A couple of months ago I wrote a post about a recent report issued by the Pacific Water Institute on the great strides that can and have been made to increase the water efficiency of agriculture in California. In particular was one example I cited from the report of a farm that reported increasing its water efficiency by 20% (which can be found on p. 33 of the report).
I just received an extensive comment to the post asking about that particular 20% number. Wayne Bossert, manager of the Northwest Kansas Groundwater Management District No. 4, asked whether the 20% increase in efficiency represented a decrease in “consumptive use” or a decrease in water “diverted and applied.” Mr. Bossert explained the question as follows:
In any hydrologic system where the water supply and the water sink (where non-consumptive water uses go) are the same, increasing irrigation application efficiency just eliminates the sink supply and provides a higher percentage of the applied water to consumptive use crop production. You can pump less water with the higher efficient irrigation system, but you can also actually consume more water.(Please read the rest of the comment here)
The 65% efficient irrigation system only makes 65% of the applied water available for crop production. The rest is non-consumptive use that returns (eventually) to the supply - at least in a traditional groundwater aquifer system. When a new 99%efficient drip system is installed, the producer pumps 75% of what he used to, but 99% of it is made available and consumed by crop production. My math tells me that 99% of 75% is more than 65% of 100%.
It is this extra water use that increases the yields so often reported when higher efficiency systems are converted to.
This comment struck me in two ways. First, I had always assumed that “efficiency” must be “good” in all circumstances – this comment has made me realize that “efficiency” is really a far more nuanced concept in water management. Second, as I have discussed in several different contexts, I believe that water management really needs to be looked at holistically, taking into consideration the entire hydrologic cycle. And that is exactly the point Mr. Bossert is making. In his example of the 65% efficient irrigation system, the other 35% of the water that does not go to the crops is not necessarily lost or destroyed. In fact, usually, that water simply returns to the natural hydrologic cycle. The same cycle that ultimately is the water supply.
To answer Mr. Bossert’s specific question, I have to say that the report doesn't provide a clear answer because as far as I can see it doesn’t squarely address the issue (though I admit I did not comb through all 75 pages). But my reading of it leads me to believe that the 20% increase in efficiency referred to a decrease in water “diverted and applied.” If the goal of water conservation is to reduce human use (i.e. consumption) of water, it seems we need to give greater thought to what it means to increase the efficiency of our water use.
This doesn’t mean that increased efficiency is a bad thing. Indeed, Mr. Bossert himself makes that point. And the Pacific Institute Report notes a number of non-consumption related environmental benefits associated with increasing irrigation efficiency. What it does mean is that increased efficiency may not be the ultimate solution for one of the largest water management challenges we face – dwindling supplies.