California’s 21st Century Megadrought

Jun 23, 2020 at 6:00pm


A recent paper on climate change in California and the West has been in the news and raising concerns. Based on extensive analysis of tree ring data—a good measure of summer soil moisture—the authors postulate that most of the region is in an unfolding “megadrought” that began in 2000 and is the second worst in the past 1,200 years.

What does this mean for California water management? If the state is in a megadrought, it means a great deal. We should plan accordingly.

Megadrought is a term of art, but essentially it refers to decades-long periods of low precipitation and soil moisture, often associated with reduced mountain snowpack. These periods are chronically dry, with less frequent wet years and drier dry years.

California’s climate signals suggest that the state is experiencing a megadrought along with much of the West, as evidenced by precipitation and, perhaps most important, temperature anomalies.

In the figure below we plot the departure from average statewide precipitation over the past 70 years (since our modern water supply system was put in place and high-resolution climate data was collected). Starting in 1999 the state entered a 22-year period (including this year) where dry years occurred three times more often than wet years. This is roughly double the frequency of dry years compared to records going back to the late 1800s.

The occurrence of significantly more dry years than wet years over the past two decades has created a cumulative precipitation “deficit” that is quite large—bigger than any 20-year period in the past 70 years (and comparable to the extended dry period that included the Dust Bowl).

But this alone does not make for a megadrought. The other key factor is temperature.

Warm years diminish snowpack, reducing an important source of spring runoff into reservoirs. Extended warm and dry periods lead to snow droughts. However, the big effect of warming may be on evaporative demand. Think of it as the “thirst of the atmosphere.”

California’s relatively dry atmosphere has a high evaporative demand year-round. On average, roughly two-thirds of precipitation that falls on the state is returned to the atmosphere through evaporation and transpiration by plants. During warm periods evaporative demand increases, reducing the proportion of precipitation that remains as soil moisture or becomes runoff.

Read more of the full original here.