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COMMENT: Sandra Postel, Global Water Policy Project (617) 723-7082
Human water use reaching upper physical limit, scientists warn
STANFORD -- Humans are using a surprisingly large - and growing - proportion of Earth's renewable water supply, according to a study reported in the Feb. 9 issue of the journal Science. The trend could lead to constraints on food production and already is placing stress on aquatic ecosystems that could decimate fish populations. "If it continues, this increasing scale of human water use threatens the integrity of Earth's life support systems," said Sandra Postel, lead author of the study.
Postel is director of the Global Water Policy Project in Cambridge, Mass. She conducted the renewable water study as a research scholar at Stanford's Center for Conservation Biology, working with Gretchen Daily, Bing Interdisciplinary Research Scientist, and Paul R. Ehrlich, the Bing Professor of Population Studies at Stanford.
They calculated that 30 percent of all the runoff from land to sea each year is accessible for use in irrigated agriculture, industries and cities. The rest is flood water not captured by dams, or water too remote geographically to be of use. The research team further calculated that human activities already make use of more than half of this accessible supply.
Human water use tripled in the years from 1950 to 1990, but this escalating water use is bumping into limits, according to the study. Projections of population size and water use per person suggest that humans could be using at least 70 percent of accessible runoff in 2025 - and possibly all of it, a troubling scenario.
"The widespread impression that globally water is too abundant to constrain future food production and economic development is ill founded," Postel said. "In this study, we have developed both an indicator of Earth's carrying capacity, and a measure of the sustainability of current water trends."
The authors suggest several actions that could slow the growth of human appropriation of water, such as greater efficiency of water use and changes in agricultural cropping patterns. Pollution prevention efforts also could have a big beneficial impact, because much of the water that humans appropriate from rivers, lakes and streams is used to dilute pollution.
However, Postel said if human population and consumption continue to increase at the current rate, food security will be threatened, because only limited opportunities remain to expand rain-fed or irrigated cropland. And with less water available for the rest of the organisms on the planet, ecosystems will be damaged along with the critical life support services they provide to humans.
The authors considered both major parts of the renewable water supply in their calculations: runoff, which is the flow from land to sea, and evapotranspiration, which is the water supply for all non-irrigated vegetation, including forests, grasslands and rain-fed croplands.
They used work by other researchers to estimate that 69,600 cubic kilometers of water each year are cycled by plants and returned to the atmosphere as evapotranspiration. An earlier Stanford study provided estimates of how much net production of plant biomass is controlled by humans; with these data, Postel's group calculated how much water those plants use.
Humans now co-opt 26 percent of total annual evapotranspiration through their use of wood, grazing land and cropland, these calculations show. "The remaining 74 percent must meet the water needs of all other land-based plants and animals," Daily said.
Other researchers have estimated that annual global runoff is roughly 40,700 cubic kilometers. To calculate how much runoff is accessible to humans, the Stanford team first subtracted water that is too remote from population centers to be used economically - including much of the Amazon's water, and the remote rivers of North America and Eurasia. Next they counted global irrigation, industrial use and municipal use based on work by other researchers, and they added in the amount of instream water, necessary to dilute pollution. They found that humans have access to 30 percent of all runoff water, and humanity keeps 54 percent for itself.
"That we are already using more than half of accessible runoff is extremely worrisome," Postel said, "because this is the water supply for irrigation, industries and cities." Irrigation accounts for about two-thirds of world water use, and is extremely important to global food security. While accounting for just 16 percent of all cropland, irrigated farms produce about 35 percent of our food. Because of water limits and other constraints, irrigated area is no longer expanding as fast as population - suggesting trouble on the food front.
The authors estimate that accessible runoff could increase by about 10 percent between now and 2025, assuming optimistically that large dams are constructed at a rate of 350 per year. Desalination, which currently supplies less than one-tenth of 1 percent (0.1 percent) of the world's water use, is unlikely to make a dent in the supply picture for the foreseeable future because of its high energy costs.
With population projected to expand by 45 percent by 2025, meeting future water needs will require a strategy to increase water efficiency - to do more with less, the authors said. Many farmers could reduce their water use by 10 to 30 percent with existing technologies. But because of government subsidies that keep water prices artificially low, farmers have little incentive to adopt these techniques.
Greater investments to prevent pollution also must be a part of a successful strategy, because industries and cities rely on fresh water to dilute pollution. Prevention of pollution has multiple benefits, Daily said, freeing up water supplies while at the same time improving the health of the human population and the aquatic environment.
"The bottom line," Postel said, "is that human well-being depends on learning to live within the limits of Earth's fresh water. And time to make the needed adjustments is short."
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