Temperature and Precipitation Trends in Lebanon’s Largest River: The Litani Basin

Abstract

This study deals with the historical hydro-climatological watershed characteristics of the Litani basin located in Lebanon, east of the Mediterranean Sea, during 1900–2008. The basin is divided into two distinct sub-basins. The Upper Litani Basin (ULB) is mostly dominated by mountainous geophysical features and the Lower Litani Basin (LLB) is closer to the Mediterranean coast. Monthly and annual temperature and precipitation data were generated from different sources, including global gridded data, satellite data, and local station data. Because long-term runoff records were not available, previously generated monthly synthetic runoff data were employed. Temperature and precipitation trends were investigated using the Mann-Kendall and Sen Slope nonparametric trend tests. The mean annual and monthly runoff were subsequently correlated with temperature and precipitation variations using a multiple linear regression approach. The results show that between 1900 and 2008, the whole Litani basin, including both sub-basins, experienced a drying trend without a significant change in temperature. However, within the period of 1970–2008, the whole basin grew notably warmer in all seasons, without being wetter; this climatic pattern was reflected in the LLB but not in the ULB, which was only slightly warmer, but also slightly wetter in this period. In addition, the results revealed distinctive seasonal and annual correlations between temperature and precipitation changes and the basin’s runoff. However, runoff for both the ULB and the LLB was shown to more frequently correlate with precipitation than with temperature. Positive correlations were found between runoff and precipitation during winter and wet seasons, and a negative correlation was detected during winter between temperature and runoff variations of the ULB. Hence, the continuation of increasing temperature and decreasing precipitation trends may pose a threat to future water resources in the Litani as a whole, and the ULB in particular.