Trends in Precipitation and April 1 Snow Water Content in California

Drought, severe drought, extreme drought, atmospheric rivers, more atmospheric rivers, floods, snow pack, full reservoirs, soggy soils, infrastructure damage, full allocations – all words and phrases permeating our most recent California water conversations. If anything, these words, all used during the 2016-17 winter season, show how quickly California’s precipitation regime staggers and lurches from dry to wet and back again. Despite the high annual variability, an analysis of long term precipitation records and snowpack observations show diverging trends. Precipitation, as indicated by the Northern Sierra Precipitation Index, shows a steady increasing trend over the past 70 years. Conversely, the April 1, snow water content in the Northern Sierra snowpack has experienced a significant decline over the same time period. Overall, the average precipitation trend shows an increase of about 0.10 inches per year whereas the April 1 snow water content has declined by 0.05 inches/year. The April 1 snow water content is particularly pronounced in the 5,000 – 7,000 ft. elevation range in the Northern Sierra. These results suggest that the snow water content decline has completely overcome the increased precipitation and experienced further losses in April 1 snow water content. The decreasing April 1 snow water content has far reaching impacts on reservoir operations and water management in California, particularly if the precipitation trend reverses direction under a future warming climate.