Earth Cycles

The Hydrologic Cycle: Reservoirs and fluxes of water on Earth

by Anne E. Egger, Ph.D.

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Did you know that rising sea levels do not mean that the amount of water on Earth is increasing? Rather, the amount of water on Earth remains the same, but with global warming, glaciers diminish and sea levels rise. With warmer temperatures comes more evaporation, which causes an increase in precipitation and extreme weather across the planet.

As recently as 12,000 years ago, you could walk from Alaska to Siberia without having to don a wetsuit. At that time, glaciers and ice sheets covered North America down to the Great Lakes and Cape Cod, though coastal areas generally remained ice-free. These extensive ice sheets occurred at a time when sea level was very low, exposing land where water now fills the Bering Strait. In fact, throughout Earth’s history, times of extensive glaciers correlate with low sea level and times when only minor ice sheets exist (like today) correlate with high sea levels. These correlations are due to the fact that the amount of water on Earth is constant, and is divided up between reservoirs in the oceans, in the air, and on the land. In addition, Earth’s water is constantly cycling through these reservoirs in a process called the hydrologic cycle. Both of these facts together lead us to the conclusion that more water stored in ice sheets means less water in the oceans.

Earth is the only planet in our solar system with extensive liquid water – other planets are too hot or too cold, too big or too small. Though Mars appears to have had water on its surface in the past and may still harbor liquid water deep below its surface, our oceans, rivers, and rain are unique as far as we know, and they are life-sustaining. Understanding the processes and reservoirs of the hydrologic cycle is fundamental to dealing with many issues, including pollution and global climate change.

As early as 800 BCE, Homer wrote in the Iliad of the ocean "from whose deeps every river and sea, every spring and well flows," suggesting the interconnectedness of all of Earth's water. It wasn't until the 17th century, however, that the poetic notion of a finite water cycle was demonstrated in the Seine River basin by two French physicists, Edmé Mariotte and Pierre Perrault, who independently determined that the snowpack in the river's headwaters was more than sufficient to account for the river's discharge. These two studies marked the beginning of hydrology, the science of water, and also the hydrologic cycle.

Hydrologic cycle

The hydrologic cycle can be thought of as a series of reservoirs, or storage areas, and a set of processes that cause water to move between those reservoirs (see Figure 1). The largest reservoir by far is the oceans, which hold about 97% of Earth’s water. The remaining 3% is the freshwater so important to our survival, but about 78% of that is stored in ice in Antarctica and Greenland. About 21% of freshwater on Earth is groundwater, stored in sediments and rocks below the surface of Earth. The freshwater that we see in rivers, streams, lakes, and rain is less than 1% of the freshwater on Earth and less than 0.1% of all the water on Earth.

Figure 1: A depiction of the hydrologic cycle showing reservoirs and their volumes in bold and fluxes between reservoirs and their rates in italics.

Comprehension Checkpoint
More freshwater is stored in ice than is found in all other freshwater sources combined.
Correct!
Incorrect.

The ocean and the atmosphere

Water moves constantly between these reservoirs through the processes of evaporation, condensation and precipitation, surface and underground flow, and others. The driving force for the hydrologic cycle is the sun, which provides the energy needed for evaporation just as the flame of a gas stove provides the energy necessary to boil water and create steam. Water changes from a liquid state to a gaseous state as it evaporates from the oceans, lakes, streams, and soil (see our Water: Properties and Behavior module for a further explanation). Because the oceans are the largest reservoir of liquid water, that is where most evaporation occurs. The amount of water vapor in the air varies widely over time and from place to place; we feel these variations as humidity.

The presence of water vapor in the atmosphere is one of the things that makes Earth livable for us. In 1859, Irish naturalist John Tyndall began studying the thermal properties of the gases in Earth's atmosphere. He found that some gases, like carbon dioxide (CO2) and water vapor, trap heat in the atmosphere (a property commonly called the greenhouse effect), while other gases like nitrogen (N2) and argon (Ar) allow heat to escape to space. The presence of water vapor in the atmosphere helps keep surface air temperatures on Earth in a range from about -40° C to 55° C. Temperatures on planets without water vapor in the atmosphere, like Mars, stay as low as -100° C.

