Cottonwood River Native Vegetation Water Quality

To provide context and to better manage our water resources, this study quantified the surface water quantity and quality and soil hydrologic characteristics of perennial vegetation on undisturbed soils in southwest Minnesota, and measured the changes that occurred following the conversion of a portion of the perennial vegetation to cropland utilizing a paired watershed design. Two small watersheds were instrumented with H-flumes and monitored year-round for four years. The perennial vegetation did not produce run-off during non-frozen soil conditions; however, it did have run-off associated with snowmelt over frozen soils. The water quality of the snowmelt run-off did have elevated levels of total phosphorus (TP), primarily in the dissolved molybdate reactive phosphorus (DMRP) form, and contained various forms of nitrogen, along with low sediment levels. The water leaving the perennial vegetation did carry nitrogen, phosphorus, and sediment although the run-off volumes annually averaged less than 0.1 inches of runoff/acre resulting in low pollutant exports. One of the watersheds was converted from perennial vegetation to cropland in May 2013. Four run-off events from the cropland were observed in June of 2013. These were the only run-off events on non-frozen soils over the duration of the project. The conversion to cropland did result in additional total nitrogen (1.8 lb/acre), total phosphorus (0.24 lb/acre), and sediment (953 lb/acre) being exported from the watershed compared to the control in June 2013. These increased losses are more reflective of a shift in hydrology rather than a shift in pollutant concentrations, due to the lack of run-off observed from the perennial vegetation during non-frozen soil conditions. Soil bulk density and hydraulic conductivity were used as indicators of changes in soil properties after conversion from perennial vegetation to cropland. It is anticipated that the hydrology and soil properties of this recently converted cropland would continue to change over time until a "new" equilibrium is reached that is consistent with lands in long-term crop production. An above and below design was also used to monitor non-point source agriculture run-off as it entered the perennial vegetation, and monitored the run-off as it exited the perennial vegetation near the bottom of a hillside. These nested watersheds provided an opportunity to quantify the changes in water quantity and quality of non-point source pollution as it moved through a perennial vegetation. The vegetation effectively captured pollutants and run-off with high infiltration rates on a transition zone between a highly productive agriculture zone and the river valley floodplain. Similar areas exist within the Cottonwood River Watershed that hold potential to serve as a possible best management practice (BMP) treatment area for agriculture run-off.
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Minnesota Water Research Digital Library
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Public Domain