Once taken for human use, water generally follows a path described in Figure 3 below. After undergoing treatment and distribution, it is used. In the broadest sense, water is constantly being re-used. Water that is taken from rivers or streams for domestic, industrial, or agricultural use was most likely also used by communities or farms up-stream, and subsequently treated and discharged. Over even longer timescales, the water in streams, lakes, and groundwater is the same water that has ever been on Earth – and those same molecules have undoubtedly cycled through many plants and animals before we were even around!
Depending on the nature of water use, it may be re-captured after treatment (“recycled water”) for re-use. As we will see later in the semester, this re-use of water resources is one strategy to cope with water scarcity. The recycled water, depending on its quality, can be used for irrigation (i.e. for parks or golf courses), or for domestic supply. Once the water leaves the “use” loop, it is treated and discharged, typically into surface water bodies. In some cases, the treated water may be used to recharge aquifers instead, either through induced recharge systems or at a smaller scale via passive filtration through soils – for example in leachfields. The discharged water, after mixing with water in the river, stream (or aquifer), becomes a water source for downstream or down-gradient users.
1. Do you know the source of domestic or municipal water in your hometown? If yes, what and where is it? If no, does it surprise you to realize that you don't know where your drinking water comes from?
2. Do you suppose that any of that water is used and then treated by others before being taken for your use?
3. Take a look at figure 3 above. Had you thought about your water as a substance that has a “life cycle” and is constantly being used, treated, released, and re-used? If not, does the idea make you uncomfortable?
Some cities are sited in areas where water is available - or was at the time they were settled - including Las Vegas, Los Angeles, Chicago, St. Louis, and Pittsburgh (Figure 4). In some cases, and as we will discuss in detail in later modules in the course, rapid development and growing demand can outpace the original and limited water source for a city or region, leading to a vicious cycle of water acquisition, growth enabled by water availability, and subsequent water stress.
Many of America’s major manufacturing centers (i.e. the rust belt) are located in areas where major rivers and canals provided a means for transport of raw materials and goods, power generation, water supply for processing and cooling, and conveyance of waste. At small scale, harnessing hydropower was accomplished by mills; at larger scales in modern dams, it is through hydroelectric power generation. Major rivers also provide the water supply for irrigation-based agriculture in some areas, where precipitation is not sufficient or consistent enough to support crops.
Indeed, for these reasons, rivers in many parts of the world are considered the “lifeblood” of society (Figure 6). For example, the Nile River valley in Egypt comprises ~5% of the land area, yet is home to nearly the entire population of 78 million, with a population density among the highest in the world (more than 1000 people per square km). Despite the obvious connection between water availability and human needs, the story of water resource distribution and population growth is not that simple! In some cases, major engineering projects in which millions of acre-feet of water are moved across states or continents have allowed cities and irrigated agricultural regions to flourish in water-scarce parts of the world. In others, major dams or new water sources (i.e. deep groundwater, reclaimed water, or desalination) have provided a means for cities to prosper in unlikely places. For example, take another look at Figure 4 above. The concentration of nighttime lights provides a reasonable proxy for population density. In many parts of the U.S., they follow the water: along the St. Lawrence, girdling the Great Lakes, and along the Mississippi River. Yet other major population centers have sprung up in perennially dry regions, mainly in the deserts of the southwest: Los Angeles, Las Vegas, Tucson, and Albuquerque.
1. Inspect Figures 4 and 5 and compare the two maps. Note 3 major cities that are near large water sources (rivers or lakes). View Figures 4 and 5above.
2. List 3 cities or regions of high population density that are not near major water sources, and/or lie in areas of low precipitation.
3. Do you know anyone who lives in one of these dry areas, or have you thought about moving there?
The distribution of water-rich and water-poor regions is of course not the whole story – access to clean water isn’t just about the amount of water that falls as precipitation. It’s also about the infrastructure needed to obtain, treat, transport, and deliver potable water. And that’s just the water supply. Disposal and sanitation of dirty water are equally important and require a means of transporting waste away from the distributed sources, collecting it and treating it, and discharging it safely. Ideally, both supply and waste conveyance systems should also be monitored for performance and for their impacts on water quality.
In some areas, water is plentiful, but access to clean water is not (Figures 7-8). The converse is also true, mainly in developed nations where water projects, desalination, or dams provide a water supply to regions that receive little precipitation. There is also a clear distinction between access to clean water in rural and urban areas (Figure 7), wherein access in rural areas, even in developed nations, lags behind that in urban areas.
1. What is the primary trend shown in Figure 7 above, with respect to urban vs. rural areas?
2. Is there a major difference in access to clean water supply and sanitation when comparing developed and developing nations?
3. Which is the bigger difference – urban vs. rural, or developed vs. developing nations?
4. Do you find your answer to question #3 surprising – or is it what you had expected?
Access to clean water differs between rural and urban areas, and between developed and developing nations. In general, in rural areas, even in developed nations, access to water and sanitation lags behind that in urban areas. Globally, the areas with the poorest access to clean drinking water are in equatorial and sub-Saharan Africa, and parts of South America and southeast Asia (Figure 8).
One might imagine that access to clean water and sanitation would be strongly correlated with water-related illnesses and death. For example, compare the maps in Figures 8 and 9.
1. Compare the maps in Figures 8 and 9. Is there a correlation between access to improved water and water-related illness? Note two areas where there is a correlation, either positive or negative. See Figures 8 and 9 above.
2. Based on Figure 8 and the distribution of water availability, do you think that these problems are related to water scarcity, or more related to water treatment and infrastructure?