Spatial and temporal variations in surface water nitrate concentration in a mixed land use catchment under humid temperate climatic conditions

Streamwater nitrate (NO-3) concentrations along the main stream and at the outlet of several subcatchments within the 114\\3 km2 Zwalm watershed in Flanders, Belgium, have been monitored regularly since 1991. Land use within the Zwalm catchment is predominantly agricultural, with forested regions in the south and urban concentrations in the north-east of the catchment. Streamwater NO-3 concentrations increased with increases in stream discharge rates, but in general, discharge rate explained only about 30 f the variation in NO-3 concentrations. The low R2 values were attributed to the observed anticlockwise hysteresis in the NO-3 concentration - discharge relationship and to differences in NO-3 concentrations between both seasonal flow and various flow regimes, with winter flow explaining 51 f the variation in NO-3 concentrations, whereas summer flow explained only 28 f the variation. A hypothesis was formulated in which flow regime accounts for the seasonal variation in NO-3 export, postulating that the catchment seasonally alternates between two hydrological stages. The first stage occurs during wet winter periods, when the catchment drains as a single source area, whereas the second stage occurs during dry summer periods, when the groundwater store disconnects into separate subcatchments. This causes NO-3 concentration peaks to be more delayed during summer storm events compared with winter storm events. Regarding flow regimes, differences between high and low flow conditions and between increasing and stable/decreasing flow were not as pronounced a differences between seasons. In contrast to the estimation of NO-3 concentrations, discharge was a strong predictor (R2= 0\\71) of the NO-3 flux within the tributaries of the Zwalm catchment. The NO-3 concentrations in the main stream increased with decreasing elevation, whereas the seasonal concentration patterns along the main channel were similar to those observed at the outlet. NO-3 concentrations varied considerably among catchments and showed a high variability over time, although in general, the variation in NO-3 concentration was higher between catchments than within catchments. The impact of land use is clearly reflected in the streamwater NO-3 concentrations, although NO-3 concentration patterns were also affected by topography and, to a lesser extent, by soil type. A gradual increase in NO-3 concentrations at the outlet of the Zwalm catchment could be observed throughout the 1991 - 1998 study period, providing evidence for the general trends of increase in Flanders, which are attributed to the intensification of agricultural activities.