Strategic Insights on the Role of Farm Ponds as Nonconventional Stormwater Management Facilities

Farm ponds are constructed for different purposes in many parts of the world, including irrigation, livestock water, and so forth; however, their impacts are not fully captured in stormwater application. As part of this study, the environmental benefits of farm ponds were assessed, and their cumulative impact at the watershed level was evaluated. The following three criteria were used to assess the impact of farm ponds: (1) the aggregate capability in reducing non-point-source pollution (measured in terms of phosphorous load reduction); (2) the cumulative impact in reducing channel erosion; and (3) the support to the ecological function of the buffer areas known as resource protection areas (RPA). The investigation was carried out on 494 farm ponds located in Fairfax County, Virginia. The results showed a profound effect of these small water storage features in protecting the downstream receiving channels from erosion. It is estimated that the level of peak flow reduction provided by 70% of the farm ponds exceeds 78% for the 1-year storm, and 55% for the 2-year storm event. This means that the presence of these farm ponds significantly reduces the stresses caused by the increase in impervious areas as a result of urbanization and its impact on channel incision. The net environmental benefit of the farm ponds in terms of their pollutant load reduction capability is found to exceed $\mathrm{USD}0.58\mathrm{million}/\mathrm{ha}$ ($\mathrm{USD}0.23\mathrm{million}/\mathrm{acre}$) of pond surface area. In Fairfax County, it is estimated that a total of 2.7 t (6,000 lbs) of phosphorous load is being trapped by farm ponds on average annual basis. The role of these water bodies in supporting the ecological function of the RPA buffer was studied by first analyzing the connectivity of farm ponds and their locations following the core functions of RPA buffer features. The study found that a total loss of $1.5{\mathrm{km}}^2$ (0.61 sq mi) in the RPA buffer would be anticipated in the event that these farm ponds are drained. The outcome of this study is intended to inform decision makers on the right policy that could be integrated as part of the adopted jurisdictional regulatory framework. Such an approach could be scalable to other jurisdictions contingent upon addressing the local contextual issues of other localities.