The water supply of La Paz is threatened by the impact of climate change through the melting of glaciers. The city receives its drinking water from three water systems: El Alto, Achachicala and Pampahasi. La Paz shares the first and largest of these systems with its sister city El Alto. All three systems are fed by glaciers and rivers in the Cordillera mountain range. 20-28 % of its water is fed by glaciers, the remainder coming from rainfall and snowmelt. The glaciers recede as a result of climate change, initially increasing water availability during the dry season, but ultimately threatening a substantial decrease in dry season run-off when they completely disappear. A small glacier, the Chacaltaya near El Alto, already disappeared in 2008. The El Alto system receives its water from the Tuni Dam and two water channels. These channels divert water that flows from the Zongo Glacier on the slopes of Huayna Potosi and from Condoriri North of El Alto. The 2.9 km long Zongo glacier retreats at a rate of about 18 meters per year. The Tuni and Condoriri glaciers have lost 39% of their area between 1983 and 2006. According to a study by the Stockholm Environment Institute (SEI), the El Alto system is the least resilient against the impact of climate change among the three systems. The study says that reducing water distribution losses is the most effective short-term strategy to deal with water scarcity. New water sources further to the North in the Cordillera include the Khara Kota and Taypicacha, but they are expensive to develop and their water supply is also affected by glacier melt.