This figure illustrates the three elements used for simulating pollutant loads in the PLRM: 1) hydrology and hydrologic source controls (HSC), 2) pollutant load generation and pollutant source controls (PSC), and 3) storm water treatment (SWT). User input is required for each element, and the results derived from each element are used in subsequent elements. Computed pollutant loads represent the combined effectiveness of the three major elements.  

The definition of each element is as follows:

• Hydrology and Hydrologic Source Controls (HSCs)
  Hydrology is reported as average annual runoff volumes from long-term continuous simulations of precipitation and runoff. Continuous hydrologic simulations include many time steps over a specified period of time; including many hydrologic events and intervening dry periods (rather than a single storm) to represent the full range of hydrologic conditions during the period of time simulated.
  
  HSCs reduce runoff volumes and minimize the concentration of storm water runoff through distributed runoff interception, infiltration, and disconnection of impervious surfaces.  HSCs primarily function to increase infiltration, which routes precipitation or surface runoff to groundwater.
• Pollutant Generation and Pollutant Source Controls (PSCs)
  Pollutant generation focuses on estimating the total pollutant load from a drainage catchment based on the characteristics of the catchment, in particular the land uses with the catchment and the condition of the land uses. The product of storm water volume and storm water pollutant concentration is the pollutant load generated from the catchment.
  
  PSCs reduce the generation of pollutants of concern at their sources by inhibiting or reducing mobilization and transport of pollutants with storm water.
• Storm Water Treatment (SWT)
  SWT removes pollutants of concern after they have entered concentrated storm water runoff flow paths. This might include treatment of flows infiltrated to groundwater as well as those discharged to surface waters.

The PLRM streamlines and automates many of the data inputs required to setup and execute a simulation. The PLRM is intended to minimize the burden on the end user for data collection and data compilation. For example, the PLRM automatically generates location-specific meteorological data needed for the long-term continuous simulation based on user input. With some minor exceptions, performing a PLRM simulation only requires data typically collected and compiled in current Tahoe Basin practice for implementing storm water quality improvement projects (i.e. Formulating and Evaluating Alternatives [SWQIC, 2004]). The PLRM is intended to standardize the methods used to estimate pollutant loads for urban areas in the Lake Tahoe Basin. The PLRM reports the pollutants of concern defined by the Lake Tahoe TMDL, specifically: TSS; fine sediment particles (<16 microns); total and dissolved phosphorus; and total and dissolved nitrogen.