Impacts on the Curve Number Runoff Hydrology due to Changes on the Initial Abstraction Ratio in the Mississippi Delta

The Natural Resources Conservation Service (NRCS) Runoff Curve Number (CN) methodology is used by practitioners and scientists to determine the amount of runoff (Q) generated by a rainfall event (P). The ratio, λ, between the initial abstraction (Ia) and the maximum potential retention (S) has been the subject of research to improve the CN method’s performance. By analyzing measured rainfall-runoff events, a study was performed to evaluate the effect of varying λ on the estimation of the representative CN values and runoff depths for six (6) agricultural watersheds in the Mississippi Delta region. The original λ value of 0.2, a modified value of 0.05, and the estimated value of λ that best fit each experimental dataset were considered for the analysis. Four (4) to six (6) years of rainfall and runoff events were available from each gaged site. Results indicated that ordered series (frequency matching) as compared to natural (observed) provided a better fit to the rainfall-runoff data. The CN values for the watersheds ranged from 43 to 99 when λ was 0.2, decreasing to 23 to 98 when λ was 0.05. Response patterns for the magnitude of the CN associated to rainfall depths were found to unexpectedly increase as rainfall exceeded a depth of 2.54 cm (1 in). The study results supported previous modeling studies for which runoff was better estimated when λ equaled 0.20.