Recent Changes in the Sierra Snowpack of California

The mountain snowpack is an important element of California’s water supply. Winter precipitation in the
high mountains is held over in the form of snow as natural water storage for the spring and summer dry season when
irrigation demands are high. Historically, snowmelt furnished about 30 percent of the water supply for irrigated and
urban users. In a warmer climate, less snow is expected and accumulation would be more limited to the higher
portion of mountain watersheds.
In a paper 5 years ago at the Western Snow Conference (Roos and Sahota, 2012), contrasting trends were
described. For a northern Sierra group of snow courses, a decline in April 1 measured water content was noted;
however, for another group of southern Sierra courses, a small increasing trend in water content was noted. In both
north and south, there was a decreasing trend in the volume of April through July runoff (mostly snowmelt)
compared to total water year nature runoff. Now, after the drought, a recheck and update of these charts show that
the southern Sierra snowpack also shows a decreasing trend, although not as much as in the north. One possible
reason for the original trend is that the extensive cloud seeding programs in the southern Sierra temporarily offset an
overall decreasing trend caused by warmer temperatures in the past two or three decades. As warming in the 21st
century has increased, the changes have more than offset the small incremental increase by the weather modification
programs.