Characterization of selected rainfall-runoff events in Lebanon and comparison to other Mediterranean catchments

This work analyzes the hydrological response characteristics at the event scale of catchments in Lebanon (eastern Mediterranean); and discusses the impact of climatic conditions and physiographic characteristics on the catchment response. Furthermore, the Lebanese catchments are compared throughout this work to catchments in other parts of the Mediterranean. The comparison was used not only to learn from similarities and differences but also to compensate the lack of high-resolution data. Hydro-meteorological data relative to 17 rainfall-runoff events (22 drainage basins ranging in area from 28.6 to 1432 km2) are collected and analysed across the country. Similar data are gathered from the literature for 137 events (137 drainage basins ranging in area from 3.8 to 4915 km2) all across the Mediterranean divided into the northwestern (NWM), eastern (EM) and southern Mediterranean (SM). All the chosen events are of at least a 10 years return period. The gathered information consist of the physiographic and climatic characteristics of the affected catchments, the date, duration and rainfall depth of the event, and the catchment response in term of peak flow, runoff depth and ratio. The analysis shows that the majority of the Lebanese catchments are mountainous with high relief ratio and drainage density which favors the rapid concentration of runoff along flow lines. Although under Mediterranean climate these catchments do not exhibit water-limited conditions, in fact annual runoff ratio/aridity index values are higher/lower than recorded in other EM catchments. Estimation of the unit peak flows are in the range of those recorded in other EM catchments however, events runoff ratios (mean = 0.39) are more in the range of the NWM catchments (mean runoff ratios are 0.4 for NWM, 0.35 for SM and only 0.12 in EM),. This could be attributed to the karstic nature of the Lebanese terrains. Finally, antecedent soil moisture conditions appear to have a major impact on the catchment response especially in term of event runoff ratio. This work shed some lights on the hydrological response characteristics of poorly-gauged Lebanese catchments at the event scale. Nevertheless, it does not omit the need for higher resolution data that permits more in-depth investigation of the mechanism that trigger Lebanese catchment response.