Application of a unit hydrograph based on subwatershed division and comparison with Nashʹs instantaneous unit hydrograph

The Unit Hydrograph (UH) technique is widely used for runoff estimation, especially for determining peak discharges. In this paper, a geomorphologically based UH has been applied. Its most remarkable characteristic is that it includes the watershed structure in its formulation. This is defined from the drainage network, each subwatershed being considered as a linear reservoir. In this method, the fact of considering this reservoir sequence according to the drainage network leads to the formulation of the model only depending on a single parameter. The Geomorphological Unit Hydrograph of Reservoirs (GUHR), proposed in this paper, is compared with Nashʹs Instantaneous Unit Hydrograph (Nashʹs IUH), by applying the two methods to the Aixola watershed. This 4.7-km2 watershed is located in Gipuzkoa (Northern Spain). It is characteristic of the headwaters watersheds of the area. Most of them are forested and have steep slopes. Annual rainfall is over 1500 mm and many intense rainfall events are observed, among which 18 were selected for this study. Both UH techniques were applied to the rainstorms. The resulting hydrographs were plotted against registered data and the best fits were determined. According to these results, the GUHR model behaved similarly to Nashʹs IUH. However, the GUHR method seemed able to reproduce a wider range of rainstorms than Nashʹs IUH. While analyzing the UHs calculated, seasonal behavior was observed in runoff generation, and different average UHs were proposed for two different periods. This variability was also observed in values adopted by the GUHR model parameter, providing some information about the watershed time response. The dynamic character of the only uncertain parameter, and the model formulation, in which the watershed morphology is reflected, together with the modelʹs simplicity, leads us to consider GUHR as being a promising UH model.