Lake Chala's average annual rainfall is about .: The lake surface has an average annual evaporation of near . Approximately 80 percent of the lake's inflow comes from groundwater, which is derived mostly from rainfall in the montane forest zone of Mount Kilimanjaro at an elevation of . It takes about 3 months for groundwater to reach the lake.: The groundwater flowed into the lake at an estimated annual volume of from 1964 through 1977.