DSPBAR028

Fig. 13.8 Instantaneous Unit Hydrographs (IUH) used in the ReFH, Flood Studies Report (FSR) and Flood Estimation Handbook (FEH) rainfall-runoff methods (after Kjeldsen, 2007, Copyright NERC (CEH)).

Time/ Tp

Fig. 13.8 Instantaneous Unit Hydrographs (IUH) used in the ReFH, Flood Studies Report (FSR) and Flood Estimation Handbook (FEH) rainfall-runoff methods (after Kjeldsen, 2007, Copyright NERC (CEH)).

The constant baseflow value used in the FEH rainfall-runoff model has been replaced in the ReFH baseflow model, which allows variation through the event. The model is based on a linear reservoir concept, with inflow to the reservoir being recharge which is assumed to come from saturated land that is also producing runoff; elsewhere it is assumed that rainfall inputs are retained as soil moisture that will become evapotranspiration and longer term recharge. The ratio of runoff to recharge is controlled by the baseflow recharge parameter, BR. Baseflow lag (BL) controls the decay of base-flow from the linear reservoir, which is an exponential function in time (see equations 12.6). Both the baseflow parameters are best calculated by fitting recession curves to flow data. Otherwise they can be estimated from catchment descriptors, as follows:

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