RUNOFF CURVE NUMBERS DETERMINED BY 3 METHODS UNDER CONVENTIONAL AND CONSERVATION TILLAGES

Event-based rainfall and the associated runoff data collected from a s mall 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 thr ee years of conventional (CvT), followed by three years of conservatio n tillage (CsT). Soils of the watershed are Decatur (clayey, kaoliniti c, thermic Rhodic Paleudults) and Emory silt loam (fine-silty, siliceo us, thermic Fluventic Umbric Dystrochrepts) which are classified in Hy drologic Soil Group B. Curve numbers (CN) were determined by the SCS m ethod and a method developed assuming a log-normal probability distrib ution of potential maximum retention, S. The published CN of the avera ge soil moisture condition or antecedent moisture condition II (AMC II ) for the study site are 78 and 75 for CvT and CsT, respectively, whic h 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 b y 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.