Understanding how much rainwater runs off fields is important for managing floods, soil erosion, and water quality. We studied a widely used method for estimating runoff, called the Curve Number (CN) method, in an agricultural watershed in eastern South Dakota. By measuring changes in soil moisture after rainstorms in fields managed with three tillage practices (conventional, vertical and no-tillage), we found that the standard CN values often overestimate runoff, especially in fields with conservation tillage. The results showed that using real field measurements and considering modern farming practices can make runoff estimates more accurate, helping farmers and engineers design better ways to protect soil and water resources.