The junkies among you can read a more technical description of field and basin efficiency here, or see a sketch of a description in this slide show (which only makes sense if you already know the topic).
The reason I don’t like to talk about field and basin efficiency is that ag advocates are known to do a sorta bullshit sleight of hand, saying that their “efficiency” is some amazingly high number (anything higher than 90%) and so they don’t need to listen to any more of this talk about new irrigation practices. If they are talking basin efficiencies, that means that for some large boundary, perhaps their water district or a watershed, the amount of water they collectively apply is only barely more than collective crop demand, and presto! Everyone in that area is a very good irrigator. Except they don’t really say “basin efficiency”. They say “efficiency” or “district-wide efficiency”, and hope that you will be suckered into thinking that they mean the average of every single grower’s field efficiency.
Field efficiency comes from matching irrigation water to crop demand on a single field or small farm. This is the realm that individual farmers can influence, with the type of techniques that the Pacific Institute talks about (irrigation scheduling, crop shifting). If every single grower had an field efficiency in the 90s and you averaged them to get some average field efficiency in the 90s, that would indeed mean that you are talking to very skilled irrigators who don’t need any advice from you. But having a basin efficiency in the 90s doesn’t mean everyone is doing a good job applying a precise amount of water. It just means that water gets used again and again within the big boundary area.
I kinda resent giving the idea of basin efficiency much credence, because some advocates use it in a sneaky way to imply that all their irrigators are doing a good job. I also don’t like that some advocates use the idea of basin efficiency to say that there is no good reason to improve their practices on the smaller field scale. This isn’t true, as both the irrigation professors and the Pacific Institute report point out. What high basin efficiency can look like in practice is that growers overapply water at the top of the basin and other growers use it again, as tailwater with pesticides or fertilizers or salts in it. Or it sinks into the ground and other growers pump it out. Or it returns to the river, salty and warm, and gets diverted again. You could still have high basinwide efficiencies, but you pay costs in water quality or pump energy or crop yield. Taking more water than you need (because it will return soon enough or so that it can percolate into groundwater) hurts the rivers it was diverted from; fish would like to live in that clean cold water until the very last second before it gets used. So yeah. There are real problems with using high basin efficiencies to give yourself a free pass.
On the other hand, there is no denying that it exists. A lot of diverted water gets used several times. It is a cheap way to move water, or at least a way to move water that externalizes some of the costs. Further, people have been living in this connected system for two generations now. I have to think they have roughly optimized their positions and come to rely upon them. Getting someone else’s water after it ran off the field may not be a good way for the system to work, but at this point, I’m inclined to think it is the best bet for that second grower. They’ve had forty years to debate sinking a well or digging a pipe directly to the river. If they thought those were better for them than taking tailwater, I think they’d have done it. Disrupting that system (like, if you had the grower at the top really cinch down on irrigation efficiency) will probably put downstream growers in a worse position. To the extent that it means that they’re internalizing their environmental costs, I’m fine with it. But as much as they perceive it to damage their interests, they’ll fight that.