Teaching about the water cycle can be made more realistic and valuable for students by incorporating what they know about water-where it comes from, what happens to it after they use it, and what problems are associated with its use. Watersheds, the land area draining into a single body of water, can be considered a basic unit of the landscape that determines water availability, movement, and quality. When students study watersheds, they learn in a personal way about the importance of water, and how land use affects surface and groundwater.
When we think about the water cycle, most of us remember the diagram we were taught in third grade. Arrows move water from one part of a stylized world to another, from alpine peaks into the big, blue ocean and students learn terms like condensation, evaporation, and precipitation. But learning the water cycle in this traditional manner does not incorporate the ways in which humans both rely on and affect their watershed. To help students feel connected to their place and invested in its future, we believe they need to understand ecological processes at the local level. By emphasizing place-based learning, students investigate topics including where drinking water comes from, how much drinking water is used, and where waste water is treated through science inquiry while building an understanding of their local environment.
The Local Water Cycle
We were inspired to develop this curriculum by a review article written by one of our scientists, David Strayer, and his colleague, David Dudgeon, on freshwater biodiversity conservation (Strayer & Dudgeon, 2010). We were struck by the disconnect between the fact that freshwater ecosystems, and thus the water cycle, are being degraded worldwide, and how students are being taught the oversimplified, generic water cycle without recognition of the challenges freshwater systems face. We believe that recognizing water’s pathways might better be depicted as a web, where water can “branch” at any “stop” along the cycle, and incorporating the ways in which humans have altered the traditional cycle.
We have built almost a million dams on the world’s rivers and streams (Jackson et al. 2001). These dams allow us to prevent floods, generate hydroelectricity, and capture water for farms and cities, but they also have had immense effects on the water cycle by modifying the flow of surface water in rivers. Because of dams, water now takes twice as long to reach the ocean from the time that it enters a stream channel as it did naturally, affecting ecosystem processes such as fish reproduction along the way (Vörösmarty and Sahagian 2000). Some rivers, such as the Colorado, regularly don’t reach the ocean anymore (Waterman, 2012). Thus, some of the dammed water breaks from its “normal” path to the sea by evaporating, infiltrating, lingering in reservoirs or being moved and then used by people. We also move water across watersheds in unnatural ways - for instance, New York City moves 390 billion gallons of water a year from watersheds in upstate New York - this amount exceeds the amount of precipitation that lands on New York City each year by more than 50%. Similarly large cross-basin transfers support large areas of irrigated agriculture in Arizona and Southern California, parts of Africa and Asia, and the Middle East.
Evaporation and transpiration=evapotranspiration
Plant transpiration (the evaporation of water from plants through its leaves) often is underestimated in textbook depictions of the water cycle – a mature tree, for example, transpires as much as 40 gallons of water on a summer day! However, humans have altered global vegetation and subsequently altered the path of water dramatically. About half of the nearly 800 trillion gallons of water used each year worldwide in irrigation is sent up to the atmosphere via evapotranspiration from irrigated fields. Much of this water would otherwise have stayed underground in ancient aquifers, or would have been part of normal stream flow (Jackson et al. 2001).
Runoff, Infiltration, and Ground Water
Anything that changes the soil will likely change how it absorbs water. The most obvious of these activities is the construction of impermeable surfaces such as roads, roofs, and parking lots. Water that falls on such surfaces does not soak in, but instead forms puddles or rapidly runs off, causing severe urban flooding and further damaging urban streams through erosion. Drainage systems redirect water to nearby water bodies instead of allowing groundwater to recharge. Lack of recharge combined with excessive withdrawals of ground water have caused many water tables to fall, leading springs and small streams fed by ground water to disappear and contributing to salinization (Falke et al. 2010)
Human activities affect the quality, amount, and timing of precipitation reaching the land. As the global climate changes, the amount and timing of precipitation that falls will change all around the world, and in some places snow will change to rain (Frumhoff et al, 2007).Acid rain is just one problem that continues to plague the world, along with numerous other ways in which people affect the quality of the water around them.
Teaching the Local Water Cycle
According to a recent survey of Environmental Literacy in the United States, a mere 13% of Americans know that only 1% of the world’s water is fresh and available for use, and only 22% of Americans know that runoff from yards, city streets, paved lots, and farm fields is the most common form of water pollution (Coyle 2005). The concepts we suggest teaching would fill these knowledge gaps for many citizens.
While the processes water goes through are similar in all places, the details of the water pathways differ significantly from place to place. Research has found that “students often portray the hydrologic cycle in the context of mountain or coastal landscapes that are common in textbooks but that are not representative of the environments where students live” (Shepardson, 2009). Knowledge of the local water cycle can increase a student’s understanding of where their own water comes from and where it goes, the factors that might affect the quality and quantity of their water, and the impacts they and their communities have on thewater cycle.
Coyle, K. J. 2005. Environmental Literacy in America: What Ten Years of
NEETF/Roper Research and Related Studies Say about Environmental
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Dickerson, D. L., and K. R. Dawkins. 2004. Eigth grade students' understandings of
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Falke, J.A., K.D. Fausch, R. Magelky, A. Aldred, D.S. Durnford, L.K. Riley, and R. Oad. 2010. The role of groundwater pumping and drought in shaping ecological futures for stream fishes in a dryland river basin of the western Great Plains, USA. Ecohydrology, 4(5):682-697.
Frumhoff, P.C., McCarthy, J.J., Melillo, J.M., Moser, S.C, and D.J. Wuebbles. 2007. Confronting Climate Change in the U.S. Northeast: Science, Impacts, and Solutions. Synthesis report of the Northeast Climate Impacts Assessment (NECIA). Cambridge, MA: Union of Concerned Scientists (UCS).
Jackson, R. B., S. R. Carpenter, C. N.Dahm, D. M. McKnight, R. J.Naiman, S. L. Postel, and S. W. Running. 2001. Water in a changing world. Ecological Applications 11: 1027–1045.
Shepardson, D. P. , B. Wee, M. Priddy , L. Schellenberger, and J. Harbor. 2009. Water transformation and storage in the mountains and at the coast: Midwest students' disconnected conceptions of the hydrologic cycle. International Journal of Science Education, 31(11): 1447-1471
Strayer, D. and D. Dudgeon. 2010. Freshwater biodiversity conservation: recent progress and future challenges. Journal of the North American Benthological Society, 29(1): 344-358. www.bioone.org/doi/pdf/10.1899/08-171.1
Vörösmarty, C.J., and D. Sahagian. 2000. Anthropogenic disturbance of the terrestrial water cycle. BioScience 50: 753-765.
Waterman, J. 2012. Where the Colorado runs dry. New York Times.