The climate in Israel is notoriously arid but when the time finally comes for rain, like buses on Jaffa Road, it all comes at once. This winter has been no exception. Heavy storms over the past few weeks produced as much as 11 mm. of rainfall in Jerusalem in one hour alone, while flash floods near the Dead Sea claimed the life of an American hiker at the end of February. One would expect that in a country with a chronic water shortage, any rainfall would be welcome, but that is not always the case, according to experts. Although sorely needed, the recent heavy downpours have been a mixed blessing. "From a hydrological point of view, light, moderate rain is best," explains Yossi Guttman, chief hydrologist for Mekorot, the national water company, which supplies 80 percent of the country's drinking water. "Heavy rain becomes mostly surface water, but we are looking for moderate rain, like in England, because it percolates more slowly into the aquifer," he says. The fate of falling rain depends on a number of factors, including the intensity of the rainfall and the terrain on which it lands, says Amir Perez, from the Environmental Protection Ministry. Some evaporates, some percolates through the soil which eventually boosts the groundwater supply (aquifer), while intense rainfall typically runs off the ground and travels to rivers and the sea. "If we have all of our rain in one day, most of it becomes run-off and goes to the sea. But if it is spread over 10 days, then it slowly percolates down to the groundwater. Why do we have floods in the Negev desert? It's because the soil in the desert creates a crust when the rain hits it and the water immediately becomes run-off and creates floods," explains Perez. Jerusalem's unique topography, surrounded by hills and situated at an altitude much higher than Israel's coastal plain, creates challenges for bringing the water supply to the city's residents. "Since Jerusalem is sitting on top of a hill, the water is not collected, it just runs down," says Micha Blum, deputy director of wastewater and drainage at Gihon, the municipal water company. "Jerusalem is on a watershed which has five river basins, or wadis. Rain is drained by the drainage system and then goes down to the wadis which take the water west to the Mediterranean or east to the Dead Sea," explains Blum. The wadis running eastwards are Arugot, Kidron and Og, while water travels westwards to the coast via Nahal Refaim and Nahal Sorek. The latter is partly fed by the large reservoir in Beit Zayit, which collects rain as well as water from Nahal Halilim, Motza Spring, Halilim Spring and other streams that fill up with the winter rains. The reservoir's stunning location, encircled by green hills and the nearby Jerusalem Forest, makes it a popular recreation site. The water captured in the reservoir recharges the aquifer and, when it is full, spills over the dam and flows into Nahal Sorek, which eventually discharges itself into the Mediterranean Sea at Palmahim. The reservoir is currently almost full, but Guttman says that it is too early to know whether it will overflow this year because it takes time for the water to flow in. He predicts that local springs with a small recharge area will rise this summer and some small villages, including Palestinian ones, will be able use the springs for domestic use. Nevertheless, it is unlikely that much of the recent rain will actually be used for drinking water and, in general, only about 35% of rain recharges the aquifer while the rest evaporates or is lost through the wadis, says Guttman. In Jerusalem, most residents get their drinking water from the National Water Carrier. But Guttman says it would be better if Jerusalemites could get more of their water supply locally. "It is much cheaper to pump water from Beit Zayit or Ein Kerem than to use the energy needed to elevate the water [from elsewhere]. Bringing water from the National Water Carrier, it has to be lifted from zero meters to an altitude of 800 meters. Ein Kerem is at around 450 meters, so that would save a lot of energy," he explains. "Theoretically, we don't use all of the potential water around Jerusalem," Guttman adds, citing the difficulty in finding appropriate places to drill boreholes as the key obstacle. "There are only four boreholes still operating [in the Jerusalem area] and we're looking for new places all the time. But it is very difficult to locate new boreholes because of the infrastructure needed, the existence of dried-up or contaminated aquifers, as well as disruption to the local population." According to the Society for the Protection of Nature in Israel, much more can be done to improve local water conservation in the Jerusalem area, such as using the 11,000 cisterns that once provided water for tens of thousands of people in the city. "Considering Jerusalem is in the desert, it's an excellent idea to have more local water collection. Homes in Israel already have solar water heaters which shows that we can adapt," says Michelle Levine, of the SPNI. "Cisterns can provide drinking water that is at least as good as tap water and could easily be used for other purposes like linking up to washing machines, which would make homes more environmentally efficient. Because tools like these are not being implemented, it is resulting in the draining of the Kinneret and the Dead Sea. "We need sustainable practice for Israel's water policy and the organization responsible for that is the Water Authority," adds Levine. "They should revise their policy so that specific amounts of water are allocated to various uses, so they don't just drain as much as they want without taking into account nature, rivers and the Dead Sea." Whether the city's drinking water comes from the Kinneret, rainfall or plastic bottles, Jerusalemites can look forward to drier weather this month. January and February are Jerusalem's wettest months, according to the Israel Meteorological Service. Average monthly rainfall in the city is 133 mm. and 118 mm. in January and February respectively, when Jerusalemites can expect to use their umbrellas on at least one in every three days. March signals the imminent arrival of drier weather, with around 20% less precipitation on average than February.