Runoff Curve Numbers Determined by Three Methods Under Conventional and Conservation Tillages

Event-based rainfall and the associated runoff data collected from a small watershed planted to cotton were analyzed to determine effects of two tillage systems on SCS runoff curve numbers. A tillage study was conducted for six years on a 3.8 ha watershed planted to cotton in the Limestone Valley region of northern Alabama. The tillage included three years of conventional (CvT), followed by three years of conservation tillage (CsT). Soils of the watershed are Decatur (clayey, kaolinitic, thermic Rhodic Paleudults) and Emory silt loam (fine-silty, siliceous, thermic Fluventic Umbric Dystrochrepts) which are classified in Hydrologic Soil Group B. Curve numbers (CN) were determined by the SCS method and a method developed assuming a log-normal probability distribution of potential maximum retention, S. The published CN of the average soil moisture condition or antecedent moisture condition II (AMC II) for the study site are 78 and 75 for CvT and CsT, respectively, which are recommended in many watershed/water quality computer simulation models such as CREAMS and AGNPS. The results showed that CN of AMC II (CN-II) calculated by the log-normal method were 83 and 88 for CvT and CsT, respectively. These were slightly higher than those calculated by the SCS method which were 82 for CvT and 86 for CsT. The calculated CN-II values were higher than the published CN-II values but the order of magnitude was reversed.