OOOH! 24 million acre-feet have been sucked out of the ground in the San Joaquin Valley in the past six years without replenishment! Hey, that’s interesting. Let’s think about that a little. I’m going to make some very rough approximations.
At six million irrigated acres in the SJV, that would be four feet of water per acre in six years (if every grower were pumping gw). Forty-eight inches in six years means each acre is using about 8 inches of gw/year.
My usual rule of thumb is three-and-a-half feet of water to grow a crop, so 42 inches of water/year.
What this information tells me, very roughly, is that every year growers in the SJV are
using overdrafting groundwater to get about 20% of the water they need to finish their crop*. This is, of course, wrong; some growers use only surface water, some use only groundwater, some mix it up in different proportions. But now I have a rough approximation. When the groundwater in the SJV gets too deep to be worth pumping (or energy gets more expensive), I expect to see something slightly less than 20% of the irrigated acres drop out of production. I am standing by my gut feeling of California ag settling at about 6 million irrigated acres by 2050, down from 9 million today.
Let’s compare 24 million-acrefeet of depleted groundwater to something else. Here is DWR’s graphic of annual water supplies and uses. (On page 6. A two-sided bar chart. I’ll try to put it here later.)
First thought. Hmmm. The state of California uses about 80 million acrefeet of surface water every year, for everything. That’s all the rivers being rivers, cities drinking, crops being irrigated. An additional (roughly) four million acre-feet of gw getting sucked out of the SJV aquifers per year sounds plausible. Second thought. That column on the very far right is a calculated closure term. It says how much more we know we’re using than supplies, and that water comes from somewhere. Reservoirs being drawndown or groundwater. The numbers there go from about 5 million acre-feet a year to 12 million acrefeet a year. Four million acrefeet of overdraft in the SJV fits right into that. (The last time we had more supplies than use was 1998). Contra Dr. Gleick here, the new Water Plan seems to report overdraft at about the same scale as the JPL results.
So, a rough estimate and a different presentation say the JPL results are on the right scale. I didn’t doubt JPL, except for their poor choice of units, but it is nice to see these all agreeing.
*Did you see how easy that was? That is because good units have factors.