Watershed nutrient yield and nutrient streamflow: quality of water courses
Figure 6(a) presents the average monthly nutrient concentration in the stream at the watershed outlet during the hydrological year, showing highly variable H2PO4 and a sharp increase for N-NH4 concentrations in February and May (Fig. 6(b)). For the study period, an increasing inverse relationship with discharge at the watershed scale was found, especially for K and N-NO3 (Fig. 6(c) and (d), respectively). The N-NH4 and N-NO3 concentrations displayed a seasonal pattern with higher values during winter and early spring, this probably being explained by microbial immobilization during the wintertime leaching (Foster et al. 1989, Bruland et al. 2008). The 3-year daily N-NO3 concentration in the runoff throughout the study averaged 4.7 mg L−1, ranging from 1.4 to 7.5 mg L−1. The maximum N-NO3 concentration rates detected for a discharge event exceeded neither 50 mg L−1, which is the allowable limit for drinking water according to the WHO (2008), nor the 10 mg L−1 upper limit recommended for irrigation waters (Ayers and Westcot 1994). Average N-NH4 concentrations in the stream ranged from 0.01 to 0.06 mg L−1, less than the natural levels for surface water and groundwater of 0.2 mg L−1 (WHO 2008) and 5 mg L−1 for irrigation (Ayers and Westcot 1994). The H2PO4 concentrations in the runoff ranged from 0.00 to 0.03 mg L−1, below the established limits for the eutrophication of surface waters of 0.01 mg P L−1 (Vollenweider and Kerekes 1980) to 0.05 mg L−1 (US EPA 1976). Likewise, the inorganic P concentration in runoff was well below the recommended level of 2 mg L−1 for agricultural use (Ayers and Westcot 1994).