Mixed Watersheds

The previous examples were the simplest kind because they had the same slope, vegetation, and soil conditions throughout the watershed. In nature, this only occurs for very small areas. Watersheds usually contain different slopes, vegetation, soil types, and farming practices. All of these variables affect the value used for C. Any changes in the value used for C will change the calculated runoff. To cope with a variable watershed it is necessary to calculate the weighted runoff coefficient (Cw) for the watershed.

The weighted runoff coefficient is determined by finding the appropriate value of C for each field or portion of the watershed that is different, multiplying that value of C by the appropriate area (ac), adding up these products for all of the different areas in the watershed, and then dividing their sum by the total watershed area.

Study Figure 17.6 and the following equation:

FIGURE 17.6. Example of a mixed watershed.

FIGURE 17.6. Example of a mixed watershed.

Problem: Determine the weighted C for the watershed in Figure 17.6. Solution:

= 0.6305 ...or 0.63 The weighted C value for this watershed is 0.63.

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