RN-20 : Rainfall-runoff relationships

Abstract

For estimating dependable yield while planning and designing water resources development projects, long period series of stream flow are required. However, since the flow data are generally available for only short periods, the stream flow series are extended using rainfall data through use of rainfall-runoff relationships.

Rainfall is the single important parameter which influences runoff directly as overland flow or indirectly as sub-surface and ground water. In the absence of direct measurements of runoff, regional rainfall-runoff relationships developed over a hydrologically homogeneous region based on long period rainfall and runoff data should be used for estimation of yields from ungauged catchments in the region.

Starting with simple techniques like runoff-coefficients and empirical formulae, the later day developments included use of graphical relationships such as simple rainfall-runoff plots, coaxial graphical techniques using antecedent precipitation index and statistical techniques such as bivariate and multivariate regression equations between rainfall and runoff.

The study and analysis of rainfall-runoff process has attained further sophistication with the introduction of high speed computers making it possible to model every aspect of the process in greater detail. In spite of the availability of conceptual models which could be run on medium and large computers, the simple rainfall-runoff relationships do have a role to play in the estimation of water yields from small and medium catchments because of the relative ease with which they could be used on micro computers. A review of simple rainfall runoff relationships has been undertaken covering empirical formulae, and graphical and statistical relationships. Some of the rainfall-runoff relationships used by the state and central water resources organisations for project planning and in research studies in India have also been reviewed.

The review has broadly indicated that except for a few, most of the rainfall-runoff relationships used only rainfall (current or previous time steps) for relating with runoff. To make the so called simple rainfall-runoff relationships more purposeful and reliable, it would be necessary to incorporate appropriate climatic and physiographic factors either directly or through their derivatives.