Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/28/2009
Publication Date: 6/28/2010
Citation: Endale, D.M., Schomberg, H.H., Fisher, D.S., Jenkins, M. 2010. Examination of curve numbers from a small piedmont catechment under 33 years of no-till crop management. Joint Federal Interagency Conference on Sedimentation and Hydrologic Modeling (JFIC2010: 9thFISC & 4thFIHMC, June 27 - July 1, 2010, Las Vegas, Nevada. CDROM.
Interpretive Summary: Estimating the amount of rain water that comes off agricultural and urban lands (runoff) following rainfall is not easy or straight forward. Runoff is influenced by many factors that are not necessarily all known or understood. Yet such estimates form the basis for assessing the impact of agricultural (or urban) activities on water quantity and/or quality. Scientists develop mathematical models based on available knowledge to estimate impacts of land management. Models must continuously be tested so that they can be used with confidence. Researchers at the USDA-ARS J. Phil Campbell Senior Natural Resource Conservation Center used rainfall and runoff data collected from 1976 to 2009 (127 events) on a 6.5-acre watershed that had been managed as a crop-field under a no-till system to test USDA’s Curve Number model to calculate runoff from rainfall. The commonly used model was not well adjusted to soils managed in this way. Runoff was overestimated by 60%. Consensus is developing from research across many other locations that this model must be adjusted to provide more realistic prediction of runoff. The data indicated that this historically eroded watershed under long-term no-till management behalves like a well kept pasture or meadow. The need for adjustment is not surprising since the original model was developed from limited data. Long-term data from field operations are essential for improving models. The Curve Number model is popular throughout the world and these observations should be of interest to teachers, farmers, environmental groups, regulators, engineers, and water resource managers.
Technical Abstract: The Curve Number (CN) method for estimating direct runoff from storm rainfall developed in the 1950’s by the USDA Soil Conservation Service has been adopted by large groups of users across the world. This has helped to gradually expose its strengths and weaknesses. Recently users have identified the need for locally defined CN values to address concerns with regional and seasonal variations. We determined CN values from rainfall-runoff data gathered from 127 storms between 1976 - 2009 on a 2.7 ha instrumented catchment (P1), managed under no-till at the USDA-ARS Watkinsville, GA. Summer crops included soybean, sorghum, millet, cotton and corn with barley, wheat, crimson clover and rye as cover crops. Mean and median CN values were 38 and 31, respectively. In contrast, CN values from standard Tables for the conditions at P1 are approximately 60 to 70 indicating an expectation of greater runoff than we measured. On average, 6.5% of the rainfall was partitioned to runoff. Half of the events partitioned <1% to runoff. Hence, CN estimates for a historically eroded watershed under long-term no-till management are similar to those of a pasture or meadow in good hydrologic condition. A proposed change to improve the CN method is to replace the current initial abstraction ratio (') value of 0.2 with 0.05. The median ' value in our study was 0.04 which is in close agreement with the proposed value. Future work will examine the variability of the CN values with season, crops, prior rainfall, and weather conditions and across time.