There has been a rather spirited discussion occurring on John Fleck's blog that I was sucked into. I just can't stand by and watch people use bad science to try to influence policy-makers.
The whole thing was prompted by a comment at a Pasadena (California) city council meeting where they were discussing the implementation of new conservation measures to deal with expected cut-backs in water deliveries from the Metropolitan Water District of So. Cal. (MWD - the main water wholesaler in the LA-San Diego region). A retired engineer made the point that convincing people to stop watering their landscaping could have unintended consequences - a loss of recharge to the aquifer that results from infiltration of over-applied water. This concept was leapt on by a local blogger in Pasadena who made it his mission to stop water conservation measures aimed at outdoor water use because it was necessary to save their local groundwater resources.
The point I sought to make is that if water is being pumped from the aquifer (Pasadena typically gets about 40% of their water supplies from wells, the remainder from MWD), pumping more water so people could continue watering their lawns would not help the aquifer - it would further deplete it. Now if imported water is used for outdoor irrigation there is a benefit to the aquifer from over-watering to the extent that some of the irrigation water will in fact recharge the aquifer, but if the goal is to recharge the aquifer there are far more efficient ways to do it than by having residents over-water their grass. And because the city probably cannot determine whether a given resident is watering with imported water or groundwater, banning or limiting outdoor watering probably has more benefit to the aquifer than encouraging wasteful watering practices.
Fortunately Pasadena appears to be listening more to people who actually understand these issues than to rebel bloggers and will be imposing watering restrictions in order to compensate for cutbacks in MWD water deliveries.