A SIMPLE NONLINEAR RAINFALL-RUNOFF MODEL WITH A VARIABLE GAIN FACTOR

A simple non-linear rainfall-runoff model, incorporating a variable ga in factor dependent on the prevailing soil moisture state of the catch ment, is developed. The simulation mode output of an auxilliary simple linear model is used as an index of the soil moisture state and simpl e and realistic functional relationships between the gain factor and t he prevailing catchment wetness index are assumed. The resulting model , referred to as the variable gain factor model (VGFM), is tested on t he data of five catchments, assuming two different functional forms of gain factor variation with the catchment wetness index. The results i ndicate significant improvement over the performance of the simple lin ear model for the test catchments. The operation of the model is shown to be equivalent to a special form of the non-linear Volterra model.