Explain the curve number method of direct runoff estimation. Compute potential maximum retention if curve number (CN) is 80.

The Curve Number (CN) method is a widely used empirical approach for estimating direct runoff from rainfall events, particularly in hydrological modeling and watershed management. It is based on the relationship between soil and land cover characteristics, rainfall intensity, and runoff generation.

The CN method assigns a dimensionless curve number (CN) to represent the hydrological properties of a watershed, which depends on factors such as land use, soil type, vegetation cover, and antecedent moisture conditions. The CN value ranges from 0 to 100, with lower values indicating high infiltration capacity and higher values representing reduced infiltration and increased runoff potential.

To compute potential maximum retention (S), which is the amount of rainfall that can be retained by the soil before runoff occurs, we can use the formula:

[ S = \frac{{(1000/CN) – 10}}{2.8} ]

Where:

• ( S ) = Potential maximum retention (in millimeters)
• ( CN ) = Curve Number

Given a curve number (CN) of 80, we can calculate the potential maximum retention as follows:

[ S = \frac{{(1000/80) – 10}}{2.8} ]
[ S = \frac{{12.5 – 10}}{2.8} ]
[ S ≈ \frac{{2.5}}{2.8} ]
[ S ≈ 0.8929 ]

Therefore, the potential maximum retention (S) for a curve number (CN) of 80 is approximately 0.8929 millimeters.

This value represents the maximum amount of rainfall that the soil can retain before runoff begins to occur. Any rainfall exceeding this retention capacity will contribute to runoff, with the excess water flowing over the land surface and eventually entering streams, rivers, or other water bodies. The Curve Number method provides a simple yet effective way to estimate direct runoff and inform hydrological modeling and watershed management decisions.