University of California Water Resources Center

[Note: see PDF for proper symbols.] In this study water quality in a set of catchments that vary from 6 ha to almost 1500 ha is investigated. Studying catchments across this large range of scales enables us to investigate the scale dependence and fundamental processes controlling catchment biogeochemical export. The Devil Canyon catchment, in the San Bernardino Mountains, California, has some of the highest atmospheric N deposition rates in the world (40-90 kg ha-1 yr-1 at the crest of the catchment). These high rates of deposition have translated into consistently high levels of NO3 - in some streams of the San Bernardino Mountains. However, the streams of the Devil Canyon catchment have widely varying dissolved inorganic nitrogen (DIN) concentrations, variability, and export. These differences are also, to a more limited extent, present for dissolved organic carbon (DOC) but not in other dissolved species (Cl-, SO4 2-, Ca2+ and other weathering products). As catchment size increases DIN and DOC export first increases until catchment area is ~150 ha but then decreases as catchment scale increases beyond that size. Inorganic nitrogen and DOC also share similar temporal variability within the catchments. The reasons for these phenomena appear to be the dominance of flushing of dissolved constituents out of soil at small scales, the groundwater exfiltration of these flushed materials at intermediate scales and the removal of biologically active materials from streamflow through riparian processes at larger scales. While the particular scale effect observed here may not occur over the same range in catchment area in other ecosystems, it is likely that other ecosystems have similar scale dependant processes. Instream removal processes are a particularly relevant process for understanding the loss processes controlling the fate and transport of nutrients derived from agricultural and urban land uses.