The compliance “yardstick” that anchors MS4 progress toward WLAs
In this article:
Overview
A baseline represents the runoff and pollutant loads your MS4 would generate if no controls were in place. Comparing baseline to current and potential scenarios quantifies your program’s impact—supporting TMDL compliance and better local decision-making.
Why Baselines Matter for Compliance
Regulators require MS4s to demonstrate how their programs reduce pollutant loads compared to their share of a TMDL wasteload allocation (WLA). The baseline provides the reference point:
- It quantifies the runoff and pollutants your MS4 would discharge in an uncontrolled condition.
- Every reduction you document in 2NFORM—whether from street sweeping, green infrastructure, or regional treatment systems—is measured relative to that baseline.
- This comparison allows you to prove progress toward meeting WLAs with clear, defensible numbers.
Without a baseline, there’s no way to measure how far you’ve come or how much work remains to meet permit or TMDL runoff and pollution reduction requirements.
How the Baseline Is Calculated
2NFORM uses TELR to calculate baselines with location-specific, high-resolution inputs:
- Raster-based computations. TELR models the landscape on a 30m grid, capturing the variability of soils, land cover, impervious surfaces, and rainfall across your MS4.
- Rainfall data. Long-term precipitation patterns from regional PRISM datasets ensure runoff estimates reflect your local climate.
- Soils and infiltration. NRCS hydrologic soil groups and survey data define how much water infiltrates vs. runs off.
- Impervious and land cover. Satellite imagery provides impervious cover and vegetation patterns that strongly influence runoff and pollutant buildup.
- Empirical pollutant concentrations. National datasets of characteristic runoff concentrations (CRCs) assign pollutant levels by land use type.
The result is a spatially detailed map of runoff and pollutant generation in your city, showing where stormwater impacts originate.
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How Baselines Are Used in 2NFORM
Baseline results are summarized by catchment, drainage to each outfall, and MS4-wide, defining the starting point for loads delivered to receiving waters.
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Compliance tracking. Compare baseline vs. current to quantify and track reductions to date.
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Planning future work. Compare baseline vs. potential to estimate reductions from planned BMPs.
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Prioritization. Identify high-contribution areas to focus resources where effective investments can make the greatest positive impact.
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Reporting. Provide audit-ready numbers for annual and five-year reports.
The Takeaway
Baselines are the yardstick for compliance and accountability. Anchoring progress to a scientifically sound baseline lets you show regulators, leadership, and the public exactly how investments reduce pollutant loads and move your MS4 toward TMDL targets.