Phosphorus reductions following riparian restoration in two agricultural watersheds in Vermont, USA

Abstract

Achievement of management goals for Lake Champlain (Vermont/New York, USA and Quebec, Canada) will require significant reductions of phosphorus (P) loads from agriculture, the dominant diffuse source in the basin. Cost-effective P reduction strategies must be based on reliable treatment techniques beyond basic erosion control and animal waste storage practices. The Lake Champlain Basin Agricultural Watersheds National Monitoring Program (NMP) Project evaluates the effectiveness of low-cost livestock exclusion, streambank protection, and riparian restoration practices in reducing concentrations and loads of diffuse-source pollutants from grazing land at the watershed level. Treatment and control watersheds in northwestern Vermont have been monitored since 1994 according to a paired-watershed design. Monitoring includes continuous stream discharge recording, flow-proportional sampling for total P and other pollutants, and documentation of land use and agricultural management activities. Strong statistical calibration between the control and treatment watersheds has been achieved. Landowner participation in the land treatment program was entirely voluntary and all treatments were 100% cost-shared by the project and cooperators. Installation of riparian fencing, alternative water supplies, protected stream crossings, and streambank bioengineering was completed in 1997 at a cost of less than US$40,000. The paired-watershed design was effective in controlling for the influence of extreme variations in precipitation and streamflow over six years of monitoring. Two years of post-treatment data have documented significant reductions in P concentrations and loads from both treated watersheds. Reductions of approximately 20% in mean total P concentration and approximately 20-50% in mean total P load have been observed, with greater reductions occurring in the watershed receiving more extensive treatment. The effectiveness of riparian zone restoration in P reduction tended to be lower during periods of very high runoff, especially outside the growing season.