Evolution of runoff and groundwater recharge in the Gaza Strip over the last four decades

Gaza Strip is experiencing a severe water crisis caused mainly by overexploitation of the groundwater source. Rainfall is the only source of freshwater replenishment for the Gaza coastal aquifer. Groundwater level has dropped to more than 10 m below mean sea level as a result of aquifer exploitation and imbalance between recharge and abstraction in the past decades, which exposes the groundwater to seawater intrusion. As the only fresh water source recharging the aquifer is from rainfall, it is essential to estimate the annual recharge volume from rainfall. This paper is focusing on recharge estimation based on historical daily rainfall data for the past 41 years (1973–2014). An estimation of surface runoff was made using soil conservation services curve number method. An estimation of groundwater recharge has been calculated using two different methods: Thornthwaite and Mather soil moisture balance approach and chloride mass balance. Four land use maps and data have been used for the entire calculation period, three of them based on actual surveys carried out in 1994, 2004 and 2010, while the fourth map was developed based on the population expansion trend to cover the period before 1993. It was found that the proportion of built-up area has expanded from 8.25% in 1982 to 25.23% in 2010, while the sand dune area has shrunk from 31.46% in 1982 to 8.64% in 2010. Runoff has doubled from 6.9 million m³ in 1982 to 13.7 million m³ in 2010, while groundwater recharge was reduced from 24.4 million m³ in 1982 to 18.1 million m³ in 2010.