Along-Stream Flood Estimation in Ungauged Locations in the Po River Basin

This thesis examines the performance of the Along-Stream approach in the prediction of flood statistics in ungauged basins and compares it with a regional approach officially in use in the Po river district in Italy. This method, introduced by Ganora (2013), is here used for a larger dataset of 175 basins with three additional distance definitions. Distance definitions based on basin area, Euclidean distance between the basins, basin mean elevation and the combination of basin area and mean elevation are investigated. The study attempts to assess how different distance-threshold levels change the uncertainty of the estimates propagated and derived compared to the regional model. It is shown through the analysis that smaller distance limits, for example those based on the basin area ratio, result in much lower errors of propagated estimates when compared to the regional model, which, in turn, indicates that the predictions may be more accurate if confined to smaller distance limits. Increasing the distance threshold continues to increase the uncertainty of propagated estimates, thus indicating a trade-off between expanding the applicability domains while retaining prediction accuracy. The analysis locates D_lim= 1.6 (corresponding the basin area ratio of 4.95) for the area-based definition, D_lim= 1.3 (corresponding to the distance of 3.67 km) for Euclidean distance, and D_lim= 1.72 (corresponding to the basin area and elevation of 5.58) on the combined definition of the elevation-area ratio as the optimal thresholds. It is concluded from the analysis that the mean elevation may be unrepresentative of the basins and is not included in the optimum distance determination. At these limits, a wider percentage of basins (up to 30%, 25%, and 10% for area ratio, area-elevation multiplication and Euclidean distance, respectively) have lower RMSE for propagated estimates than the "regional" model. Thus, these thresholds provide a good balance between the applicability of the models and prediction uncertainty, hence making the Along-Stream approach a proper method for discharge prediction in ungauged basins along with regional models.