Perhaps the most persuasive piece of the irrigation professors’ critique was this text box:
If so much water is being wasted as implied by the estimates of potential savings in the PacInst Paper, it would have to be going somewhere. That somewhere could only be into the ground or out through rivers. But we know that there is a huge groundwater overdraft (perhaps 2 million acrefeet/year) in the San Joaquin Valley and the San Joaquin River runs dry near Dos Palos in the summer. (emphasis in the original)
It sounds facile, but it is true. The Pacific Institute efficiency argument says, in effect, that 3.5 million acrefeet of water is sloshing around California agriculture, not being taken up by plants*. If so, where is it?
There aren’t many choices. The Tule Lakebed doesn’t hold a three foot deep lake in August. Excess irrigation water sure isn’t draining to the San Joaquin River. In fact, look. The western part of the San Joaquin Valley sends about 50,000 acrefeet/year of really gross water to the Grasslands Bypass Project (16,000 af this dry year). Fifty thousand acrefeet a year? Draining the better part of the west side of the San Joaquin Valley? You’ve got to show me a whole bunch more bypass projects to come up with the other 3.45* 1.65 million acrefeet the Pacific Institute says is loose water. Or you could show me rising groundwater levels, as the ground soaks up excess irrigation water. Except you can’t, cause they aren’t there.
The only explanation I can come up with for such a huge gap between building upward from models of field irrigation efficiency and working downward from a valley-wide water budget is double counting. I think the Pacific Institute method of adding up efficiency improvements from all those farms double (and multiple) counts the same chunk of water each time it gets re-used as if it were separate chunks of water from which you can extract 10% every time. That’s why I don’t think there is potential to get 3.5 maf of water from Great Valley agriculture and have ag stay at current levels.
*Naw, after reading more closely, that’s not what they’re saying. The 0.6 maf they claim could come from crop shifting is presumably now being taken up by field crops. But that still leaves 2.9 maf. Well, I guess they also say that 1.2 maf are being applied to trees and vines now that aren’t necessary for constant crop yields because you could use regulated deficit irrigation instead. 1.7 maf! Show me 1.7 maf of water loose in the San Joaquin and Sac Valleys!