Predicting seasonal stream chemistry from catchment topography: the importance of the near-stream zone.

English Abstract:

Catchment topography influences stream nutrient export through regulation of such factors as hydrological flow paths and distribution of organic soils, and by determining connections between sources of nutrients across the landscape and the stream. Recent work has demonstrated that biogeochemical processes occurring in the near-stream zone exert a stronger influence on stream chemistry compared with catchment-averaged processes. This may be particularly true in dry seasons when there is less hydrological connectivity between upland source areas and the stream channel. In this study, we tested the hypothesis that near-stream topography is a better predictor of seasonal stream chemistry compared with whole-catchment averages. We evaluated relationships between catchment topography and 20-year average seasonal concentrations of nutrients at 10 forested catchments in south-central Ontario. 1 m resolution LiDAR derived digital elevation models were used to calculate average topographic metrics within 20m, 50m and 100m of the stream, as well as for the entire catchment. Dissolved organic carbon and nitrogen were strongly related to wetland coverage averaged across the entire catchment and these models were not improved by considering only the near-stream zone, perhaps because wetlands in these catchments are generally associated with the stream channel. Regression models predicting sulphate and nitrate concentrations using catchment slope and wetness indices were often improved by considering only the area within 50 m or 100 m of the stream network, particularly during the summer. This was in contrast to wetter seasons (e.g. spring and winter), when larger proportions of the catchment contributed to stream chemistry. In these forested headwater catchments, variable hydrologic connectivity of source areas to streams alters the role of the near-stream zone environment in regulating sulphate and nitrate concentrations in streamwater. During dry periods, the near-stream-zone catchment morphology should be included in landscape models to improve predictions of streamwater sulphate and nitrate concentrations.