Testing the Effectiveness of Targeted Conservation With a Paired Watershed Approach.

Two 19 mi² agricultural watersheds in SW Wisconsin have been continuously monitored for sediment, phosphorus and flow for six years. Field management and soil phosphorus were inventoried in both watersheds to provide an assessment of runoff phosphorus loss risk using the Wisconsin Phosphorus Index for each field and pasture. Following a three-year baseline period, conservation practices were implemented in one of the watersheds using a strategy focused on fields and farms that were estimated to contribute the highest amounts of phosphorus to the stream.  Management changes implemented over three years are estimated to result in a greater than 25% reduction in phosphorus and sediment delivery from agricultural land to the stream. Although changes implemented at the field level may take time to result in changes at the watershed outlet, there appears to be a trend of suspended sediment and total phosphorus load reduction in the monitoring data. Analysis of the baseline paired watershed monitoring data indicates that reductions of 36% in sediment and 28% in phosphorus are needed to be statistically significant and attributable to conservation practice implementation. A SWAT analysis of the treatment watershed provided a ranking of subbasins by total P contributions that was consistent with the ranking provided by aggregating field P Index values for each of the subasins.