Precipitation at Hetch Hetchy – 2018 A new water year (WY) starts every October. The graph charts cumulative precipitation at Hetch Hetchy Reservoir as the WY progresses. Precipitation is shown as a percentage of average, and curves for the current year and past year are shown. Cumulative precipitation curves for both dry and wet years are also shown, as well as a median. Why 1977? – It is the driest year on record. 2 Why 1983? – It is the wettest year on record.