California's snow-covered Sierra Nevada, as seen from space. Courtesy NASA

One of the less appreciated effects of warming temperatures on water supply is the shift in the timing of runoff. Warming springs mean earlier melt. This is as much an infrastructure problem as it is an overall water volume problem, because the dams and ditches built to manage water during the 20th century were based on climate normals of the 20th century. Some water is stored behind dams, and some us presumed to be stored in the snowpack itself.

That’s the message of a new BAMS paper by Sarah Kapnick and Alex Hall, which found (not surprisingly) a shift to earlier runoff from California’s Sierra Nevada in the spring:

Given future scenarios of warming in California, one can expect acceleration in the trend toward earlier peak timing; this will reduce the warm season storage capacity of the California snowpack.

Given our apparent inability to reduce greenhouse gas emissions, this is a pattern California’s water managers will likely need to learn to cope with.