Does water conservation make more water available for use?

Interesting point from the California WaterBlog:

"Water conservation does not always make more water available for use. Consider this example of washing a boat:

Last week I sailed from Vallejo to the Delta. While docked in Vallejo, I resisted the temptation to hose off the boat with fresh municipal water because the city is encouraging water conservation during the drought. By not washing the boat in Vallejo, about 20 gallons of fresh water became available for other urban uses rather than ending up in San Francisco Bay.

But then, while anchored off Mildred atoll, I washed the boat by heaving buckets of fresh water onto the deck. Almost all the water returned to the Delta for eventual use by farms or cities, or flowed out to San Francisco Bay. (I’m debating how much less evaporation occurred from the Delta because the surface area of my boat evaporates less than water surface area.)

Likewise, because water has further use downstream, water conservation in inland cities is less effective than water conserved in coastal cities. Switching from thirsty lawns to drought-tolerant plants frees up water for other uses.

To estimate the likely benefit of urban water conservation in California, former UC Davis graduate student Ryan Cahill and I recently examined the effect of reducing California’s per capita urban water use to levels common in Australia — some of the lowest in the world for prosperous economies with dry climates. We found it would reduce gross water use in California by about 2.1 maf/year and reduce net water use (making new water available for others) by about 1.5 maf/yr. This, again, is not a trivial amount. But it is less than 6 percent of the state’s agricultural water use.

In agriculture, most net water losses are to “evapotranspiration” – the amount of water consumed to grow crops. Almost all the excess water applied to fields already returns to recharge aquifers or supply rivers downstream where, in most cases, it is used again to grow crops or supply cities.
Improved plant breeding and farming has increased crop yields for decades, but has not greatly changed net water use per acre of agriculture. The large increases in agricultural “efficiency” often called for would mostly reduce aquifer recharge in wet years, jeopardizing water storage for dry years and not make much water available for other uses (Lund et al. 2011; Lin 2013). It is possible to reduce unproductive water losses from evaporation in fields, but only by a small amount and at a substantial management expense. Without reducing crop production — meaning fallowing land — surprisingly little net water can be saved from agriculture."