As Governor Jerry Brown stood in a field of dry grass near the shore of Lake Tahoe in April 2015, California was enduring its fourth straight year of drought, and it was clear that water-use patterns had to change if the Golden State was to survive and prosper. “This is the new normal,” Brown said, referring to his state’s current drought, “and we’ll have to deal with it.” At that press conference, the governor announced mandatory water restrictions that required cities and towns in California to reduce their water use by 25%—an aggressive target intended to help the United States’ most populous state and the world’s eighth largest economy deal with one of its worst droughts in recorded history.
Record-low levels of precipitation coupled with record-high heat have kept California in drought conditions since 2012 and made water scarcity one of the greatest challenges faced by the state in recent memory. The drought has largely been driven by the lack of intense “atmospheric river” precipitation events, which can supply up to 50% of the state’s annual precipitation. In these events, moist air from the Pacific rushes inland over California, to the Sierra Nevada Mountains. As the air rises over the mountains, it cools, resulting in torrential rains and heavy mountain snow, which eventually supplies water to rivers during snowmelt in spring.
Since 2011, however, a high-pressure atmospheric system in the eastern Pacific has interfered with these events, resulting in reduced precipitation. The Sierra Nevada snowpack in 2014, for example, was a mere 18% of its normal volume after years of reduced snowfall coupled with elevated temperatures. According to a National Aeronautics and Space Administration (NASA) study, the accumulated precipitation deficit in 2015 was 50 cm (20 in.)—the equivalent of an entire year’s average rainfall.