DETERMINATION OF RUNOFF CURVE NUMBER AND SEDIMENT YIELD FOR MAIZE GROWN ON A SOIL WITH DIFFERENT SLOPES

Abstract

The rainfall-runoff-sediment erosion process is a complex, dynamic and nonlinear process, which is affected by many factors. Reliable predictions of runoff and sediment yield from ungauged watersheds are difficult and time-consuming. Such predictions are often required in the design of hydraulic structures and formulation of watershed management strategies. The Soil Conservation Service Curve Number (SCS-CN) method is a well accepted tool for the estimation of volume of surface runoff from small watershed for a given rainfall. It converts rainfall to surface runoff using its parameter curve number (CN) which represents the runoff potential of watershed characteristics. CN values for different combinations of soil, land use and treatment classes are given in Section 4 of the National Engineering Handbook (NEH- 4) (SCS, 1985). These tables were derived from the analysis of data from small watersheds in USA. Hence, it is preferable to derive the CN values for respective watersheds from recorded rainfall-runoff data. This study aims at to determine the runoff CN and sediment yield in the Indian watersheds and to investigate experimentally the effect of watershed slope on generation of runoff and sediment yield. The SCS-CN method was used to determine the runoff curve number and SCS-CN based sediment yield model (coupling the SCSCN method with USLE) was used to determine the sediment yield from the watershed. For this study, three plots (each of size 22m x Sm) having maize crop with three different slopes (viz., 1%, 3% and 5%) were established near Roorkee, Uttarakhand State, India. The soil of all field plots when tested for infiltration using double ring infiltrometer was found to fall in 1-lydrologic Soil Group 'C'. In-situ data were collected through two types of events (viz., natural rainfall-runoff events and artificial rainfall-runoff events) separately. As expected, the CN-values and sediment yield were found to increase with slope of watershed, and vice versa, for the same soil, land use, and rainfall. Estimated runoff depth and sediment yield were compared with the corresponding observed data. The results showed that positive correlations were detected between observed and computed data. Also, the present study explores the relationships of antecedent soil moisture content (e0) with SCS-CN parameter potential maximum retention (5) and with potential soil w erosion (A) by using the in-situ experimental data collected from the field plots. Besides the data expectedly exhibiting the plot of higher slope to generate the larger amount of sediment yield than that due to smaller slopes for the same rainfall, soil, and land use, the study finds improved model results with use of the proposed S - Oo and A - Oo relations.