RN-17 : Regional unit hydrograph

Abstract

The estimation of runoff from the watershed is needed for comprehensive water resources planning, flood flow forecast, adequate design of hydraulic structures etc. The climatic and physical characteristics of the watershed are the main factors affecting runoff. The climatic factors include nature of precipitation, evapotranspiration and interception, rainfall intensity, duration of rainfall, areal and temporal distribution of rainfall and direction of storm movement. The primary physical characteristics of the watershed which influence runoff are its area, length, shape, elevation, slope, orientation, soil type, drainage or channel system, water storage capability and vegetal cover etc.

Unit hydrograph is one of the most popular simple technique for the computation of runoff from the watershed. It is characteristics for a given watershed and it represents the integrated effect of various physical features on the routing of the rainfall input through the catchment system. The unit hydrograph for gauged catchments can be derived by analysing the available rainfall-runoff data. However, for many small catchments the stream flow data are limited and for ungauged catchments it is not at all available. Therefore, the unit hydrograph for such catchments can only be derived using their physical and storm characteristics. This necessitates the development of suitable regional relationship for unit hydrograph derivation. The procedure used for this purpose involves the derivation of the parameters that describe the unit hydrograph for gauged catchments and then the development of the regional relationships between the unit hydrograph parameters with pertinent physiographic and storm characteristics of the catchments. The catchments considered for such regional study have to be similar in hydrological and meteorological characteristics.

In this report, the basic steps involved for developing such regional unit hydrograph relationships are described in detail. Various regional unit hydrograph studies conducted in India as well as abroad have also been reviewed. In general, multiple linear regression analysis have been used by many investigators for developing the regional unit hydrograph relationships. Conceptual models have also been used by many investigators abroad for developing the regional unit hydrograph relationships. However, in India very little work has been done for developing such relationships using the conceptual models. Some investigators have studied the effects of forest, agricultural practices and urbanisation on the shape of the unit hydrograph. There is need for similar studies for Indian basins.