Hydrology of Lebanese catchments in the Mediterranean context

Lebanon is a small mountainous country with a typical Mediterranean climate and a high spatial variability of precipitation with a substantial amount occurring as snow. Moreover, the majority of Lebanese terrains are karstic. It is a heavily urbanized country with increasing anthropogenic pressure on water resources. Furthermore, the Lebanese catchments are poorly-gauged due mainly to a large gap of data (1975 - 2000) caused by the civil war (1975 - 1990). However, previous studies on regionalization suggested that across Lebanon when physical characteristics are not changing, hydrologic parameters shows a remarkable degree of spatial coherence. Thus, with an integration of more physical and functional characteristics, a regionalization procedure could be considered as a framework for defining the physical and hydrological characteristics of the Lebanese catchments. Consequently, this work aims to understand the hydrological response characteristics of Lebanese catchments in the Mediterranean context and to classify these catchments according to their physical and hydrological properties. It is structured into three parts: (i) a review on Mediterranean catchments hydrology, (ii) a datal analysis of the Lebanese catchments and comparison to the Mediterranean context, and finally (iii) a classification and modeling of the Lebanese catchments. The first part reviews 152 hydrological studies conducted in the Mediterranean region at various time scales (paper accepted in Hydrological Sciences Journal). This study also compares methods and modeling approaches used for individual-catchment or regionalization studies. The study area is divided into the northwestern (NWM), eastern (EM) and southern (SM) Mediterranean. The analysis indicates regional discrepancies in which the NWM shows the most extreme rainfall regime. A tendency for reduced water resources driven by both anthropogenic and climatic pressures and a more extreme rainfall regime are also noticeable. Catchments show very heterogeneous responses over time and space, resulting in limitations in hydrological modelling and large uncertainties in predictions. However, few models have been developed to address these issues. Regionalization studies are scarce and inconsistent in the Mediterranean. Additional studies are necessary to improve the knowledge of Mediterranean hydrological features and to account for regional specificities. In the second part, an inventory of the available spatial and temporal data was carried out and followed by a detailed data analysis of twenty eight Lebanese catchments through extracting the physical and hydrological response characteristics for the period 2001 – 2011. The spatial data concerns the morphometry, drainage system, geology, karst, soils and land cover. The temporal data concerns the precipitation (32 stations at a daily –when available- and monthly time step), evapotranspiration (remote sensing data at a monthly time step) and discharge data (24 gauging stations at a daily time step and 4 at a monthly time step). Gathering the available temporal data was a real challenge since these data are not always available for the same period and precipitation data is expensive. The 2001 - 2011 temporal data was analyzed and compared with a database from the pre-war period. Afterwards, the hydrological responses of the Lebanese catchments are compared with other Mediterranean catchments at different time scales. At the annual water balance level, runoff ratio values are found to be high across the country. At the event scale, the amount of rainfall in a given event represents a substantial amount of the total annual rainfall. Moreover, unit peak flow [maximum daily discharge] decreases with the catchment area and is not correlated with the rainfall depth of an event. Event runoff ratio is high; in fact it is much higher than values recorded in the EM and is the range of the NWM catchments. The third part concludes with a new clustering of the Lebanese catchments according to their physical and hydrological characteristics. The variables defined from the data analysis were used for the classification. Three classifications were carried out using catchments physical descriptors and runoff signatures separately. Catchments holding simultaneously the same physical and hydrological similarities were grouped together forming five “physically and hydrologically similar” catchments’ classes. Finally, a simple modeling approach at a monthly time step was tested using GR2M model. Different regionalization approaches were also compared. The discussion focuses on the analysis of the hydrological functioning of each basin and each class of basins.