UNIT-HYDROGRAPH DERIVED FROM A SPATIALLY DISTRIBUTED VELOCITY-FIELD

A unit hydrograph model is proposed in which the watershed is decompos ed into subareas which are individual cells or zones of neighbouring c ells. The unit hydrograph is found for each subarea and the response a t the outlet to excess rainfall on each subarea is summed to produce t he watershed runoff hydrograph. The cell to cell flow path to the wate rshed outlet is determined from a digital elevation model. A constant flow velocity is assigned to each cell and the time lag between subare a input and response at the watershed outlet is found by integrating t he flow time along the path from the subarea to the outlet. The respon se function for a subarea is modelled as a lagged linear reservoir in which the flow time is equal to the sum of a time of translation and a n average residence time in the reservoir. It is shown that the assump tion of a spatially varying, but time-invariant, velocity held underly ing this model produces a linear system model for all subareas whose o utputs can be summed in the manner indicated. An example application i s presented for the 8.70 km(2) Severn watershed at Plynlimon in Wales using a 50 m digital elevation model in which the cell velocity is cal culated by modifying an average watershed velocity according to the te rrain slope and the drainage area of each cell. The resulting model re asonably reproduces the observed unit hydrograph.