Effect of Land Use on Curve Number in Steep Watersheds

Abstract

Runoff is an important and valuable variable used in planning of water resources and design of hydraulic structures. A number of models have been developed to calculate runoff from a rainfall event. The Soil Conservation Service Curve Number (SCS-CN) methodology is one of the most widely accepted event-based methods and is extensively used for estimation of surface runoff for a known precipitation event from small un-gauged agricultural watersheds. The model is satisfactorily established in hydrologic engineering. The main cause for its wide applicability lies in the fact that it is easy to use, and it incorporates major runoff generating watershed characteristics: soil type, land use, surface condition, and AMC. The only parameter curve number CN is critical for exact runoff prediction. According to the SCS-CN concept, the runoff quantity agricultural watershed depends on the above four major watershed characteristics. The CN values resulting from exhaustive investigations in the United States for all soil and land uses have been investigated and reported in National Engineering HandbookChapter-4 (NEH-4). Since the inception of SCS-CN method, only a few or no efforts appear to have been made to justify curve number rationality to watersheds in other countries. The slope was excluded in its original development, and it is included as a factor in the recently developed new models. Investigations were carried out on agricultural plot of size (12.0 × 3.0 m²) located Toda Kalyanpur, Uttarakhand, India to calculate the effect of slope, soil type, AMC, and land use on the runoff and runoff curve number in selected three grades of 8, 12, and 16% with same Hydrologic Soil Group (HSG) “A.” There were nine0 plots of three land uses, maize, finger millet, and fallow lands for investigation. As expected, the conclusion of land use on runoff curve number was such that the fallow lands showed the largest runoff and the CN values. With the increase of slope, the CN and runoff quantity increased which we got in 16% slope, fallow land. The effect of soil was more prominent than the slope. The soil was, however, the same for all experimental plots, i.e. HSG-A. The SCS-CN parameter potential maximum retention (S) showed an inverse relation with the measured AMC value.