Modelling stream recession flows

The curvature in semilog plots of hydrograph recessions has been explained previously by the assumption of a power relationship between groundwater storage and its outflow to the stream, with no recharge occurring during the recession period. The current work uses an alternative hypothesis of a linear groundwater system with a continuing inflow from the vadose zone. This leads to the development of recession equations with time-varying inputs of various forms. Comparison of these and the ‘no recharge’ models, using data from 22 Australian benchmark catchments, leads to the conclusion that significant recharge does continue through recession periods, and should be accounted for in conceptual models of the rainfall-runoff process. While the initial value of the recharge is closely related to the stream flow, the time constants for all the models vary widely between events, which led to the development of master recession curves. The recession equation of the IHACRES model, which takes the form of the sum of two exponential functions, was found to provide a very good fit to the data. Evaporation and seepage losses can be incorporated in the recession equation, and the magnitude of the losses can be quantified.