UBC Theses and Dissertations
Analysis of the response and the rainfall distribution in a mountainous watershed Loukas, Athanasios
The hydrologic response of a small, steep and forested watershed is investigated with the use of a simplified watershed model. The watershed model utilises the linear storage routing technique and treats the runoff as fast and slow. The separation of the rainfall input to each of the components of runoff is achieved with the use of a non-linear exponential equation (Eq. 3.1). Hourly rainfall data averaged from five raingauges and hourly streamflow are used as input to the model. The model is found to perform well. Analysis of the variation of model parameters and the resulting linear form of the rainfall separation equation (Eq. 5.1), after the application of the model, indicates that the hydrologic response of the watersed is reasonably linear except for intense summer rainstorms under dry soil conditions. The linearity of the response of the watershed can be attributed to the runoff generation mechanism which is the subsurface pipeflow. Studies of the dynamics of pipeflow are reviewed and validated for the study watershed and it is shown that the parameters of the model take values that are consistent with their physical representation. Reviewing the literature, it has been recognized that the rainfall distribution with elevation is different from linear for mountainous areas with humid climate. Analysis of the rainfall distribution in the study watershed reveals that the linear increase of precipitation with elevation is not valid for rainfall. The rainfall depth per event increases up to the mid-elevation and then decreases at the upper elevations. Furthermore, the hourly intensity decreases with height and the two dominant hillslopes do not receive the same amount of rain. This variation in rainfall is explained by meteorological precesses and the physiographic features of the watershed. The performance of the model is highly affected by the accuracy and representativeness of the rainfall data. For example if weighted averages of several precipitation stations are used, the model performs well. However, if only a single point rainfall is used, the model performance becomes less good. As a result, unless the rainfall and runoff are known with great precision, it is difficult to make an accurate determination of the storage characteristics of the basin, and consequently it is argued that it may not be possible to distinguish between the accuracy of different routing models.
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