Augmented hydrograph hypothesis: discussion of principles

Traditional catchment models split the rainfall-runoff process into two major components, namely volume reduction and shape transformation, assume that these two components are independent of each other, and apply them to the input in the order volume reduction followed by shape transformation. When an example of such a model, composed of three subsystems, namely runoff ratio, time area diagram and non-linear reservoir, was applied to 66 isolated storm events from the River Ray catchment a relationship was found between the size of the non-linear reservoir and the value of the runoff ratio, thus making the two major components of the model no longer independent of each other. In this report a new model is proposed which supports the idea of splitting the rainfall-runoff relation into two major independent components, but challenges the traditional order in which they are applied. The new model contains the same three subsystems but rearranged so that the non-linear reservoir precedes the runoff ratio, instead of following it. In this new model the gross rainfall hyetograph is applied first to the shape transformation component rather than to that of the volume reduction, giving as intermediate output from the model a hydrograph, which is termed the augmented hydrograph. This augmented hydrograph provides an alternative concept to that of the net rainfall hyetograph, which is the intermediate output obtained from traditional models.