RN-13 : Comparative study of components of watershed models

Abstract

The rainfall runoff process in a catchment is a complex and complicated phenomenon governed by large number of known and unknown physiographic factors that vary both in space and time. The rain or snow falling on a catchment undergoes number of transformations and abstractions through various component processes such as interception, detention, evapotranspiration, overland flow, infiltration, interflow, percolation, sub-base flow, base flow etc. and emerges as runoff at catchment outlet. Application of mathematical modelling techniques to the constituent processes involved in the physical processes of runoff generation have led tc better understanding of the processes and their interaction. Different types of watershed models have been developed depending on the purpose such as flood forecasting, simulation of hourly or daily runoff or estimation of water yield etc.

Each watershed comprises of different types of soil cover, vegetation, land use, topography, drainage pattern and density, slopes etc. The processes that take place are not uniform throughout the basin, moreover they are also not uniform in time, eg.interception loss depends on type of vegetation cover and its density and also on -7,4.rfall amount, its intensity and duration. Interception loss is high at the beginning of rainfall but reduces gradually to a constant value equal to potential, evaporation rate till rainfall continues. Similarly infiltration rate varies space and time and also depends on initial soil moisture condition. As such exact analysis of these complex component processes is very difficult.

To simplify analysis of these complex processes different water- shed models have adopted different laid out approaches, mohair or approximations for each process and the developed model as a those is capable to simulate observed runoff. A comparative study of model structures for various processes considered in different watershed models has been done to ascertain suitable model structure for each component process for typical physiographic and hydrometeorological conditions of river basins in India.

The models that have been included in this review are standard watershed model(SWM IV) developed by Crawford and Linsley of USA, UBC model(University of British Columbia model) developed at the Civil Engineering Department of the University, Vancouver, Canada, stream flow synthesis and Reservoir Regulation(SSARR) model of the U.S.Army Engineers, Sacramento model-of the Sacramento river forecast Centre, California, USA, TVA daily stream flow model of Tennessee Valley Authority, USDAHL-74 hydrologic model of U.S.Department of Agriculture, HBV-model of the U.S.Department of Agriculture, OBV"model of the-Swedish Meteorological and Hydrological Institute, USGS Peak flow synthesis model of U.S.Geological Survey, RORB(Version-3) model of Bionash University, Australia and Leavesley model of George,H.Leavesley of Colorado State University, USA. The component processes that have been considered in the review are interception, evapotranspiration, overland flow, infiltration, percolation, interflow and base flow.