Canadian Forest Service Publications
Searching for similarity in topographic controls on carbon,nitrogen and phopsphorus export from forest headwater catchments. 2013. Mengistu, S.G.; Creed, I.F.; Webster, K.L.; Enanga, E.; Beall, F.D. Hydrological Processes. 28: 3201-3216.
Issued by: Great Lakes Forestry Centre
Catalog ID: 34763
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Topography influences hydrological processes that in turn affect biogeochemical export to surface waters on forested landscapes. The difference in long-term average annual dissolved organic carbon (DOC), organic and inorganic nitrogen (NO3 --N, DON), and phosphorus (TDP) export from catchments in the Algoma Highlands of Ontario, Canada with similar climate, geology, forest and soil were established. Topographic indicators were designed to represent topographically-regulated hydrological processes that influence nutrient export, including: (a) hydrological storage potential (i.e., effect of topographic flats/depressions water storage); and (b) hydrological flushing potential (i.e., effect of topographic slopes on potential for variable source area to expand and tap into previously untapped areas). Variations in NO3--N export among catchments could be explained both by indicators representing hydrological flushing potential (91%, p<0.001) and hydrological storage potential (65%, p<0.001), suggesting the importance of hydrological flushing in regulating NO3--N export as well as surface saturated areas in intercepting NO3 --N loaded runoff. In contrast, hydrological storage potential explained the majority of variations among catchments in DON (69%, p < 0.001), DOC (94%, p < 0.001) and TDP (82%, p < 0.001) exports. The lower explanatory power of DON (about 15% less) compared to DOC and TDP suggests another mechanism influencing N export, such as controls related to alternative fates of nitrogen (e.g., as gas). This study shows that simple topographic indicators can be used to track nutrient sources, sinks, and their transport and export to surface waters from catchments on forest landscapes.
Plain Language Summary
The objective of this work was to study catchment areas that are monitored with streamflow gauges to find similarities in nutrient export that could be applied to other (i.e., ungauged) areas on the landscape. This approach recognizes that topography is an important driver of hydrological and biogeochemical processes on forest landscapes. Using the 30+ year data record from the 13 gauged catchments at the Turkey Lakes Watershed we observed that the catchments varied in the dissolved nutrients essential for maintaining forest and aquatic ecosystem health. In particular, dissolved organic carbon, organic and inorganic nitrogen and phosphorus export varied among the catchments, although all had similar climate, geology and forest type and age. We used remote sensing technology to derive topographic metrics and tested them for their role in explaining the variation in nutrient export. We learned that wetlands that serve as interceptors and transformers of nutrients and the frequently flushed variable source area regions that serve as conduits of nutrients were the two most important topographic metrics regulating nutrient export behaviour. Our conclusion is that a clear understanding of topographic metrics, such as wetlands and connected variable source areas, gained from long-term monitoring of catchments provides a clear way forward to characterizing nutrient responses and fates in ungauged catchments. This research has important implications for scaling our understanding from our study catchments to the landscape where policy and management decisions are made.