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How Humans Affect the Water Cycle, a Complete Guide

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In part one of this two-part post on the water cycle, we learned how the water cycle makes life possible for land-dwelling plants and animals, including humans. The water cycle provides the constant source of freshwater that we need to survive. The sun powers the process of evaporation, which separates fresh water vapor form our oceans. This can fall as precipitation on land.

The water cycle diagram labeled drawing, hand-drawn with transpiration and infiltration of groundwater.
Hand drawn and labeled Water Cycle Diagram, with transpiration, runoff, infiltration, and more!

There is no doubt that the water cycle impacts our lives, as humans. Humans, however, impact the water cycle too. Our actions as a species have effects on every step of the water cycle. In this post, let us dive deeper into the ways humans are affecting the water cycle. Although it can be overwhelming to think about the numerous ways that we impact this natural cycle – and I certainly felt a little overwhelmed in doing the research to write this post – my goal here is not to dishearten anyone about water resources. Change starts with education, and ultimately requires effort on both an individual and larger scale. At the end of this post, I share some simple and inexpensive ways we can save water resources and reduce our impact on the water cycle. I also made a Saving Water Bingo game to help make learning about water resources fun for kids and to encourage simple steps in families’ daily routines to help save water!

Glaciers and Ice Sheets

        Due to human caused (“anthropogenic”) climate change, the total amount of ice in glaciers, snowcaps, and semi-permanent snowfields has decreased each year, for more than 30 years. This means that every year the amount of new glacial ice forming is less than the amount of glacial ice melting, on average, across the planet. 

         Glaciers and ice sheets are the largest storage of fresh water on the planet, making up over two thirds of all the freshwater on earth! Glacial ice also serves an important role in regulating our planets’ temperature. Compared to water or land, ice is very effective at reflecting solar radiation. Ice’s reflectiveness helps keep earth’s temperatures cool. However, as glaciers melt, less radiation is reflected, which causes warming, and consequently more glaciers melting. This sort of amplifying effect is called a positive feedback cycle

I visited Byron Glacier in Alaska, which is an example of a glacier in retreat – one that shrinks more than it grows each year

Oceans

         Saltwater currently makes up about 97.5% of the water on earth! And this amount is increasing. As glaciers, ice sheets, and semi-permanent snowfields melt, their waters often flow into the oceans, mixing with salt water. The result is sea level rise. This can cause (and already is causing) an upsetting impact on humans that live in low-lying areas near oceans, such as some island nations and coastal communities. 

         Rising ocean levels also threaten coastal wetland and estuary ecosystems. These are unique marshy areas along the coast with abundant and diverse plant and animal life. In addition to providing habitat for wildlife, coastal wetlands provide a buffer to coastal communities during weather events like floods. Also, the roots of the many plants in these wetlands help prevent erosion along coasts. Losing these unique coastal ecosystems also means losing their ecosystem services – the benefits that they provide to humans. 

The outer banks of North Carolina

Precipitation

        Climate change doesn’t just impact water in glaciers – it also impacts the water that falls from the sky as precipitation. Changes to the global climate can impact when, where, and how much precipitation falls. It can also impact whether precipitation falls as rain or snow.

Runoff

         A lot of the water we use comes from diverted runoff – meaning we change the path that water takes back to the ocean, so that it goes where we need it most. This can happen on a very small scale, such as diverting the water that runs off of our roofs into a rain barrel. Or an a very large scale – like the Grand Ditch in Colorado, which takes water from the west side of the Rocky Mountains and brings it to the Front Range, on the opposite side of the continental divide.

         Runoff begins high in the mountains where snowmelt and rain on mountain peaks start the headwaters of streams. Runoff ends when water flows into the oceans at estuaries and deltas. As humans, we have impacts the runoff process from the mountains to the oceans, and everywhere in between. Warmer winter temperatures in the mountains due to climate change may cause less winter snowpack than mountain regions experienced historically. Less winter snowpack means less springtime runoff.

         Building reservoirs is one of the most noticeable impacts that humans have on the runoff process. Reservoirs are necessary in mountain and arid regions to maintain a year-round water supply. In these areas, most of the runoff each year arrives in a fairly short time frame in the spring, as snow melts in mountains. Agriculture in these areas would not be possible without stopping saving some of that runoff to use later during the summer and fall growing seasons. Reservoirs are the way we stop and save that water. Reservoirs have great benefits to our ability to live in arid regions but have some downsides too. Storing water in a reservoir in an arid region means having that water open to the atmosphere. Consequently, over several months of storage, significant amounts of reservoir water are lost to evaporation. 

         Downstream of reservoirs, the discharge, or volume of water running through a river, will be less than normal. Downstream aquatic ecosystems can be harmed by lower flows in rivers and streams. Changing a swift, snowmelt fed river to a slow, reservoir outflow can impact the water quality and water temperature. Some fish species are adapted for life in cold, fast flowing water, so changes in their home stream habitat can cause them harm.

My rain barrel diverts runoff from my roof, which I then use to water my garden.

Infiltration

         As water travels across the earth’s surface, naturally some of it seeps into the tiny pore spaces of the underground rocks and sediments, in a process called infiltration. In cities, large areas of the ground are covered in pavement, which is less permeable than soils – it does not allow water to pass through as easily. This prevents infiltration from occurring in these areas. Consequently, flooding may occur because there is no place for the water to go so it stays on the surface. Water may also become contaminated as it flows across paved surfaces, carrying pollution into nearby lakes and streams.

