California Groundwater Difficult to Replenish

California's past winter was a lot wetter than the last few, replenishing reservoirs and snowpack in a drought-afflicted state. "Thomas Harter, a professor with the [University of California, Davis] Department of Land, Air and Water Resources, says the rainy winter will make a dent in the state's groundwater deficit, especially for the Central Valley."

However, Alastair Bland reports, groundwater levels are still declining over the long term and it's hard to refill deep aquifers. "The challenge, then, will be for Californians to identify methods for diverting surplus surface water during wet winters into areas where it can sink into the ground and recharge overdrawn aquifers."

These groundwater reserves are much larger than the total volume of snowpack and reservoirs. But last winter, a lot of precipitation drained into the ocean rather than the aquifers (which can collapse when they're overdrawn, permanently reducing capacity). Harter says the best way to refill them would be to let surplus flows drain through agricultural lands.