Annual Loads

Regressions of total load versus total runoff from a family of storms give a slope in concentration units, which can be used to predict pollutant load for a specific quantity of runoff. To better represent the actual runoff process, base flows were abstracted from storm runoff or low-flow loads from storm load. Good correlations have been found using log-load versus log runoff volume, reflecting log-normal distribution of the concentration data. Nonpoint source data are usually log-normal distributed, as are hy-drologic events.

After mass loadings for a given land-use type are accumulated over a considerable period of time, results can be expressed in terms used to estimate loadings from that type of land use for the rest of the watershed(s) of interest. The approaches below are commonly used:

1. Annual loading/area of given land use, lbs/acre/yr

2. Annual loading/curb mi of given land use, lb/mi/yr

3. Annual loading/traffic volume, lbs/vehicle/yr

4. Annual loading/air pollution index, lbs/avg in/yr

5. Annual loading/runoff volume, lbs/million gal

6. Annual loading/precipitation amount, lbs/in (for specific area)

Number 1 assumes that pollution varies according to land use. This is the most commonly used method of predicting loading under future conditions. Number 2 assumes that pollution loading varies with the number of curb mi in various stages of development. Numbers 3 and 4 make similar assumptions regarding automobile traffic and air pollution. Numbers 5 and 6 are designed to convert loading data from specific storm events to annual average loadings, which are then converted to relationships with land use for predictive purposes.

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