Abstract: Throughout the years, Gaza Strip has been experiencing an increasing shortage of water, due to growing demands and its location in a semi-arid region with a small average rainfall of 300 mm/year. The main source of water in the Gaza Strip is the groundwater aquifer that is naturally recharged from rainfall. Over the time, continuous increase of population resulted in dramatic urban expansion, which has a direct influence in reducing groundwater recharge and increasing the surface run-off. To quantify the losses due to run-off, Geographical Information Systems (GIS) as a measurement tool, in addition to the soil conditions and metrological data for more than 20 years is used. The urbanized area represents around 16% in the year of 1998 and 21% in the year of 2005 and is expected to increase in the next years due to the rapid increase in population to represent 33% for the year of 2015. In the meantime, the water demand will increase due to the expansion of water supply systems. The total amount of rainwater losses due to urbanization as surface run-off is estimated 14.5 Mm3 in the year of 1998 and expected to increase to about 20, 35 Mm3, for the years of 2005, 2015, respectively. These will results in an increasing pressure on underground water resources, which has lead to an irreversible depletion of the aquifer. Mitigation measures, such as rainwater collection and areas for natural recharge should be protected.