As shown in the Freshwater Resources section, water demand varies by culture and country, while water availability is dependent on climate and geography (see also Module 2). Some areas of the world are already experiencing freshwater shortages and/or their water supplies are unsanitary as the result of improper treatment of waste and inadequate infrastructure to transport and store potable water. The combined specters of climate change and rapid population growth create uncertainty in planning future water supplies.
What will the future bring? Good question, right? How can we gauge what water demand and availability will be in the future, particularly with projected large increases in population and potential climate change superimposed? Not to alarm you, but to inform you, we will go through the exercise of making such projections, both for the U.S. and, on a more limited basis, for the world. What do we need to know for making such estimates? First, let's jot down some ideas. Then we will continue the process below.
1. What do you think we would need to know in order to predict future demand for water? Take a minute to jot down what you think one would need to take the first crack at this.
ANSWER: Answers will vary. Clearly, we will need to know something about population growth and climate.
First, here is an expert opinion as to how the future will go…
In Human Population and the Environmental Crisis Ben Zuckerman and David Jefferson write: “At a low population density, a society may be able to derive its water from rivers, natural lakes, or from the sustainable use of groundwater. As the population grows, so does the volume of water needed (we will assume demand is proportional to population size). Moreover, levels of waste discharge into the environment will grow as the population rises. Thus, the available unmanaged supplies deteriorate at the same time that demand on them is increasing…A destructive synergy is at work: population size affects the water resource in a manner that is not one of simple proportionality.”
What was it Yogi Berra (N.Y. Yankees catcher and later Manager) infamously said…"It's tough to make predictions, especially about the future." Well, that is a truism, but let's see what projections are being made regarding future population growth, because, clearly, that's one of the inputs we need to determine potential future water use globally. The present global population (as of 2014) is approximately 7.25 billion people. Interestingly, the top three countries, in terms of population, are China, India, and, yes, the United States, in that order (Figure 17). But, by 2050 the global population is estimated to be 9.6 billion people by the United Nations—a staggering 33% increase in the next, say, 35 years! So, at the minimum, if we assume that water use will increase linearly on a per person basis, we would expect that this rate of growth will require 33% more fresh water by 2050. Is that a problem? Do we have excess capacity to supply this water?
Do we have excess capacity to supply this water? That is an important question, but you have probably already determined that the real issue is where the population growth occurs and what water resources are available there. The major growth is projected to occur in developing countries (Figure 17). African nations are likely regions for greater than average growth. Interestingly, much of Africa is estimated to have significant groundwater resources (BGS, 2013) that could be developed if necessary. In fact, Nigeria is projected to surpass the population of the U.S. by 2050 (Figures 17-19). One must examine the population density and rate of projected growth vs. water needs. In addition, climate change impacts must be considered.
1. What is the relationship between Total Fertility and Per Capita Income shown in Figure 19 above?
ANSWER: Fertility is inversely related to income worldwide. There are several drivers of this relationship, including infant mortality, need for agrarian labor, etc.
2. Why might this be an important consideration when considering future demand for water?
ANSWER: The greatest growth is likely to occur in areas with the least access to infrastructure for accessing, treating, and distributing fresh water.
We would probably be better off examining the impacts of climate change on water availability that would increase "water stress," then compare these stresses with those caused by increasing demand, either by population growth in a given region (personal or agricultural demands) or increased water usage resulting from new demands (e.g., energy production) (Figure 20). A number of studies have predicted water supply vs. water demand relationships resulting from climate change. A study by MIT (Massachusetts Institute of Technology) researchers (Schlosser et al., 2014) compared the potential impacts of climate change, on the basis of projected greenhouse gas emission increases in a complex Earth-system model, on water stress in 282 assessment regions (large or multiple watersheds) globally, holding demand constant, to the potential impacts of population growth in the same regions.
They found that, in most regions, projected population growth with increased demand to 2050 was the greater stressor. These researchers use a Water Stress Index (WSI) defined as WSI = TWR/RUN+INF (TWR is total water required for a given watershed region, i.e. all consumptive uses, RUN is available runoff within the watershed, and INF is inflow to the watershed from adjacent regions. The cutoffs used for interpreting water stress are: WSI<0.3 is slightly exploited, 0.3≤WSI<0.6 moderately exploited, 0.6≤WSI<1 heavily exploited, 1≤WSI<2 overly exploited, and WSI≥2 extremely exploited as originally set out by Smakhtin et al. (2005).
It appears that a substantial proportion of Africa, all of the middle East, India, and central Asia will see increased water stress in the next few decades, largely due to projected population increases. Even the southwestern U.S. is projected to experience expansion and intensification of water stress, but, in this case, mostly as the result of climate change and longer-term drought. Interestingly, the major central U.S. groundwater source, the Ogallala Aquifer, does not appear to be a candidate for significant stress except at its southern end in Texas. However, other studies (see Module 7) suggest that depletion of this aquifer will be more severe.
There are a number of possible methods to enhance supplies of fresh water, each of which has an economic, political, and/or environmental impact.
1. Provide three examples of potential ways to increase fresh water supplies.
ANSWER: Answers may vary. There are a number of potential strategies, including 1) Build large dams to increase water storage; 2) Bank water in groundwater storage; 3) Encourage transfers from other consumptive uses and/or conservation; 4) Increase recycling and reuse of wastewater; 5) Desalination of seawater or shallow saline groundwater.
Some of these strategies have been alluded to previously (e.g., encouraging transfers from agricultural use to drinking water supplies). Water storage behind dams is an old strategy and problematic in a number of ways (see Module 6), including high costs, environmental impacts, and political issues that arise when major rivers flow through multiple countries. Nonetheless, there is still major proposed and ongoing dam building in China and other countries.
Groundwater banking is a newer strategy that requires replenishment of aquifers with treated wastewater and/or with runoff available during times of excess. Costs are associated with treating, impounding, and injecting the water (see Module 7). This will mainly benefit regions with significant groundwater resources.
Recycling and reuse are gaining support with successful projects in the U.S. and elsewhere. Penn State University recycles and reinjects nearly 98% of its treated wastewater and has done so since the 1960s. Orange County, CA, has another successful system (see Module 8). Such systems must overcome consumer opposition, however, because of the perception that consumers will be drinking, well, toilet water! Nonetheless, the water quality in such systems is as good or better than that in municipalities that draw water from rivers downstream from other municipalities that discharge treated wastewater into the same river. Another form of reuse is to employ "gray" water (only partially treated) for irrigation of golf courses in arid to semiarid, water-stressed regions. Las Vegas, NV, has implemented such a system, coupled with the removal of water-hungry turf, for which the economics work and conservation is encouraged.
Desalination may be a last resort because of the costs of energy required to remove salts from seawater or water pumped from saline aquifers in non-coastal regions. However, in water-poor but hydrocarbon-rich middle-Eastern countries the economics may support the desalination of seawater. Alternative energy sources (e.g., solar) or emerging processes such as chemical reverse osmosis may be economical in the future as they become more efficient and less costly. And, of course, if water is deemed to have significant value in the future, the high costs may be more acceptable.
Finally, there are still proposals to import or export water from regions replete with fresh water resources (e.g. Alaska) to severely water-stressed regions (e.g. India). However, the costs of transporting such a commodity across the oceans would appear to exceed the value of that water at its terminus.
All of these strategies will be explored in later modules in more detail.