Recently, I found there are three equations using in calculating CN(curve number) that area "S= 25400/CN - 254" and "S = 1000/CN - 10" and "S= 2540/CN - 25.4". In fact, many papers used different equations in their research(just more than these three). I guess maye "S= 2540/CN - 25.4" can be misspelling or not. But I'm wondering the difference bettween "S= 25400/CN - 254" and "S = 1000/CN - 10" and whichis the original one.
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Dear Yue Ma,
All the equations are correct. The discrepancy is because of the unit conversion.,
CN is a function of hydrologic soil group, land use, land
treatment and hydrologic condition.
S = a measure of the watershed hydrologic response potential
S=watershed storage, inches
1000/CN-10
S=watershed storage, mm
25400/CN-254
S=watershed storage, cm
2540/CN-25.4
The Soil Conservation Service Curve Number (SCS-CN) method is a widely used empirical method for estimating direct runoff from rainfall events in a watershed. The equations you've mentioned for the calculation of S (potential maximum retention after runoff begins) are variations of the original SCS-CN method equations. However, the original and most widely recognized equation for calculating S in the SCS-CN method is:
S = (25400 / CN) - 254
This equation is commonly used in many applications and is considered the standard equation for SCS-CN calculations. In this equation:
- S represents the potential maximum retention after runoff begins.
- CN is the curve number, a dimensionless parameter that depends on land use, hydrologic soil group, and antecedent moisture condition (AMC).
The equation you mentioned, S = (1000 / CN) - 10, is another variation of the SCS-CN method that uses different constants. It's not the standard SCS-CN equation, but it may be used in some applications.
The equation S = (2540 / CN) - 25.4 appears to be another variation. It's possible that it was derived with different units or regional adjustments, but without specific context or documentation, it's challenging to confirm its origin or validity.
In practice, the choice of which equation to use for S should align with the specific data and regional conditions you are working with. It's crucial to ensure consistency within your study or application and, when possible, to reference established guidelines or local standards for SCS-CN calculations. The equation S = (25400 / CN) - 254 is the most widely accepted and recognized equation for S in the SCS-CN method, so it's often a safe choice unless there are specific reasons to use an alternative equation based on regional or project-specific considerations
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