The need for a strong and impactful El Niño is more crucial than ever as the Tuolumne River Watershed concluded the 2015 precipitation year with only 20.92 inches of rainfall, a total that comes in as little more than half of the historical average of 36.03 inches.
One significant contributor to the arid 2015 precipitation year, which spans from September to August, was a historically dry January. During this month, which on average achieves 6.32 inches of precipitation, the region only experienced .09 inches of rainfall.
This dry January had serious implications on the precipitation year overall according to Turlock Irrigation District Utility Analyst Jason Carkeet, who said that an average January would have produced approximately 100,000 acre-feet of much-needed runoff.
“To emphasize how dry it was this year, consider July,” said Carkeet. “July watershed precipitation was just over 700 percent of average at 1.21 inches.”
“However, that is equal to only 19 percent of an average January,” continued Carkeet.
Additionally, Carkeet said that 1.21 inches of precipitation in July was greater than precipitation in October, January and March combined.
July was not the only month of the 2015 precipitation year that exceeded the historical average rainfall amount. During the month of May, the region experienced 1.80 inches of precipitation, a total that exceeded the historical May average by 0.19 inches.
According to Carkeet, what TID considers an average year does not depend on the amount of precipitation the city receives. Rather the deciding factor lies in the amount of accumulated runoff, or precipitation that is either not evaporated or absorbed in the ground.
Carkeet said that due to conditions caused by the recent string of dry years, the Tuolumne River Watershed would need more than an average year’s amount of rainfall to achieve the average amount of runoff.
“In order to get the 1,955,000 acre-feet of runoff needed for an average year, we would need at least 44 inches of watershed precipitation, which is approximately 122 percent of the average of 36 inches,” said Carkeet.
With the 2016 precipitation year officially underway, Carkeet said on Tuesday that September has experienced .05 inches of rainfall. For a month that on average achieves 0.59 inches, this total so far is 12.2 percent of average for this date.