Groundwater

         Groundwater provides much of the water we use in our homes, for irrigating crops, and for livestock drinking water. We get groundwater out of aquifers, underground geologic units containing water in their pore spaces. To extract the water from an aquifer to the surface, we install wells which are pumped. Pumping can have many impacts on the water cycle’s groundwater movement and storage. In some cases, pumping will lead to lowering the water table and depleting aquifers – in other words taking groundwater out of aquifers faster than water is infiltrating into aquifers. Because this type of use will eventually lead to a groundwater well running dry, it is unsustainable water use.

         Ideally, groundwater is pumped at slower, more sustainable rates, allowing time for aquifers to replenish and refill with water. Even so, pumping can have impacts on the water cycle. Any amount of pumping can change the way that water flows underground, impacting the amount of water that feeds springs and gaining streams. This can have both ecological impacts and impacts to downstream human water users. 

A groundwater monitoring well extends tens of feet below the ground and allows scientists to track changes in the water table at a location of interest

Transpiration

         Transpiration is the process of plants taking up liquid water through their roots and releasing water vapor through their leaves. Transpiration is an important part of the water cycle, moving water from the soil to the atmosphere. Humans tend to change the natural plant communities of an area – whether by replacing plants with buildings and pavement, by removing natural plant communities to grow food crops, or by introducing invasive plant species. A change in the plant community with change the amount of transpiration. 

         It seems hard to imagine that transpiration has much of an impact on the water cycle because it is a process that we cannot see – water vapor is invisible to us. However, a single tree can transpire thousands of gallons of water each year. Furthermore, the amount of water used by different plants varies significantly. Growing a high-water use crop in an arid region can deplete local water resources of an entire area! One recent example of this is in Harney county, Oregon, USA, where hay farming is a main industry, despite the difficulties of growing this high-water use crop in a naturally dry environment. Since the region’s precipitation does not provide enough water for hay to grow, groundwater is pumped to irrigate the hay fields, at a rate the aquifer cannot sustain. (see further reading and references at the bottom of this article to read more about Harney county’s water resources). 

From tiny wildflowers to towering trees, all plants must transpire water to perform photosynthesis!

Ten ways you can help save water!

Everyone needs clean, freshwater every day – to drink, clean with, and to grow food. Clean water is a human right; but something that not everyone has access to.  If you do have access to plentiful, clean water, you have the great opportunity to start considering some ways to use less water and lessen your impact on the water cycle. There are many creative ways to reduce water use while still having enough water for our basic needs. Small actions are the first steps towards lessening our impacts on the water cycle and conserving water resources. 

  1. Time your shower, then commit to decreasing your shower time by two minutes.
  2. Reuse drinking glasses to decrease the number of dishes to be washed.
  3. If emptying a water bottle, consider dumping the water out on a house plant, or using it to soak dishes, or flush the toilet, etc.
  4. Research your food – find out how much water it takes to grow the different plant and animal foods that you eat. 
  5. Inspect your home’s faucets and hoses for leaks.
  6. Consider areas of your yard where you do not really need grass, like the margins next to sidewalks – even replacing even a small area with low-water use plants, gravel or wood chips can help decrease water usage in the yard. Yard improvement projects can also be done in a way that improves your backyard’s wildlife habitat.
  7. Turn off the tap while scrubbing dishes.
  8. Turn off the tap while brushing teeth and lathering hands.
  9. Encourage your household to skip toilet flushes for #1’s.
  10. Talk to your family and friends about the importance of using less water! To help make saving water fun, I created this free, printable save water bingo game! Save the file on your computer or print and post it on your fridge – see if you can get five in a row, or better yet, every square!

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References and Further Reading

  1. Cureton, E. (2020). Water crisis puts Oregon community at a crossroads. NPR. Available: https://www.npr.org/2020/01/01/792692254/water-crisis-puts-oregon-community-at-a-crossroads
  2. Lindsey, R. (2020). Climate change: glacier mass balance. NOAA Climate.gov. Available: https://www.climate.gov/news-features/understanding-climate/climate-change-glacier-mass-balance
  3. National Oceanic and Atmospheric Administration (n.d.). Understand conserving coastal wetlands for sea level rise adaptation. Available: https://coast.noaa.gov/applyit/wetlands/understand.html
  4. NOAA Fisheries. (n.d.). How Dams Affect Water and Habitat on the West Coast. Available: https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/how-dams-affect-water-and-habitat-west-coast
  5. Plummer, C. C., Carlson, D. H., & Hammersley L. (2019) Physical Geology. New York, NY: McGraw-Hill Education. (16th ed., pp. 232 – 320).
  6. University of Colorado Boulder. (2015). Reservoir evaporation poses a big challenge for water managers in the west. Available: https://www.colorado.edu/today/2015/12/28/reservoir-evaporation-big-challenge-water-managers-west
  7. University of Maine. (2011). Loops of change: the positive feedback loops that drive climate change (part 1). Maine Climate News. Available: https://extension.umaine.edu/maineclimatenews/2011/07/06/loops-of-change-the-positive-feedback-loops-that-drive-climate-change-part-i/
  8. U.S. Geological Survey. (n.d.). Evaluating the potential benefits of permeable pavement on the quantity and quality of stormwater runoff. Available: https://www.usgs.gov/science/evaluating-potential-benefits-permeable-pavement-quantity-and-quality-stormwater-runoff
  9. U.S. Geological Survey. (n.d). Groundwater Use in the United States. Available: https://www.usgs.gov/special-topic/water-science-school/science/groundwater-use-united-states

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