Nearly every part of the state experienced at least average amounts of precipitation, with the amount of rainfall in many areas being significantly above average. In contrast to eight of the last ten water years being exceptionally dry. The water years between 2012 and 2015 set a record for the driest consecutive four-year period in California history.
Many of the hydrologic impacts of the extended period of dry conditions have been relieved considerably. Runoff in major river basins exceeded 150 percent of the average for the first time in six years. The abundant runoff helped to replenish depleted soil moisture and surface water storage in the majority of California’s major reservoirs.
The San Joaquin Valley Index that is based on runoff from the Stanislaus, Tuolumne, Merced, and San Joaquin Rivers, showed that runoff from the San Joaquin Basin was more the last five drought years combined, at 14.5 million acre-feet (MAF). The Sacramento Valley Index, which is comprised of runoff from the Sacramento, Feather, Yuba, and American Rivers, showed the Sacramento Basin at nearly double the amount of an average year at 36.9 MAF.
The considerable amount of rainfall in the water year 2017 allowed many parts of California access to excess surface waters for groundwater recharge. The fall 2017 groundwater data which will become available after the end of the calendar year is forecasted to illustrate the effects of the substantial recharge water available.
Great video on the years of Drought link