First, let's define what we're talking about. The impact of our water use somewhat depends on what happens to the water afterwards. On one extreme, water is not very "used" in a swimming pool, running river, or hydroelectric project. Many people can share the same pool water, look at or fish in a river, or benefit from water flowing below a dam. These kinds of non-consumptive uses do not really deplete the quantity of water currently available in an area. On the other extreme, you have irrigation with sprinklers that results in high evaporation and leaves little water behind. That kind of "consumptive use" makes it much harder to stretch a lot of water across many uses and users.
The point that Chris wants to emphasize (and I agree with this) is that it is sometimes more important to manage consumptive use more than use per se. It's in this context that people in Singapore, Orange Country, California and other places with recycled water facilities should not worry about their showers or flushed toilets. Their water use is non-consumptive in the sense that the water can be captured, cleaned and used again. The same is true (to a degree) for farmers in Palo Verde Irrigation District who use flood irrigation. They apply about 9 feet of water per acre over the year and about 4 feet run off, back into the Colorado River. Assuming their soils are well-drained, their technique does not "waste" water because the excess runs back into the river, instead of evaporating.
The trouble is that most urban utilities and irrigation districts do NOT handle their water in a non-consumptive way. On the urban level, you have lawn irrigation that is consumptive as well as a tradition (based on cost, yuck and "abundance") of discharging wastewater into rivers or oceans. On the agricultural level, you have a tradition of "use it or lose it" water rights that are generous enough to deter investments in improving water efficiency -- especially when improvements that "yield" water cannot be rewarded by selling that water.
So, why or how do these differences matter? Well, they are very important when considering the definitions of water rights. They also matter for water managers facing supply shortfalls. Orange Country and Singapore count on recycled water to meet demand; Las Vegas has been intensively recycling water for re-use, but the problem there is consumptive use.
Which brings me to the question of whether these definitions matter. Yes, they do in the long run because -- as Chris rightly points out -- you need to understand and manage all water flows. In the short run, however the differences do not matter.
In my post last week, I suggested short run and long run actions that California can take to reduce risk of water shortages. In the short run, I said to reduce urban irrigation and agricultural groundwater pumping (as well as raise prices, to maintain fiscal health), but I did not discuss consumptive vs non-consumptive use. Why? Because that difference is not important in the short run. Technology and infrastructure are fixed, and return flows and recycling rates are known. I was targeting "wasteful" uses on lawns (rather than drinking water in restaurants) and unsustainable (unknown) groundwater pumping because both reductions would leave water for other uses now and in the future. In my long-run actions, I got into market, regulation and water quality improvements where "consumptive differences" matter more.
Bottom Line: Water can be managed for efficiency, equity or physicality, for minimal energy demand, maximal environmental health, or targets of justice. Decide your community's priority and then manage in that direction, but don't forget to monitor impacts on your "less important" goals.
- I'm ignoring the fact that water in pools and behind dams evaporates, as well as the fact that dams disrupt flows and water temperatures.
- There some exceptions to this. San Diego paid Imperial Irrigation District $millions for water "saved" after a canal was lined (reducing seepage). The trouble with that deal (and others like it), is that some of the "lost" water was non-consumptive, i.e., it was recharging aquifers that other, Mexican farmers were using. They lost a lawsuit claiming damages from the lining because they lacked standing (=not American).
- Australia's rights are evolving as farmers find ways to use less water; the idea is to extract those savings from their licenses, rather than let them sell the savings, because their "savings" end up reducing the quantity necessary to keep other licenses "wet.