Curve Numbers for No-till: Field Data Versus Standard Tables.

The Curve Number procedure developed by Soil Conservation Service (Now Natural Resources Conservation Service) in the mid-1950s for estimating direct runoff from rainstorms has not been extensively tested in cropping systems under no-till. Analysis of CNs using the frequency matching and asymptotic approaches and derived from 35 years (1976-2010) of rainfall-runoff data gathered from a 2.7 ha Georgia Piedmont catchment managed under no-till throughout, produced an average CN (CN_II –reference) of 57.7. In contrast, the average of the range of CNs given in standard handbook tables for the catchment is 73. When examined seasonally, average CNs varied from 49 to 63. Similarly, average CNs varied from 50 to 67 when examined by cropping system (3 spring-summer and 4 fall-winter each). The derived median value of the initial abstraction ratio (λ) was 0.033, compared to 0.2, the standard value. Many researchers recommend 0.05 for λ. Mean runoff coefficient (percent rainfall partitioned to runoff) was 7.0 while the median was 0.65. Use of standard CN coefficients and values for fields managed in no-till, and possibly other conservation tillage systems, likely would lead to overestimation of runoff.