Planning Beyond California's Three-Year Drought - A 2015 Hydroelectric Planning Perspective

Following three dry years, precipitation for the month of December 2014 was 131% of the Sierra Region’s 120-year December average; however October-December 2014 as a 3-month period continued to remain below average. On December 31, 2014, precipitation for the California Department of Water Resources (DWR) northern California 8-station index was 129% of its historical 30-year average. However DWR’s southern California 5- station index was only 70% of its 30-year historical average. Precipitation was much above average in northern Sierra and drier than average for the southern Sierra. In spite of this unbalanced situation statewide, the overall increase in precipitation appeared to many as a dramatic turnaround from one of the most severe droughts in California’s recorded history. However, in spite of this dramatic precipitation recovery, California’s snowpack at the end of December statewide was only 49% of average, thus remaining much below average, mostly due to the higher snowline accompanying the relatively warm December storms. For Pacific Gas & Electric Company which has historically produced about 38 percent of its conventional hydroelectric energy from the aquifer outflow of large volcanic springs in northern California and approximately 37% of its conventional hydroelectric energy from the snowpack, the fall rains after satisfying an unusually large soil moisture deficit made little difference in the outflow rates and added only meager amounts of recharge to northern California’s porous volcanic aquifer storage which had lost approximately four million acre feet from underground storage since 1999. The 2014 calendar year was highly unusual beyond its single characteristic of being dry. Minimum daily temperatures for the 12-month period for the Sierra Region were the warmest in 120 years of record. This additional heating resulted in some of the driest soils moisture conditions for California in the past 1200 years. While rain dominated precipitation caused flood damage in December for parts of California, river flows quickly declined following the December rains. Aquifer outflows in northern California continued to remain much below normal following December’s wetness. As of January 1, 2015, precipitation for the remainder of the water year would need to increase to about 135% of average to get 100% of average annual conventional hydro generation.

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