On The Relationship Between Runoff Coefficient And Catchment Initial Conditions

Abstract: The runoff coefficient, estimated as the ratio between quick flow and rainfall volume, on an event basis, has been analyzed in an empirical framework, as a function of the initial catchment state conditions prior to an event, such as pre-event soil moisture and pre-event base flow. The resulting relationships have been tested for their utility in runoff prediction, focusing on the variability of the catchment response, dependent on the initial condition itself. The variability of the catchment response is also addressed in a conceptually based stochastic model for stream flow simulation and forecasting. The watershed response to effective rainfall is considered as deriving from the response of linear reservoirs in parallel, representing contribution to stream flow from the fast (surface flow and shallow subsurface flow) and slow (deep subsurface flow and groundwater flow) components. The parameter that indicates how the hydrograph is split between these components, responsible for the non linearity in the rainfall-runoff transformation and for difficulties in the runoff ratio estimation, has been estimated. This shows a seasonal dependence and, generally, a dependence on the initial state, prior to an event