A characterization of soil erosion in cultivated watersheds in Manitoba's Red River Valley using sediment budgeting, and its implications for managing soil erosion’s impacts
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Soil erosion accelerated by agriculture reduces agricultural productivity and compromises the function of drainage infrastructure and downstream water quality. In Manitoba, the relationship between soil erosion and water quality is of particular concern, following measurable declines in Lake Winnipeg’s water quality since the 1990s. Understanding the state of soil erosion, transportation, and deposition in the Red River Valley is of interest, due to the extensive cultivation of the river’s watershed and its contribution to total flow (and by extension, sediment flux) into the lake. Sediment budgets were drafted for two sub-watersheds of the well-studied Boyne-Morris and La Salle River watersheds, located in the Red River Valley, and coded 05OF024 and 05OG008 by the Water Survey of Canada (WSC), respectively. To characterize soil erosion, the sediment budgets used published National Agri-Environmental Health Analysis and Reporting Program (NAHARP) soil erosion risk estimates calculated with the SoilERI model. Sediment transportation was quantified using flow and total suspended solids (TSS) measurements made by the WSC and other organizations. Deposition within the sub-watersheds was quantified through measurements made in road-side ditches (a common sediment sink in the Red River watershed) and inferred through imbalances in the sediment budgets. Rates of soil erosion, deposition within, and transportation out of the 05OF024 sub-watershed were an order of magnitude greater than in the 05OG008 sub-watershed due to differences in basin scale, but relative differences in their rates in each basin were the same. Rates of erosion were 1 order of magnitude greater than rates of deposition in road-side ditches and 3 orders of magnitude greater than transportation past the sub-watershed outlets. Differences between rates of soil erosion and deposition in road-side ditches were noted and attributed to unmeasured deposition in cultivated fields. Rates of deposition in such settings were of the same order of magnitude but less than rates of erosion. Both sediment budgets quantified rates of road-side ditch dredging, which were 1 order of magnitude greater than rates of deposition in road-side ditches and directed soil back into cultivated fields. Rates of water, wind, and tillage erosion characterized by NAHARP erosion risk estimates were not mirrored by related, measured rates of deposition in roadside ditches in either watershed. The stark differences between rates of soil erosion and rates of sediment transportation past the outlets of both sub-watersheds suggested the downstream impacts of eroded soil on water quality may be minimal at coarse temporal scales in watersheds of similar or greater size in the Red River Valley. Greater degrees of sediment delivery to road-side ditches suggested that sediment may have more meaningful impacts on the function of drainage infrastructure, especially at the same temporal scales in smaller watersheds in the region. Differences in estimated rates of water, wind, and tillage erosion and related rates of deposition in road-side ditches suggest the SoilERI model may not adequately characterize rates of soil erosion. This does not invalidate the SoilERI model, but highlights its limitations which should be considered when it is used to estimate rates of soil erosion in such settings.
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