Quantitatively Tracking and Assessing BMP Implementation
To address a watershed TMDL, long-term planning of BMPs is often the first step. However, over the years programmatic budgets fluctuate, priorities change, new technologies develop, and suddenly the plan developed all those years ago is out-of-date. How should a stormwater program assess pollutant reduction achievements?
To address this data gap, the County of San Diego Department of Public Works Watershed Protection Program (County) along with Geosyntec Consultants (Geosyntec) developed a BMP Dry Weather Load Reduction Calculator (Calculator) to track and assess their nutrient TMDL progress in the Rainbow Creek Watershed. The Calculator, partially funded by a 319(h) grant from the State Water Resources Control Board, numerically assesses anticipated flow and pollutant load reductions as a result of distributed non-structural and structural BMPs implemented throughout the watershed at the parcel level.
Rainbow Creek is a rural watershed in northern San Diego County within the Santa Margarita River Watershed. The Rainbow Creek watershed has a small residential population, a high percentage of nursery and agricultural land uses, and a high water table. The San Diego Regional Board and the County have been working on a TMDL implementation plan for several years and the County is now responsible for addressing municipal nutrient loads in the watershed.
The Calculator consists of three main components, all accessible from an easy to use Excel interface: 1) dry weather baseline load/runoff estimates; 2) BMP dry weather load/runoff reduction estimates; and 3) watershed-wide BMP implementation and benefit tracking. The dry weather average annual baseline loads and runoff volume used in the Calculator were estimated for watershed land-cover based on over 10 years of monitoring data, using multiple regression optimization. The estimated baseline loads and runoff volumes were then verified and supplemented with values from relevant regional and national studies and monitoring data.
Over 25 non-structural and structural BMPs being implemented in the watershed were evaluated for their dry weather pollutant load and/or flow reductions. An extensive literature review of BMP effectiveness, including San Diego MS4 Water Quality Improvement Plans (WQIP), and TMDL implementation plans, and the International BMP database, provided BMP dry weather flow reductions and load reductions for nutrients, bacteria, and sediment.
The Calculator’s watershed progress tracking interface allows for County staff to easily input parcel information collected during field visits. Parcel based baseline flows, loads, and reductions are automatically calculated based on land-cover identified and BMPs implemented. The Calculator keeps a cumulative total of the baseline flows, loads, and reductions for parcels that have been visited within the watershed.
The Calculator was developed with flexibility to allow for the evolving needs and priorities of the County. Future customization could include the addition of new BMPs, the estimation of wet weather benefits, or the application across other watersheds assessing not just TMDLs, but MS4 compliance. Using the dry weather BMP load reduction Calculator has allowed the County to quantitatively demonstrate their progress in achieving compliance with the Rainbow Creek Nutrient TMDL and adaptively manage their future BMP implementation.