: 0, 55, 82 and 109 kg N ha(-1). Due to the NO3- content in the irrigation water, however, the actual N content was 30 kg ha(-1) higher in all four treatments repeated in two different years.

The results showed correlation between melon plant N uptake and drainage (Dr), which in turn affects the amount of N leached, as well as correlation between Dr and LAI (leaf area index) for each treatment. A fertilizer factor (alpha) was estimated through two methods, from difference in Dr and in LAI ratio with respect to the maximum N dose, to correct ETc based on N doses. The difference was found in the adjusted evapotranspiration in both years using the corresponding alpha achieved 42-49 mm at vegetative period, depending on the method, and it was not significant at senescent period. Finally, a growth curve between N uptake and plant dry weight (DW) for each treatment was defined to confirm that the observed higher plant vigour, showing higher LAI and reduced Dr, was due mainly to higher N doses. (C) 2012 Elsevier B.V. All rights reserved.