Chart: Western Snowpack April 1 Snow Water Equivalent, 1981 - 2017
Methodology: Western snowpack data are from U.S. Department of Agriculture Western U.S. SNOTEL network. Values are expressed as a percentage above or below the median normal (1981-2010) snow water equivalent for the major western river basins in the SNOTEL network on March 31. California runoff estimates from the U.S. Geological Survey California Water Science Center.
The start of April marks the end of the winter wet season in the western U.S., the time of year when the region gets most of its precipitation. Measuring the snowpack at this time indicates how much water will melt from the snowpack during the hot and dry summer months that follow, making it a critical resource for agricultural,residential, commercial, and recreational users.
For only the second time this decade, the last being in 2011, the snowpack at the start of April is above normal, providing a respite from a drought which had been especially bad in California. In the Sierra Nevada, the snowpack was 164 percent of normal at the start of the month. The snow water equivalent, or the amount of liquid water locked away in the snow, was about 46 inches.
This year's barrage of western storms and the increase in meltwater has already sped up runoff into California's reservoirs, the majority of which are above their historical averages, although not by much. Only the San Luis Reservoir in central California is within a few percent of capacity, but even so, that reservoir is twice as high as it was at this time last year.
While this year's snowpack is fairly healthy, in a world warming from increasing greenhouse gases, the early spring snowpack has been declining over the long run. And snowpack isn't the only important water source: deeper groundwater, which is also an component of the region's water supply, takes much longer than a single wet season to replenish.
Although the western U.S. has a long history of droughts and wet periods,the droughts have been getting worse over the last century, meaning that one beneficially wet year is not a cure-all for the long- term water supply for the West.