EME 807
Technologies for Sustainability Systems

6.1 Understanding water cycle


6.1 Understanding water cycle

River flowing through pieces of land
Kissimmee River wetland area, Florida
Credit: South Florida Water Management District (via Flickr)

Water is often envisioned as bloodstream of biosphere. It is a universal medium that is crucial for sustainability of both ecological and human societies. There is no substitute for water. More than 70% of the earth surface is covered by water. However, only 3% of this reserve is fresh water that can be used for human consumption. 90% of the earth's fresh water resources is contained in groundwater and ice, and only 10% is water is contained in surface reservoirs - rivers, lakes, wetlands, and streams. [Girard, 2013].

Although sustaining life is one of the main key purposes of the water, present-day agriculture and many industry branches heavily rely on the abundance of the water resource. For example, water is used as heat transport fluid in thermoelectric energy systems, such as nuclear and fossil fuel fired power plants and concentrating solar power farms. It is used as solvent and raw material in chemical manufacturing. Mining industry utilizes significant amount of water in hydraulic fracturing and oil recovery. Those industries are important parts of modern infrastructure; hence, the water demand must be met to keep the power and food production at the necessary level.

To plan sustainable utilization of water resources, we must understand how the water cycle works at the global and local scales. The amount of water on earth is finite, and the natural water cycle is a system that controls the circulation and redistribution of that resource. You must be familiar with the water cycle concept from your early science classes. But you can get a refresher from the following short video:

Click for transcript.

PRESENTER: All the water on Earth today, every drop, is all the water there has ever been on the planet. Fresh water is actually, millions of years old. The same water flowing in a continuous loop. Falling as rain and snow from clouds to the Earth's surface. Running in rivers. Pooling in ponds. Flowing from faucets. Irrigating crops. Traveling through plants. Generating power. Eventually, evaporating into the air and condensing into clouds, again. ANNA MICHALAK: Why is there life on Earth? And the reason there is life on Earth is because Earth has this perfect water cycle. PRESENTER: The water cycle, so simple even small children understand the basics. Yet, so complex, the most advanced Earth scientists-- hydrologists, geologists, and biogeochemists-- are studying every part and process. MARTHA CONKLIN: The water cycle is fascinating. It's something that's around us all the time. And yet, we don't really understand it. PRESENTER: How to summarize what is known about the water cycle? With two words-- flows and stores. The water cycle is a series of flows of water between various water stores, or storages-- clouds in the atmosphere. TOM HARMON: There's always a little bit of water in the atmosphere. We talk about relative humidity. It's a humid day. It's a dry day. Either way, there's water-- sometimes, a little. Sometimes, a lot. PRESENTER: There's a lot of water in the ocean-- 70% of all the water on Earth. In the ice sheets and glaciers, 2/3 of all the freshwater on Earth. In the snow packs atop mountains like the Sierra Nevada. In the Great Lakes. In rivers and streams. In reservoirs and watersheds. In wetlands. In the soil. In and on plants and trees rooted in the soil. And beneath the soil, in water tables and underground aquifers like the Ogallala-High Plains, which runs underneath parts of eight states, from South Dakota to Texas. All this storage is temporary. Water in all its forms is always in flux and always moving. And there is a name for every kind of movement in the water cycle, starting with precipitation. ANNA MICHALAK: Precipitation is the process of water falling onto the surface of the Earth. You could have precipitation in many forms-- rain, snow, hail. PRESENTER: Rain is falling water in liquid form. Snow, ice, hail, and sleet are falling water in solid or frozen form. Fog and mist, falling water in gas or vapor form. Precipitation that falls directly onto the oceans becomes part of Surface Ocean and can be churned by wave and wind action into ocean currents. Rain and snow that falls directly on rivers and streams becomes one part of stream flow. Rain that falls onto land takes a different path to the river. As does the snow and ice that falls and collects on mountain tops when temperatures warm. MARTHA CONKLIN: When snow melts, some of it runs through the snow pack and goes into small streams, tributaries that feed into large rivers. PRESENTER: What about that precipitation that falls on and over land? Some is intercepted by vegetation-- plants and trees. TOM HARMON: Like you might imagine, someone in the game of football intercepting a pass, these are raindrops trying to come to the ground and the leaves on the tree intercept them before they hit the ground. PRESENTER: And the precipitation that does hit the ground, it can run off if the ground is hardscape, covered with asphalt or concrete, or if the soil is too wet or saturated to absorb more water, like an over-soaked sponge. Otherwise, precipitation infiltrates the soil surface, percolates into the ground. TOM HARMON: Think of it as the water percolating through your coffee grounds in the morning. Gravity continues to pull it downwards so it will move through. PRESENTER: Through the topsoil. Into spaces between soil and rock particles. Down to bedrock and further into fractures. Into deep underground aquifers. Even ground water here is moving sideways or laterally, discharging toward a river, lake, or the sea. Generally, the deeper the flow, the slower the flow. MARTHA CONKLIN: Some of that fractured water might take a very long time-- thousands to millions of years-- to get out. PRESENTER: And how does water get back out into the atmosphere? It evaporates. It's turned from a liquid into a gas or vapor, by the heat of the sun. ANNA MICHALAK: If you put a bit of water into a bowl and you set it outside on a sunny day, it's going to disappear. It's still water, it's just in the form of a gas rather than the form of a liquid. PRESENTER: Water evaporates from every wet surface, even from wet air. Some rain and snow evaporates into the air while falling. Water evaporates through our respiration and perspiration. And from plants through transpiration. Trans means through or across. Plant roots draw up groundwater. And plants pull that water up through their stems into the leaves and then, release them back out through evapotranspiration. Evapotranspiration-- a spelling bee worthy term for evaporation from soil and water surfaces, plus transpiration from plants. Evaporated water molecules are tiny enough to flow into the air. Mix with smoke and dirt particles in the atmosphere. Cool. Condense into visible masses of water vapor-- clouds. Winds move clouds into colder air, water droplets collide and merge, grow bigger and heavier until they are so heavy, they fall again, as rain or snow, sleet or hail. Precipitation, collection, runoff, interception, infiltration, percolation, discharge, transpiration, evaporation, condensation-- the water cycle.

