Land Use Buildup Tab
Pollutant Buildup that accumulates over a category of land use is described by either a mass per unit of subcatchment area or per unit of curb length, as specified by the Normalizer field described below.
The buildup is usually given as mass of pollutant per unit area although they may be given as counts (in the case of coliforms) per unit curb length. Therefore B (and C1 in the functions below) can be given as lbs/acre, kg/hectare, coliform/acre, lbs/ft curb length, etc. Time in the functions refers to the number of antecedent dry days. Buildup is only used when the exponential washoff function is used to determine quality of runoff quality from catchments.
Three different functions can be used to describe buildup. The functions are all monotonically increasing. The figure below gives their relative shape.
Buildup Function Types
The amount of buildup as a function of days of dry weather can be computed using one of the following functions:
- Power Function: Pollutant Buildup (B) accumulates proportional to time (t) raised to some power, until a maximum limit is achieved.
| B | = | buildup, mass(count/area(length) | |
| C 1 | = | maximum possible build-up, mass (count)/area(length) | |
| C 2 | = | build-up rate | |
| C 3 | = | time exponent |
The effect of each coefficient is shown in the figure below.
Effect of Power Function Coefficients
- Exponential Function: Buildup follows an exponential growth curve that approaches a maximum limit asymptotically.
| B | = | buildup, mass(count)/area(length) | |
| C 1 | = | maximum possible build-up, mass(count)/area(length) | |
| C 2 | = | build-up rate constant, 1/day |
The effect of each coefficient is shown in the figure below.
Effect of Exponential Function Coefficients
- Saturation Function: Buildup begins at a linear rate which proceeds to decline constantly over time until a saturation value is reached.
| B | = | buildup, mass(count)/area(length) | |
| C 1 | = | maximum possible build-up, mass(count)/area(length) | |
| C 2 | = | build-up rate constant, 1/day |
The effect of each coefficient is shown in the figure below.
Effect of Saturation Function Coefficients
This tab contains a table with the following attribute columns:
| Column | Description |
|---|---|
| Pollutant | This menu contains all of the pollutants that have been defined in the Pollutants dialog box for the current model. Select the one that should be used for the land use currently highlighted in the list pane. |
| Max. Buildup | This field allows you to define the value of C 1 in the above equations. |
| Rate Constant | This field allows you to define C 2 in the Power and Exponential equations described above. |
| Buildup Function | This menu allows you to specify which of the buildup functions described above will be used for the land use currently highlighted in the list pane. |
| Power Constant | This field allows you to define C 3 in the Power equation described above. |
| Half Saturation Constant | This field allows you to define C 2 in the Saturation equation described above. |
| Normalizer | Allows you to specify the method by which pollutant buildup is described for the land use currently highlighted in the list pane. Choices include Area (mass per unit of subcatchment area) or Curb (mass per unit of curb length). |
| Buildup Time Series | The name of the Time Series that contains buildup rates (as mass per normalizer per day). |
| Scaling Factor | A multiplier used to adjust the buildup rates listed in the time series. |