Distinguishing between anthropogenic and natural sources of sediment and nutrients is important for water resource management in irrigated basins. Water quality of flood irrigation was monitored at the field scale in the upper Klamath Basin, Oregon, on two unfertilized cattle pastures that were 2 ha (Site 1) and 70 ha (Site 2) in area. Water samples were analyzed for concentrations of sediment, total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), orthophosphate, ammonium-N (NH z 4-N), and nitrate-N (NO { 3-N). At both sites the TDN concentration was significantly greater in surface runoff than in applied irrigation water (P , 0.05). Site 1 sediment and TDP concentrations were significantly greater in irrigation surface runoff than in applied irrigation water (P , 0.05). A first flush during irrigation was observed at Site 1 where nutrient concentration was at maximum value during the first 3 h of surface runoff. At Site 2 the surface runoff sediment and TDP concentrations were not significantly (P. 0.05) higher than the applied irrigation, except when cattle were present. When export was measured, the mean yield of sediment and TDN per irrigation was 23.9 kg N ? ha 21 and 0.26 kg N ? ha 21 , respectively, and there was a net retention of TDP of 0.04 kg P ? ha 21. NH z 4-N export occurred during one irrigation event yielding 0.15 kg N ? ha 21. NO { 3-N export was minimal or undetected. A late summer storm event resulted in pasture surface runoff concentrations of TDN and TDP that were 33 and 3 times higher, respectively, than irrigation source water concentrations. The TDN was significantly higher in subsurface runoff than it was in applied irrigation water (P , 0.05). Improved irrigation efficiency might prevent many of the nutrient and sediment transport mechanisms observed during this study. Resumen La habilidad de distinguir entre fuentes de nutrientes y sedimentos antropogénicos y naturales es importante para el manejo de recursos hídricos de cuencas irrigadas. Se monitoreó la calidad del agua de riego por inundación medida a escala de potrero en la cuenca superior del rio Klamath en Oregon, EE.UU., en dos pasturas sin fertilizar pastoreadas por bovinos, una de 2 (Sitio 1) y otra de 70 ha (Sitio 2) de superficie. Se analizaron muestras de agua para determinar contenido de sedimentos, nitrógeno disuelto total (NDT), fósforo disuelto total (PDT), ortofosfato (OP), nitrógeno en amonio (NH z 4-N), y nitrógeno en nitratos (NO { 3-N). En ambos sitios la concentración de NDT fue significativamente más alta en aguas de escorrentía superficial que en el agua de riego aplicada (P , 0.05). En el Sitio 1, la concentración de sedimentos y NDT fue significativamente más alta en escorrentía superficial de agua de irrigación que en el agua de riego (P , 0.05). Se observó un pulso inicial durante las primeras 3 h de escurrimiento superficial durante el riego en el Sitio 1 en el que la concentración de nutrientes alcanzó valores máximos. En el Sitio 2 la concentración de sedimentos y PDT en aguas de escurrimiento ...
