DETERMINATION OF RUNOFF CURVE NUMBER AND SEDIMENT YIELD FOR SUGARCANE GROWN ON A SOIL WITH DIFFERENT GRADES

Abstract

Runoff is one of the most important variables used in planning and design of hydraulic structures, and therefore, a number of models for its computation from a given rainfall event have been developed. The Soil Conservation Service Curve Number (SCS-CN) method is one of the most popular event-based methods and is widely used for estimation of direct surface runoff for a given storm rainfall event from small watersheds. This method is well established in hydrologic engineering. The primary reason for its wide applicability lies in the fact that it accounts for most runoff producing watershed characteristics: soil type, land use, surface condition and antecedent moisture condition. The only parameter of this methodology, i.e. the Curve Number (CN), is crucial for accurate runoff prediction. 4 Evidently, most studies have concentrated on the application of the existing SCS-CN method utilizing CN derived from NEH-4 tables or using GIS for watershed characteristics. No systematic effort appears to have been made for experimental verification of the effect of watershed slope and land use on CN, particularly for Indian watersheds. Thus employing the in-situ rainfall-runoff data, the present study derives parameter CN of the SCS-CN methodology for the experimental plots (size: 22 m x 5 m) of different slopes (viz., 1%, 3%, and 5%) and land use of sugarcane located in Roorkee, Uttarakhand, India. As expected, the plot of 5% slope yielded the largest runoff and, in turn, CN compared to those due to the plots of 3% and 1% grades, for the same rainfall, soil, and land use. The CN values derived from the observed data for AMC II condition and for three grades of 1%, 3% and 5% are 86.00, 88.25, and 91.42 for natural rainfall datasets. The derived CN values are fairly close to those from NEH-4 CN-values, supporting the applicability of NEH-4 CN values to Indian watersheds. CN was seen to continually increase with rainfall to a peak value and then decreased for all three grades of field plots, indicating all field plots to fall in violent category of watersheds. Another crucial and important aspect of soil erosion deals with the removal of soil from land surface by wind or water. When rain drop falls on a surface, the soil particles are splashed. 1-ligher is the velocity of impact, greater is the amount of soil splashed. The detached soil particles are then carried further, either by runoff or wind. This whole process is known as erosion, and sediment yield from a watershed is the resulting output of the erosion process. Thus, the process of rainfall-runoff-sediment yield in a watershed is a very complicated phenomenon that is controlled by a large number of known and unknown climatic, geologic and physiographic factors that vary both in time and space. In the present experimental work, an attempt has been made to determine event-based sediment yield using the model derived from coupling the SCS-CN method with Universal Soil Loss Equation ir (USLE). As expected, the plot of 5% slope yielded the largest sediment yield compared to those due to the plots of 3% and 1% grades, for the same rainfall, soil, and land use. To signify the role of antecedent moisture content (8o), the present study explored the existence of its relationships with SCS-CN model parameter potential maximum retention (5) (or CN) (used for determination of runoff) and with potential soil loss (A) (used for determination of sediment yield) by using in-situ experimental data. A and S were found to increase and decrease, respectively, with increasing Qo, and vice versa