# Computing Runoff Volumes by hcj

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```									NRS 361 Computing Runoff Volumes for Small Watersheds (less than 1000 acres) Method: NRCS (formerly known as the Soil Conservation Service) Curve Number Method (also known as TR 55). Readings: Dunne and Leopold (handout): pages 291-297, Problems 10-3 and 10-4, pages 368-369. Overview: The curve number method estimates the depth of runoff from a 24 hour “design” storm for small watersheds. The volume of runoff is determined by multiplying the depth of runoff (converted to ft) by the area of the watershed (expressed as ft2). The method is frequently used to compare the effects of land use change on watershed runoff volumes and it is the first step for modeling peak rates of runoff from large storm events. Critical data: 1) Delineation of the watershed contributing to the “design” point. 2) A map that includes the soil hydrologic group for each land use 3) The area of each combination of land use and soil hydrologic group. The different areas must add up to the total area of the watershed. 4) The magnitude of the design storm. This will be a 24 hour storm. The method is not to be used to estimate runoff based on weekly, monthly or yearly rainfall. Resources: Tables 10-3 and 10-8 in Dunne and Leopold. These tables have the runoff curve numbers for different combinations of land use and hydrologic soil groups. Figure 10-8. Chart for estimating t he volume of storm runoff for different curve numbers.

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Example 1: Using the Curve Number method, what is the runoff volume predicted for the following conditions? Design Storm: 10 year, 24 hour storm in RI (5 inches) Step 1: Obtain Watershed Characteristics 1 2 3 Land use Hydrologic Hydrologic Condition Soil Group Woodland Good B Pasture Fair C Pasture poor B Column sum

4 Curve Number

5 Area (acres) 30 40 30 100 acres

4x5

Step 2: Using Tables 10-3 and 10-8 obtain Curve Numbers and weighted CN for watershed. 1 2 3 4 5 6 Multiply column 4 by column 5 Land use Hydrologic Hydrologic Curve Area (acres) Condition Soil Group Number Woodland Good B 55 30 1650 Pasture Fair C 79 40 3160 Pasture poor B 81 30 2430 Column sum 100 7240 Obtain area weighted curve number for the entire watershed. This is computed as the sum of column 6 divided by the total area: Weighted Curve Number for the Watershed: 7240/100 = 72.4 Step 3: Go to Figure 10-8 and find the storm runoff, Q, (inches) on the Y axis that corresponds to the value for a CN of 72.4 on a vertical line arising from a 5” Rainfall on the X axis: Answer: 2.25 inches of runoff. What is the volume of runoff? = [2.25 inches * 1ft/12inches] * [100 acres * 43,560 ft2/acre] = 816,750 ft3

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Example 2: What would be the runoff from a 2 inch, 24 hour storm on the watershed described above? Because the land use did not change, the weighted CN is still 72.4. We simply need to go back to Figure 10-8 in Dunne and Leopold and read off the runoff depth for a 2” storm with a CN of 72.4. Q = 0.3 inches. Total runoff volume: 108,900 ft3 Note that the rainfall magnitude of 2” is 40% of the magnitude of a 5” rainfall --- but the runoff produced by that 2” rainfall is only 13% of the runoff produced on the same watershed by a 5” storm.

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Example 3: What would be the predicted runoff volume from a 5 inch, 24 hour storm, if the 30 acre woodland was developed? The developed land would consist of 27 acres of ¼ acre residential lots and 3 acres of paved roads? If land use changes, we must recomputed the CN for the watershed! Step 1: Obtain Watershed Characteristics 1 Land use Paved Roads ¼ acre residential Pasture Pasture Column sum 2 Hydrologic Condition N/A N/A Fair poor 3 Hydrologic Soil Group B B C B 4 Curve Number 5 Area (acres) 3 27 40 30 100 acres 4x5

Step 2: Using Tables 10-3 and 10-8 obtain Curve Numbers and weighted CN for watershed. 1 2 3 4 5 4x5 Land use Hydrologic Hydrologic Curve Area (acres) Condition Soil Group Number Paved Roads N/A B 98 3 294 ¼ acre N/A B 75 27 2025 residential Pasture Fair C 79 40 3160 Pasture poor B 81 30 2430 Column sum 100 acres 7909 Obtain area weighted curve number for the entire watershed. This is computed as the sum of column 6 divided by the total area: Weighted Curve Number for the Watershed: 7909/100 = 79.1 Step 3: Go to Figure 10-8 and find the storm runoff, Q, (inches) on the Y axis that corresponds to the value for a CN of 79.1 on a vertical line arising from a 5” Rainfall on the X axis:

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Answer: 2.8 inches of runoff. What is the volume of runoff? [2.8 inches * 1ft/12inches] * [100 acres * 43,560 ft2/acre] = 1,016,400 ft3 Changing the watershed increased the runoff volume by 24% For a 2 year storm the runoff volume for this developed watershed would be: 0.53 inches This is an increase of 74%.The increase in runoff from development is more pronounced for smaller storms.

Web Information and Tools: Annual Runoff Estimates: Tools exist to predict annual runoff based on the Curve Number method. Go to the L-Thia website for more information on long term impacts of land use on hydrology: http://www.ecn.purdue.edu/runoff/index.html

TR-55: Available on the web at: http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html

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