RAINFALL RUNOFF MODELING OF RAMGANGA CATCHMENT

Abstract

Information regarding flow rates at any point of interest along a stream is necessary in the analysis and design of many types of water resource projects. Although many streams have been gauged to provide continuous records of stream flow, planners and engineers are some times faced with little or no available stream flow information and must rely on synthesis and simulation as tools to generate artificial flow sequences for use in rationalizing decisions regarding structure size, the effect of land use, flood control measures, water supplies, and the effect of natural or induced watershed or climatic change. Simulation is defined as the mathematical description of the response of a hydrologic water resource system to a series of events during a selected time period. For example, simulation can mean calculating daily, monthly, or seasonal stream flow based on rainfall or computing the discharge hydrograph resulting from a known or hypothetical storm, or simply filling in the missing values in a stream flow record. The importance of rainfall—runoff modeling in planning and management of water resources systems can hardly be overemphasized. The long- term daily rainfall — runoff models are primarily developed for determination of continuous daily flow series from the available precipitation and other meteorological data for augmentation of record for use in water availability analysis useful in planning and management of water resources projects. Since the rainfall data is generally abundant compared to runoff data, it is necessary to develop models which convert rainfall to runoff. Water yield is generally estimated from rainfall-runoff relationships. Thus rainfall-runoff (R-R) relation is a conceptual simplification of the systematic hydrologic cycle and it is major complex processes in the hydrologic cycle. Many researchers have developed rainfall-runoff models for gauged and ungauged catchments separately that could accurately predict runoff hydrographs, peak flow rates, and times to peak. As every model has got its own limitations, the selection of appropriate method for predicting runoff from a catchment is difficult and most often there is a scope for further improvement. Hence, evaluation, refinements and modifications related to rainfall-runoff modeling remain classical. x The Soil Conservation Service Curve Number (SCS — CN) method is widely used for computation of direct surface runoff from given storm rainfall. In this study this method is first explored for its potential and limitations and then used for long term daily flow simulation. When the method is expressed as an infiltration equation, the infiltration rate becomes dependent on both total storm rainfall and rainfall intensity. When expressed as a spatially varied saturation overland flow model, the method implies that some part of any "catchment has infinite surface storage capacity. SCS curve numbers are used to estimate the amount of precipitation which becomes runoff, and the amount which infiltrates into the soil. The concept behind the SCS — CN method can be applied to determination of surface drainage flow from rainfall. When accumulated subsurface drainage flow is plotted against accumulated infiltration, subsurface drainage flow starts after some infiltration has accumulated and the relationship becomes asymptotic to a line _ of 45° slope, similar to the SCS rainfall — runoff relationship. Extending the basic proportionality concept of the SCS — CN method, an SCS — CN based procedure is suggested for determination of baseflow and it is employed in long term daily flow simulation. For daily flow simulation, a simple 4 parameter SCS — CN based daily flow simulation model is proposed and applied to the 5 years daily data of rainfall, runoff and evaporation, of Ramganga catchment. To check the versatility of the proposed model, this also tested on the data of catchments from different climatic regions of India and the results are analyzed. The simulation results were compared with the results obtained from the application of an existing 6- parameter, SCS — CN based hydrologic model and the proposed model is found to perform better on the high runoff producing watersheds, such as the Ramganga catchment, than the less runoff producing watersheds, such as Manot, Hridayanagar, Ghodahado watersheds.