Description
A catchment is an area which discharges runoff water to a designated location of a surface water or groundwater. Within or at the discharge location, there is potential for management by a BMP. A watershed can be separated into many catchments.
Input Data
The catchment is defined by the following inputs:
- Land use
- Total area
- DCIA percentage of total area
- Non-DCIA curve number (CN)
- Potential wet pond area
- Loading from other sources (e.g., pumped or groundwater base flow)
Each land use has an associated Event Mean Concentration (EMC). Additional information for land use selection, annual runoff coefficient (ROC), and loading calculations is available through the help icons
Be sure to define the catchment name before performing calculations.
Event Means Concentrations (EMCs)
If pre- or post-development EMCs are not known, user-defined values may be entered. Open (white) entry fields allow direct input.
For mixed land uses within a catchment, a weighted EMC based on annual runoff can be calculated using the calculator support button. The calculator is only visible when “User Defined Values” is selected. Nitrogen and Phosphorus values must both be entered for user-defined categories.
Many programmed land uses automatically populate EMC values.
DCIA Disconnection and Curve Number (CN) Adjustment
If directly connected impervious area (DCIA) is disconnected and routed to pervious areas before discharge, adjustments are made to the curve number calculations using the CN calculator support button adjacent to the Non-DCIA CN input.
In this example, the post-development CN is higher than the pre-development CN because the disconnected impervious area is assigned a CN of 95. This CN value reflects runoff from rainfall greater than 0.1 inches.
When 1.813 acres of DCIA are disconnected:
- Pervious area = 15.222 acres
- DCIA = 3.715 acres (17.9% of total area; 0.179 × 20.75 acres)
- Composite CN weighted by runoff volume = 76.12
A simple average CN (68 + 95) / 2 = 81.5 would produce a higher runoff volume than the composite weighted runoff CN. An area-weighted CN of 70.87 would produce a lower runoff volume than the composite weighted runoff CN. Therefore, composite weighting by runoff volume is required.
Input Parameter Definitions
Catchment Name
Any alphanumeric text, this typically matches the stormwater professional’s designation for the project site’s catchment(s). Catchments are the smallest hydrologic designation for which runoff occurs in a year and where one or more specific BMPs can provide treatment.
Pre- and post-development land uses
25 typical land use categories are available via the drop-down menu based upon current recognized values. Each has an arithmetic average EMC value. A user defined category is available for other EMCs and for composite land uses. A calculator is available for a composite EMC when there are multiple land uses based on average annual runoff for each area CN value. The calculator is only visible when the “User Defined Values” is selected. For “User Defined Values”, EMC values for both Nitrogen and Phosphorus must be entered. For many land uses programmed into the model, the EMC are automatically entered.
Total pre-development catchment area
The total area in acres for the catchment in the pre-development condition. The catchment is a contiguous area where all the runoff will accumulate.
Total post-development catchment area
the total area in acres for the catchment defined at a point where there is a discharge as well as a place for possible BMPs in the post-development condition.
Pre-development Non-DCIA curve number
the weighted runoff curve number for the portion of the catchment which is not directly connected impervious area. A composite weighted CN can be calculated using average annual runoff. This is an option in place of simple area averages. If this value is below 30, the SCS/NRCS runoff curve number method is not valid and the program will change the input to 29.9.
Pre-development DCIA percentage
the percentage of the total catchment area that is directly-connected impervious area. Note, areas wet all year are not included.
Post-development Non-DCIA curve number
the weighted runoff curve number for the portion of the catchment that is not directly connected impervious area in the post-development state. A composite weighted CN can be calculated using average annual runoff. This is an option in place of simple area averages. As noted above for pre-development Non-DCIA curve numbers, if this value is below 30, the program will change the input to 29.9.
Post-development DCIA percentage
the percentage of the total catchment area that is directly-connected impervious area in the post-development state. Areas wet all year (detention ponds, wetlands) are not included in the DCIA calculation.
Estimated Wet Pond area
is the area at the top of the permanent pool or also known as the control elevation. This area is subtracted from the overall development catchment area when the wet pond area is considered part of the total catchment area. As an alternative, the wet pond area is not included in the catchment area. No nutrient loading is included for this area. This is normally assigned to an area which remains wet all-year with no discharge. For Florida sites, the average annual runoff pond coefficient is zero because Evapotranspiration is about equivalent to rainfall on wet ponds.
Calculated and Support Buttons
Weighted EMC and CN calculator support buttons. Copy and paste is available.
Calculate Function
The “Calculate” button provides:
- Pre- and post-development runoff coefficients (C values)
- Annual runoff volume (acre-feet)
- Annual Nitrogen loading (kg/year)
- Annual Phosphorus loading (kg/year)
Multiple Catchments
For sites with multiple catchments, additional catchment worksheets may be created using the “Add Catchment” button located in the top left-hand corner of the worksheet.
