Curve Numbers from Conventional and No-Till Cropping: A 39-Year Dataset from a Small Georgia Piedmont Watershed

Abstract. The curve number (CN) method for estimating direct runoff from a rainfall event is commonly used in hydrologic investigations, but currently accepted CNs for no-till cropping might need revising. We derived CNs from ranked rainfall-runoff data gathered from 1972 to 2010 from a 2.7 ha, zero-order watershed (P1) near Athens, Georgia. The watershed was initially managed under conventional tillage for 2.5 years, followed by double-cropped continuous no-till rotations. During conventional tillage, on average 19% of the rainfall was partitioned into runoff (median 13%). The mean and median CN was 82 and 85, respectively. Least square fitting of CN versus rainfall produced an asymptotic CN of 81 (R 2 = 0.61). In contrast, the mean and median runoff during no-till was 7% and 0.6% of the rainfall, respectively. The mean CN equaled the median CN of 62. The asymptotic CN was 58 (R 2 = 0.78), compared with 72 from standard tables. When estimated using CN of 72, the mean runoff during no-till was 142% greater than the measured mean runoff. These results support the hypothesis that CNs for no-till cropping systems should be smaller than currently accepted values for comparable watersheds in the region. This has implications for hydrologic modeling for water quantity and quality where the use of the CN method is ubiquitous.