The district may establish an annual reserve fund in an amount not to exceed ten million dollars ($10,000,000) commencing with the 2000–01 fiscal year. The maximum allowable reserve fund may be adjusted annually commencing with the 2001–02 fiscal year to reflect percentage increases or decreases in the blended cost of water from district supply sources. Beginning in the 2019–20 fiscal year, a minimum of 80 percent of the reserve shall be for water purchases.