Updated May 1, 2017
After more than five years of drought in California, water year 2017 has seen above-average precipitation and snowpack, inspiring many to ask, "is the drought over?" The answer to this question requires consideration of California's three primary sources of water: surface water, snowpack, and groundwater.
The long-term outlook for California’s drought can be better assessed in the months ahead. On April 1, 2017, the California Department of Water Resources conducted its annual snowpack measurement. This benchmark is important because it provides a comprehensive examination of the snowpack’s water content. Typically, April 1st marks the start of significant snowmelt, producing runoff that recharges reservoirs and groundwater systems slowly throughout the ensuing months. Unless there is excessive heat – which has been the case for the last three years – this slower runoff provides significant usable supply for the year, and can enhance the aquifer-recharge process. On April 1, 2017, DWR measured the snowpack to be 164 percent of average for the date, and determined the snowpack water content to be 183 percent of average. This measurement likely signifies that runoff in Water Year 2017 will be above average, particularly in comparison to recent drought years. However, the rate at which that runoff occurs is particularly sensitive to temperature. A rapid rise in temperatures may cause snowpack to melt too quickly, contributing to excessive runoff in a short period of time; which could lead to flooding. A slow, steady rise in spring and summer temperatures is ideal from a water supply perspective because the snowpack will melt slowly, allowing for optimal replenishment of aquifers and reservoirs.
When compared with historical, long-term data, analysis of surface-water runoff data now being collected by the USGS streamgage network will also help scientists better understand the effects that winter storms of 2017 have had upon California’s drought conditions. Runoff is an important component in maintaining healthy waterways and ecosystems and also contributes to groundwater replenishment through groundwater surface-water interactions. The USGS California Water Science Center – along with cooperating local, state, and federal agencies - continues to collect long-term data that are needed to assess the effects of climate variability on water resources.
On average, the Sierra Nevada snowpack supplies about 30 percent of California's water needs as it melts in the spring and summer. A series of back-to-back atmospheric river storms blanketed the Sierra Nevada in January and February 2017. As of May 1, 2017, statewide snow accumulation data indicate that snowpack in the Northern, Central, and Southern Sierra is 196 percent of normal for this date.
Precipitation in water year 2017 has filled the majority of California's major reservoirs to above-historic average levels. Likewise, as the USGS streamgage network shows, flows in the majority of the streams have been at or above average for most of the last 4 months. This indicates that most of California's rivers, creeks, lakes and reservoirs are in good condition.
Groundwater aquifers recover much more slowly than surface water and are limited, amoung other things, by how much and how fast water can recharge. Unlike surface water, which can recover during a few days of heavy precipitation, groundwater aquifer recovery often takes years or decades. Groundwater systems are also relied upon more heavily during times of drought. In addition, in many areas of the state, groundwater systems have been depleted for long periods - even between droughts - that they have not recovered from. Excessive, long-term groundwater over-use resulting in groundwater depletion can cause subsidence and permanent loss of groundwater storage as well as water quality degradation and seawater intrusion. These long-term impacts on groundwater have not been remedied by the recent weather. If recovery is possible, it will likely take several to many years to accomplish.