This quite general and deceivingly simple concept of water cycle has a number of limitations which are important to understand:

  • Capacities of the reservoirs vary dramatically.
  • Flow rates between the reservoirs vary dramatically (for example replenishment of a surface stream via precipitation can take days, while replenishment of a deep aquifer may take decades).
  • This concept does not directly reflect possible delays or discontinuities.
  • Cycle kinetics depends on climate, time of the year, and geographic location.
  • This concept does not portray fluctuations in storage zones.

Check out this website!

To add some quantitative information to the picture, please take a look at the US Geological Survey (USGS) website.

If you scroll down and click on any of the components of the water cycle, you will get comprehensive information on that reservoir. Note the dramatic difference in water capacity of different reservoirs. Try to remember at least the order of magnitude of the specific water reserves since such quantitative perception can be quite useful in sustainability analysis.

Annual evaporation from the ocean is about 80,000 cubic miles versus 15,000 cubic miles from the land. Given the amounts of water evaporated and precipitated are almost equal, the total amount of water exchanged between the atmosphere and the earth surface is about 95,000 cubic miles. Out of the water evaporated and then returned by rain storms, 24,000 cubic miles fall on land as precipitation. The average annual precipitation over the land is 26 inches, but it is not evenly distributed. Arid locations may get under 1 inch of precipitation whereas some others can get more than 400 inches. The total annual precipitation in the United States is about 30 inches per year, which accounts for about 4300 billion gallons per day. The total water flow from surface and subsurface sources is about 8.5 inches per year, i.e., about 1200 billion gallons a day. This amount is available for human use, including domestic, industrial, agricultural, and recreational use. Considering that the difference between precipitation and stream flow is -21.5 inches per year (3100 billion gallons per day), this amount is assumed to return to the atmosphere (through evaporation and transpiration). This returned volume roughly accounts for 70 % of the total water supply. [Source: USDA, 2001]

In nature, the hydrological cycle is well-balanced, and fluctuations of environmental water stocks are reversible. But when some of the parts of the system are interfered, resilience of the system may be jeopardized. This can happen when the anthropogenic water consumption cycle is plugged in to the natural water cycle. The main troubles currently experienced because of mismatch of the anthropogenic and natural cycles include:

  • groundwater depletion;
  • chemical pollution of surface waters and groundwaters;
  • lake drying;
  • droughts;
  • desertification;
  • eutrophication
  • loss of habitat
  • water and food shortages.

