How California could save up its rain to ease future droughts

California has seen so much rain over the past few weeks that farm fields are inundated and normally dry creeks and drainage ditches have become torrents of water racing toward the ocean. Yet, most of the state remains in droughtmonitor.unl.edu/data/png/20230103/20230103_west_text.png">severe drought.
All that runoff in the middle of a drought begs the question--why can't more rainwater be collected and stored for the long, dry spring and summer when it's needed?
As a hydrogeologist at the University of California at Santa Cruz, I'm interested in what can be done to collect runoff from storms like this on a large scale. There are two primary sources of large-scale water storage that could help make a dent in the drought: holding that water behind dams and putting it in the ground.
Why isn't California capturing more runoff now?
When California gets storms like the california">atmospheric rivers that hit in December 2022 and January 2023, water managers around the state probably shake their heads and ask why they can't hold on to more of that water. The reality is, it's a complicated issue.
California has big dams and reservoirs that can store large volumes of water, but they tend to be in the mountains. And once they're near capacity, water has to be released to be ready for the next storm. Unless there's another reservoir downstream, a lot of that water is going out to the ocean.
In more populated areas, one of the reasons storm water runoff isn't automatically collected for use on a large scale is because the first runoff from roads is often contaminated. Flooding can also cause septic system overflows. So, that water would have to be treated.
You might say, well, the captured water doesn't have to be drinking water, we could just use it on golf courses. But then you would need a place to store the water, and you would need a way to distribute it, with separate pipes and pumps, because you can't put it in the same pipes as drinking water.