Atmospheric rivers in California and their responses

Since the 27th of December 2022, following three years of drought which triggered exceptional cutbacks on water allocations throughout the Colorado River basin, the state of California experienced a series of intense moisture laden storms known as ‘atmospheric rivers’.  Depending on the location, rainfall across California has been between 200% and 600% above normal. The result has been catastrophic, with 19 lives lost, roads swept away, bridges damaged, homes flooded, and flood watches in place for some 8 million Californians. In total, at the time of writing, the U.S, geological survey had reported almost 600 landslides across the state since the 30th of December.

On the 14th of January President Biden declared a major disaster in California, ordering Federal funding to supplement state, tribal and local recovery efforts in the Counties of Merced, Sacramento and Santa Cruz. Federal funding is also available for hazard mitigation measures across the state.  

Atmospheric rivers of this kind can under normal circumstances be expected in California once in 150 years on average, although climate change is increasing the likelihood of such events. In California, flood control reservoirs exist in the Sierra Nevada upstream: nearly all the rivers draining from the region, which accounts for 60% of California’s precipitation, have been impounded by dams. However a recent study led by Xingying Wang at University of California Santa Barbara’s School of Environmental Science & Management indicates that under current climate scenario’s, average precipitation during atmospheric rivers will increase by around 25%, but a much larger proportion will fall as rain than as snow, increasing peak runoff.   

The reservoirs in the Sierra Nevada cannot sufficiently reduce the effects of atmospheric rivers such as have recently been experienced, and therefore other catchment management measures need to be considered, such as the protection of the upper catchment areas and introducing disincentives for settlement and other forms of construction on the floodplain. The floodplain can be of use to encourage increased groundwater infiltration and storage. In addition, disaster risk reduction measures are indeed necessary, as are insurance systems that can cope with such sudden emergencies.