During the rainy season, the streets of many cities flood in minutes; in the north of the country, on the other hand, there are dams that are almost empty. If the planet is covered in water, why is there sometimes too little and sometimes too much? To find out, we are going to follow water's journey through the Earth: the hydrosphere.
Earth's water reserves
From space the Earth looks blue, but on dry land millions of people suffer from droughts. How is the planet's water distributed, and how much of it can we use?
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Classify these bodies of water as saltwater or freshwater: the Atlantic Ocean, a glacier, a lake, a river, the Mediterranean Sea, and water from an aquifer.
Classify these bodies of water as saltwater or freshwater: the Atlantic Ocean, a glacier, a lake, a river, the Mediterranean Sea, and water from an aquifer.
In a debate on sustainability, a municipal representative states: "Given that 71 % of the Earth's surface is covered by water, the fear of water scarcity is unfounded:…
In a debate on sustainability, a municipal representative states: "Given that 71 % of the Earth's surface is covered by water, the fear of water scarcity is unfounded: water is a virtually inexhaustible resource."\na) Using data from the hydrosphere (97.5 % of the planet's water is salt water and only 2.5 % is fresh water; and of that fresh water, 69 % is frozen in glaciers and barely 0.3 % is found in accessible surface rivers and lakes), calculate what exact percentage of the planet's total water is represented by liquid surface fresh water.\nb) Based on your numerical calculation, reasonably refute the representative's statement and predict the hydrological and ecological consequences of managing a region's water resources under the assumption that available water is limitless.
Groundwater does not stay still forever: it moves slowly through the pores of the rock, always downhill. When the aquifer reaches the surface, for example on the slope of a hill or where an impermeable layer forces it out, the water flows out on its own. This natural outlet is called a spring (or "ojo de agua").
That is why the water in a spring does not "begin" there: it is rainwater that seeped in at higher elevations, traveled through the aquifer for days, months, or years, and finally emerged. Many towns in Mexico get their water from springs, and wells do the opposite: they drill into the ground to draw water out of the aquifer.
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Where does the water from a spring come from? Relate it to rainfall, infiltration, and aquifers.
Where does the water from a spring come from? Relate it to rainfall, infiltration, and aquifers.
A puddle dries up in a few hours under the Sun, but the oceans never run dry and the air never fills up with water forever. Where does the water go, and how does it come back?
In the diagram, the five processes are numbered: 1 evaporation, 2 condensation, 3 precipitation, 4 infiltration, and 5 runoff.
The water cycle, droughts, torrential rains, and cyclones
Since the Sun and temperature drive the cycle, any change in the planet's heat alters it. When the atmosphere warms, water evaporates more and warm air can carry more vapor. This has two effects at the same time:
Where too much water evaporates and it rains little, the soil dries out and droughts get longer, like those in northern Mexico.
When that vapor finally condenses, it can fall all at once as torrential rain.
Tropical cyclones (hurricanes in the Atlantic and the northeastern Pacific) feed on the vapor rising from a very warm sea: the warmer the water, the more energy and rain they can have. That is why scientists link global warming, caused largely by human activity, to a more extreme water cycle: longer droughts and more intense downpours.
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The diagram shows the water cycle with five numbered processes. Name the processes labelled 1 to 5 and briefly describe what happens in each.
The diagram shows the water cycle with five numbered processes. Name the processes labelled 1 to 5 and briefly describe what happens in each.
After the construction of a housing development with paved streets and impermeable roofs on a forested hillside, technicians find that during heavy rainfall, it takes…
After the construction of a housing development with paved streets and impermeable roofs on a forested hillside, technicians find that during heavy rainfall, it takes only 30 minutes for water to reach the river instead of the usual 4 hours, and that the peak flow rate has tripled, flooding the town located downstream.\na) Justify this hydrological change by analyzing how paving the ground alters infiltration, evapotranspiration, and surface runoff.\nb) To prevent further flooding, a company proposes channeling the river with concrete in a straight line to the river mouth. Critically evaluate this proposal by predicting what will happen in areas located even further downstream, and design two alternative measures based on sustainable drainage or environmental restoration to promote aquifer recharge.
At the top of a permeable mountain, an accidental spill of a biodegradable, water-soluble dye occurs. Weeks later, dyed water is detected in an agricultural well in the…
At the top of a permeable mountain, an accidental spill of a biodegradable, water-soluble dye occurs. Weeks later, dyed water is detected in an agricultural well in the valley and, months later, at the mouth of a river in the ocean, without the dye having been observed in the surface torrents on the hillside.\na) Reasonably reconstruct the path followed by the dyed water from the summit to the sea, naming and explaining the processes of the water cycle involved (infiltration, groundwater flow, spring emergence or discharge, and surface runoff).\nb) Explain through which physical processes of the water cycle that same seawater will be able to return to the mountain peak, and justify whether the dye would travel with it in water vapor and precipitation.
Back to the beginning: water is sometimes too plentiful and sometimes too scarce because the planet has a lot of water, but almost all of it is salty or frozen, and its circulation depends on the terrain and the climate: where the soil is impermeable the water runs off quickly and causes floods, and where it is neither infiltrated nor stored, it is lacking in the dry season.
Summary
Let's practice! 3 medium exercises in this section