More on that drip irrigation thing I Nibbled yesterday. David Zetland, the aquablogger sans pareil, has blogged a bit about this. On the PNAS paper, he had this to say:
Water has to go somewhere, and drip irrigation just controls that flow. Be a good cost-accountant and find out where else it goes. (There are losers and gainers on an individual basis, but society as a whole should just try to maximize overall benefit from water.)
I’m puzzled by “society as a whole”. Is that some overweening society, or just the outcome of individual actions?
An earlier post, in response to an email, revealed that Zetland, like me, had always though that drip would be more “efficient,” but that there are many factors that come into play.
It’s all about cost and benefit. When water is cheaper, it’s not too important to conserve it, but expensive water doesn’t necessarily mean that the “best” irrigation method is the one that uses the least water. (Although drip-irrigated rice uses less water, it also has a lower yield.)
So, it’s complex. Now, there’s a surprise. Wouldn’t it be nice if there were some decision-making tool that could tell the grower which system would provide the most crop per drop, taking into account, of course, the cost per drop?
And in related news, Reuters reports that Iraq plans to revive 2.5 million hectares of agricultural land by “sucking out the salt”. I’m not sure I understand the project fully. It seems to involve “pumping out the groundwater beneath the soil over several years”. Then what? Natural rainfall replenishes the groundwater? We shall see, but it sounds like a huge undertaking with no guarantees of success or even a reasonable return.
OK, so irrigation isn’t really about agrobiodiversity, but one can use agricultural biodiversity to take advantage of what water is available, and that’s a good enough reason to post.
@I’m puzzled by “society as a whole should just try to maximize overall benefit from water”.
Seems like a simple idea to me. If a resource is scarce, optimize its use to maximize benefits. No “overweening” in that. The question is how you do it. Some might prefer the free market (farmers with entrenched water rights, typically), others might prefer a 5 year plan (Marxist-Leninist, typically).
How to do this then? On a single small field, drip irrigation is more water efficient than surface irrigation. Period. Economic efficiency is something else; there is no point in dripping when there is plenty of water to go around. You seem to be asking for “most crop per dollar”. Well, that is something farmers have been looking at all along.
What I understand the point to be of the Ward and Pulido-Velazquez paper is the distinction between field level and aggregated level efficiency.
Water “lost” in one place, i.e. through percolation, has to re-appear somewhere else. In-efficient field level water use leads to water more ground water which can (and will) be reused. By using the same drop more than once water use efficiency, on the catchment scale, goes up (depending on timing, lags, whatever).
That idea is the basis, I think, for their conclusion that
The concept of changes in efficiency at different scales is important for other resources as well. A hectare of an organic farming may leach less nitrogen to the ground water than a conventional farm. But if organic yield is lower, than the total amount of nitrogen leached can be higher than with conventional farming (not to mention –from a simplistic optimization perspective– the additional land lost to farming). You choose, a little pollution everywhere, or more pollution in fewer places combined with more protected areas.
I was aking myself: the research is about Rio Bravo, are results expected to be the same with different soil-vegetation composition? I’m not sure…
What’s more, what about soil erosion and nutrient leaching by flooding irrigation? I think this is a good research to focus on water use but not so enlightning… I will keep on running my drip irrigation system (rain catchment!)