Since the stormwater program advocates an accounting process for achieving the WLA, with a percent reduction in loading being the target, it is important to define the baseline condition from which calculations will be made. The baseline must include a year but may include additional information, such as a summary of BMPs considered in deriving the WLA. Baseline year is the year from which stormwater practices can be credited toward meeting a total maximum daily load (TMDL) wasteload allocation (WLA). That is, any best management practice (BMPs) or activity implemented during or after the baseline year that results in a reduction in pollutant loads from a municipal separate storm sewer system (MS4) to the impaired waterbody can be considered as progress towards meeting the MS4 wasteload allocation (WLA).
Most TMDL reports, TMDL Implementation Plans, or Watershed Restoration and Protection Strategy (WRAPs) reports now provide a baseline year. Many earlier TMDL reports did not provide this information. For TMDLs developed with short-term monitoring data, such as two or three years, the baseline year will be one of those years. When long-term monitoring data is used it is difficult to determine the appropriate baseline year. TMDL authors, TMDL project managers, Stormwater Program staff, and stakeholders work together to determine an appropriate baseline year.
Examples of baselines from TMDLs developed in Minnesota are provided below. In general, more recently-approved TMDLs provide clearer target loads and baselines.
- Hardwood Creek – this TMDL does not clearly define the target load from a baseline condition. The TMDL provides categorical WLAs for TSS and BOD across five flow regimes. The TMDL defines 2002 as the baseline year and provides a watershed TSS load for that year. Subtracting the WLA from the 2002 load and dividing by the 2002 load gives a reduction of 15.7 percent.
- Minnesota River Dissolved Oxygen TMDL – this TMDL provides a categorical WLA on a lb/day basis. The TMDL report states the reduction goal is 30 percent. The report also states that no BMPs were considered in the modeling. The model scenario chosen for setting the allocations was based on 1988 data. The target load is therefore assumed to be 30 percent from a no-BMP situation in 1988.
- Long-Farquar Lakes – Individual WLAs are provided for each MS4 on a lbs/day basis. Current watershed loads are given. Thus, a percent reduction can be calculated as well as an overall mass reduction. The TMDL does not define a specific year. The TMDL was based on data for three years – 2001, 2003, and 2005. The target load for the MS4s is thus either their WLA on a lb/day basis, or a percent or mass reduction from 2003, which represents an intermediate year.
- Kohlmann Lake TMDL – Individual WLAs are given to each MS4s. The WLA is expressed as lbs/day and as a percent reduction. The TMDL states that 2001 provides a reasonable starting point for tracking reductions.
The List of Approved TMDLs with MS4 WLAs (updated November 2019) includes the baseline year identified in the TMDL, or the MPCA recommended baseline year. Please click here to download the baseline year guidance document: File:TMDLbaselinesMS4.xlsx