The reliable flood forecasting is essential for the reservoir operation to prevent flood damage and meet water demands.
The unit hydrograph is a traditional means of representing the linear system response at the watershed outlet to rainfall over the watershed, but it suffers from the limitation that the response function is lumped over the whole watershed and does not explicitly account for the spatially distributed nature of watershed properties.
The instantaneous unit hydrograph(IUH) derived from spatial distribution of the flow time produce the hydrograph at the outlet by subdividing the basin into segments as cells, assigning a representative velocity to each cell and determining the flow time from each cell to the outlet.
This is theoretically feasible and produces a unit hydrograph which fairly closely matches the observed hydrograph, but its applications can be recommended for small basins (less than 500 km2)
If it is applied for large basins greater than 500 km2, it should be combined with channel flow model while subdividing the study area into smaller catchments.
The approach proposed here in which the watershed is partitioned into overland and channel, produces flood hydrograph of the outlet by applying the IUH derived from spatial distribution of the flow time for overland flow, kinematic wave model for channel flow.
The kinematic wave model for channel flow can be solved by using implicit scheme. And the lateral inflow per unit length of river can be computed by using IUH derived from spatial distribution of the flow time.
The case study for applying the proposed method demonstrates that if rain gauge information is available in real time, the error in the peak flow is less than 12% and the error in the peak time is less than two hours.
The proposed approach has been applied in the operation of cascade reservoirs on Chongchon River and can be recommended for flood forecasting in large scale watersheds.
