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California, Nevada missing about two years of normal precipitation

Merdies Hayes | 9/24/2021, 6 a.m.
The latest reports from the U.S. Drought Monitor reveal that over the last six to 10 weeks drought...

The latest reports from the U.S. Drought Monitor reveal that over the last six to 10 weeks drought conditions continued to worsen across all of California. There was some improvement in parts of Nevada due to the above-normal monsoonal precipitation. Both California and Nevada continue to be 100 percent in drought.

The California-Nevada region had record daily maximum and minimum temperature averaged over June and July. These record monthly averages were punctuated by numerous heat waves.

Since October 2019, much of California and Nevada are missing 0.5 to 1+ years' worth of precipitation. California has 1.5+ years of missing streamflow. Streamflow is “missing” more than precipitation, illustrating the non-linearities between precipitation and streamflow.

The current drought is a combination of a precipitation deficit and a surplus of evaporative demand (the atmospheric conditions leading to the drying of the landscape). The excess evaporative demand is the dominant driver of drought across 23 percent of California and Nevada, and in 37 percent of the region the evaporative demand excess is approximately equal to the precipitation deficit.

The total storage in 28 Western Sierra Reservoirs in July this year was 8.6 million acre-feet, whereas the total storage in the same 28 reservoirs was 7.8 million acre-feet in July 2015. On average for this time of year the same 28 reservoirs have slightly over 10 million acre-feet.

California’s Lake Oroville is below the previous record low level, and Nevada’s Lake Lahontan is near record low levels. Most of the 10 largest reservoirs in Northern California are below the 25th percentile levels for this time of year.

Water flows remain much below to extremely below normal, and high soil moisture deficits remain.

Evaporative demand (the atmospheric thirst) remains high as it has been throughout this water year, with the exception of Southern Nevada recently, contributing to vegetation stress.

The current drought conditions have led to concern for the wildfire season. As of late August, more than 1.6 million acres have burned in California. This includes the Dixie Fire that has burned in excess of 735,000 acres (as of late August)—making it the largest single fire in the modern era.