EFFECT OF LAND USE, SOIL TYPE AND ANTECEDENT MOISTURE CONDITION (AMC) ON RUNOFF CURVE NUMBER

Abstract

Runoff from watersheds depends on a topography, geology, climate, soil, vegetation, and land use etc. It is one of the most important variables used in planning of water resources and design of hydraulic structures. Several models have been developed to compute runoff from a rainfall event. 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 depth for a given rainfall depth from small ungauged agricultural and urban watersheds. According to the SCS-CN concept, the runoff generation in an agricultural watershed depends on four major watershed characteristics, viz, soil, land use, hydrologic condition, and antecedent moisture condition (AMC) and the only parameter required for this methodology is the Curve Number (CN). The CN ialues resulting from exhaustive investigations in the United States for various soils and land uses have been reported in National Engineering 1-land Book (NEI-l-4). This is very reliable and popular method in USA and effectively used in many other countries. No systematic efforts have been made to verify CN validity to watersheds in other countries. A study has been carried out on an agricultural experimental farm (plot size: 22mx5m) located near Roorkee, Uttarakhand, India, to evaluate the effect of soil type, land use and AMC on the runoff curve number for the selected grades of 5%, 3% and 1%. Twelve plots of different land uses such as sugarcane, maize, black gram, and fallow land were investigated. Double ring infiltrometer test had been carried out to determine infiltration capacity. It was found that the entire plot was having infiltration capacity greater than 7.62 mm/hr. and as per SCN-CN method, the entire 12 plot had fallen under HSG A. As expected, the effect of land use on CN was such th..t the sugarcane land use showed the highest runoff CN (86.28) at 3% slope and black gram land use showed the lowest runoff CN (79.44) at 5% slope. Though the l-ISG of the entire experimental plot was having HSG A but infiltration capacity was found different which greatly affected the - runoff CN. The infiltration capacity (fc) of sugarcane plot at slope 3% was 8mm/hr. and that of black gram plot 22 mm/hr. in spite of the above reason, CN was also influenced by AMC. From the experimental - result it was found that as AMC was increased, CN was also increased and vice-versa. Runoff predicted from NEH-4 handbook tables do not match well with the observed Q and correlation was not significant between them. The main conclusions of this experimental work are that CNs estimated from the handbook tables do not match well with the P-Q based CN estimates. Regression models (linear, exponential, power and logarithmic) for SCS-CN based on previous day moisture condition Oo for each experimental land use and over all plots have been developed and tested. The performances of proposed models were found to be satisfactory. Among them, performance of exponential model found to be better than others.