While the above-listed factors may have acute local effect, recent research also shows that large-scale hydraulic engineering produces global-scale impact on the earth's water cycle, raising the global sea level.

Reading Assignment:

Read through the following article that discusses the main man-made factors that affect the natural hydrological balance. While you are welcome to read the whole article, put the main focus on Table 1, which quantifies those effects, and sections on "Major classes of water engineering" and "Impacts of Human control...", which explain the specific mechanisms within the cycle.

Journal article: Vorosmarty, C.J., Sahagian, D., Anthropogenic Disturbance of the Terrestrial Water Cycle, BioScience, vol. 50, pp.753-763. (Full article can be accessed via Library e-Reserves in Canvas.)

Some of the things to reflect on in this reading:

  • Water management is closely dependent on soil management: overuse of soil, deforestation increase storm water runoff, decreasing absorption to soil and hence decreasing the continental water storage. Water does not have a chance to return to aquifers.
  • While no net losses occur in the water cycle ("closed" system), loss of continental (fresh) water from continents to the ocean (salt water), results in shrinking of the usable water reservoirs - e.g., depletion of aquifers.
  • Irrigation accounts for over 80% of all 'irretrievable' water consumption. This constitutes ~56% of all water withdrawn from the natural sources in some areas. Loss of water during irrigation is due to intense evaporation and plant uptake.
  • Large artificial reservoirs (dammed rivers) have high evaporation rates (compared to unmodified rivers) and therefore can reduce the continental runoff. At the same time, they cause water transfer to the atmosphere and to the groundwater storage.

The idea of sustainability in water management implies matching the natural water cycle and technical (anthropogenic) water use cycle together with minimal damage and maximum mutual support. A new approach to integrated managing water resources is known as total water cycle management, where water supply, stormwater, and wastewater are all considered during the design process.

The diagram in Figure 6.1. presents the water cycle in terms of stocks and flows. It illustrates the connections between different natural processes and reservoirs and also introduces the anthropogenic water paths into the system. The diagram is quite busy, so it would be useful to walk through it step by step. The video embedded below provides commentary to different parts of the diagram and also shows the links where water-treatment technologies must be applied to provide compatibility between the environmental and anthropogenic spheres. While watching, you may need to switch to 'full-screen' and HD quality setting to better see smaller details.

Click on the image to view the large version

Contact the instructor if you have difficulty viewing this image
Figure 6.1. A flow diagram matching the natural and human-controlled water cycles (click on image to open the large version). The boxes show different water reservoirs - natural (blue) and human-made (peach). The circles denote fluxes between the reservoirs. Blue arrows show the water motion within the natural hydrological cycle; red arrows show the water motion between the environmental and technical reservoirs.
Credit: Mark Fedkin

As you can see in the diagram in Figure 6.1, the boundaries between the natural and human-controlled water systems are where the sustainable water treatment technologies should come into action. The bottom line is that the role sustainable water technology is to reconcile the natural and anthropogenic cycles and to alleviate mutual harm and system misbalance.

The following list gives you some examples of possible actions that help to keep combined water system sustainable (can vary with location):

Water regime management

  • keep the aquifer levels within appropriate range
  • prevent flood damage in developed areas
  • prevent excessive erosion

Water quality

  • minimize the export of pollutants to surface water or groundwater
  • minimize waterborne sediment loading
  • minimize pollution from sewage protect existing vegetation

Water conservation

  • control water extraction and use
  • promote the use of rainwater and stormwater where such use does not adversely affect existing environmental values
  • promote the reuse of wastewater effluent
  • reduce irrigation requirements
  • promote opportunities for localized supply

Water value

  • enhance water related environmental values
  • enhance water related recreational and cultural values
  • add value while minimizing development costs

Many of these actions require efficient technologies of water control and water treatment. The following sections of this lesson provide you with some examples and technical details on current practice of water treatment and prospective technologies for the future.

Check Your Understanding

Which of the following continental water storage reserves has the largest global capacity?

Soil water
Glaciers, snow
Biological water

Click for answer.

Glaciers and snow account for 5,773,000 cubic miles; second largest - groundwater accounts for 5,614,000 cubic miles

Check Your Understanding

What processes in the water cycle are responsible for depletion of continental water storage?

Click for answer.

increased evaporation (often observed in artificial reservoirs and on irrigated areas); increased surface runoff (often caused by deforestation, urbanization, and soil damage)