Developing a watershed screening tool to estimate relative contribution of phosphorus to guide management planning

To support the development of clean water plans, as required by the federal Clean Water Act 303(d) program, the New York State Department of Environmental Conservation (DEC) developed the Loading Estimator of Nutrient Sources (LENS) tool. DEC has prioritized clean water planning for fresh waterbodies experiencing negative impacts due to excessive phosphorus levels. LENS, an Excel based tool, combines several simple steady state models into a single screening tool that may be used to estimate the relative contribution of phosphorus sources within a watershed, waterbody response, and recovery potential of a waterbody. To validate that LENS results are reasonable approximations, LENS loading estimates were statistically compared with loading estimates from more complex watershed models that were used to develop existing clean water plans using simple linear regressions. For this comparison, DEC selected a variety of completed models that have modeled watershed phosphorus loads with different land use compositions and loading from both point and nonpoint sources. This analysis shows that LENS performs reasonably well at estimating the relative loading from a range of source sectors, though cannot replace more robust watershed models. DEC has used LENS to prioritize waterbodies for clean water plans and to guide management actions in watersheds where data is lacking to support more complex modeling efforts. Future expansions of LENS may include modifying the tool to estimate other pollutants (i.e. nitrogen), add the ability to account for internal loading of nutrients within waterbodies; and estimate the contribution of nutrients from groundwater.