Impact of climate change on the flow of El Kalb River and water demand of Beirut

Abstract

The general circulation models (GCMs) projections of the Intergovernmental Panel on Climate Change (IPCC) indicate an increasing temperature and a drying trend in the Middle East and North African (MENA) region by the end of the 21st century. Accordingly, Lebanon’s water resources are expected to be heavily affected, and concomitantly, urban water demand is expected to increase. In this context, this thesis focuses on the impact of climate change on water demand in Greater Beirut Area (GBA) and the water flow in El Kalb basin – which feeds 70% of GBA population. This study is the first to: (1) simulate the impact of climatic changes on El Kalb basin using a physically-based model, and (2) model the anticipated changes in water demand in GBA under the forecasted changes in weather parameters. Part 1 of this thesis used the Soil and Water Assessment Tool (SWAT) to simulate the impact of climate change on the streamflow of El Kalb river, a major perennial river in Lebanon. The model performance was tested for monthly flow at two stations (F.1 and F.2) under a nine-year calibration period (2003 – 2011) and a four-year validation period (2012 – 2015). By comparing statistical parameters, the model results indicated satisfactory precision in fitting observed and simulated output. During the calibration and validation periods at station F.1, the calculated Nash-Sutcliffe Efficiency (NSE) values were found to be 0.57 and 0.82; and the percent bias values (PBIAS) were found to be -0.06 and -2.81, respectively. At station F.2, NSE values of 0.78 and 0.75 and PBIAS values of -8.3 and 25.6 were obtained during the calibration and validation periods, respectively. Future projections of climate change were obtained from the Coordinated Regional Downscaling Experiment (CORDEX) project under the conditions of low, intermediate and high XIII emissions of greenhouse gases (GHGs) manifested in three Representative Concentration Pathways (RCP) (2.6, 4.5 and 8.5). The model indicates that the average annual discharge of El Kalb river in the near future (2021 – 2040), will decrease by around 29 %, 28 %, and 30 % under RCP 2.6, RCP 4.5, and RCP 8.5 respectively; while, around the end of the century (2081 – 2100), it will decrease by 24 %, 28 %, and 45% under RCP 2.6, RCP 4.5, and RCP 8.5 respectively. Greater Beirut Area (GBA) is expected to have a service area population increase of 45.83% reaching 3.5 million inhabitants by 2035. As such, water resources management in the GBA should become a priority to address multiple challenges including climate change. However, research modeling the impact of climatic changes on water demand in Lebanon is still very limited. Part 2 of this study tested the impact of seven weather parameters on water demand. The most statistically significant models (R2>0.5) relate to atmospheric pressure and temperature (average, minimum, and maximum). Those were used to forecast future water demand under RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5. The results showed an increase of 32 % in water demand for every 1oC increase in average temperature. Considering a future demand of 240 million cubic meters anticipated by 2035, a water deficit in GBA of 47.5 % is expected. In terms of monthly pattern, while June is projected to experience the greatest increase in water demand, August is dominantly the month with maximum demand by 2035. End of century (2080 – 2099) simulations project a maximum increase in demand under RCP 8.5.