In the Middle East, water is scarce and population growth causes a rapid rise of urban centers. Since many towns of the Palestinian Authority (PA) suffer from water shortage, the use of rainwater harvesting (RWH) as an alternative to conventional water supply has gained increasing interest among water resources planners. This study quantifies actual volumes of urban RWH to be expected from highly variable Mediterranean rainfall. A one-parameter model uses measured potential evaporation and high resolution rainfall data as input to calculate RWH volumes from rooftops inside Ramallah, a traditional Arab town. While during average seasons a 87% runoff harvest (480 from 550 mm of rainfall) can be expected, this value decreases to about 75% (190 from 250 mm of rainfall) during drought seasons. A survey comprising more than 500 questionnaires suggests that approximately 40% of the houses are equipped with RWH systems from which one third are out of use. Although water quality is perceived to be favourable, only 3% of the active RWH systems are actually used for drinking and only 18% for domestic purposes. All active RWH systems investigated may harvest approximately 16 × 10 ~3 m ~3 of rooftop runoff during an average season and 6 × 10 ~3 m ~3 during droughts. When these numbers are extrapolated to all houses in Ramallah, theoretical maximum potentials increase to approximately 298 × 10 ~3 m ~3 during average seasons and 118 × 10 ~3 m ~3 during droughts. A part of this potential can easily be exhausted by rehabilitation of installed RWH systems. The use of RWH for emergency water supply should be advocated, although care is needed because of hygienic risks. Regional estimates for the entire Lower Jordan River Basin yielded RWH potentials of 20 × 10 ~6 m ~3 during the average season 2002/2003 but only 3 × 10 ~6 m ~3 during the drought season 1998/1999. Thus, urban RWH is a relatively small contribution to overcome water scarcity in the region and decreases significantly during droughts. Yet it is a sustainable water resource, which is available on spot for everybody. Due to population growth and ongoing urbanization it will be more important in future.
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