Once water vapor is in the air, it circulates within the atmosphere. When an air package rises and cools, the water vapor condenses back to liquid water around particulates like dust, called condensation nuclei. Initially these condensation droplets are much smaller than raindrops and are not heavy enough to fall as precipitation. These tiny water droplets create clouds. As the droplets continue to circulate within the clouds, they collide and form larger droplets, which eventually become heavy enough to fall as rain, snow, or hail. Though the amount of precipitation varies widely over Earth's surface, evaporation and precipitation are globally balanced. In other words, if evaporation increases, precipitation also increases; rising global temperature is one factor that can cause a worldwide increase in evaporation from the world’s oceans, leading to higher overall precipitation.

Since oceans cover around 70% of Earth’s surface, most precipitation falls right back into the ocean and the cycle begins again. A portion of precipitation falls on land, however, and it takes one of several paths through the hydrologic cycle. Some water is taken up by soil and plants, some runs off into streams and lakes, some percolates into the groundwater reservoir, and some falls on glaciers and accumulates as glacial ice.

Comprehension Checkpoint
What drives the hydrologic cycle?
Incorrect.
Correct!

The hydrologic cycle on land

The amount of precipitation that soaks into the soil depends on several factors: the amount and intensity of the precipitation, the prior condition of the soil, the slope of the landscape, and the presence of vegetation. These factors can interact in sometimes surprising ways – a very intense rainfall onto very dry soil, typical of the desert southwest, often will not soak into the ground at all, creating flash-flood conditions. Water that does soak in becomes available to plants through soil moisture and groundwater (see Figure 2). Plants take up water through their root systems, which mostly draw water from soil moisture; the water is then pulled up through all parts of the plant and evaporates from the surface of the leaves, a process called transpiration. Water that soaks into the soil can also continue to percolate down through the soil profile below the water table into groundwater reservoirs, called aquifers. Aquifers are often mistakenly visualized as great underground lakes; in reality, groundwater saturates the pore spaces within sediments or rocks (see Figure 2).

Figure 2: Groundwater exists below the water table, which divides unsaturated soil, rock, and sediments from saturated.

Water that doesn’t soak into the soil collects and moves across the surface as runoff, eventually flowing into streams and rivers to get back to the ocean. Precipitation that falls as snow in glacial regions takes a somewhat different journey through the water cycle, accumulating at the head of glaciers and causing them to flow slowly down valleys.

Comprehension Checkpoint
Flash-flooding can result from intense rainfall
Incorrect.
Correct!

Humans and the hydrologic cycle

The properties of water and the hydrologic cycle are largely responsible for the circulation patterns we see in the atmosphere and the oceans on Earth. Atmospheric and oceanic circulation are two of the major factors that determine the distribution of climatic zones over the Earth. Changes in the cycle or circulation can result in major climatic shifts. For example, if average global temperatures continue to increase as they have in recent decades, water that is currently trapped as ice in the polar ice sheets will melt, causing a rise in sea level. Water also expands as it gets warmer, further exacerbating sea level rise. Many heavily populated coastal areas like New Orleans, Miami, and Bangladesh will be inundated by a mere 1.5 meter increase in sea level (see Figure 3). Additionally, the acceleration of the hydrologic cycle (higher temperatures mean more evaporation and thus more precipitation) may result in more severe weather and extreme conditions. Some scientists believe that the increased frequency and severity of El Niño events in recent decades is due to the acceleration of the hydrologic cycle induced by global warming.

Figure 3: Areas in red would be flooded with a 1.5 m rise in sea level; areas in blue would be flooded by a 3.5 m rise in sea level. Image has been modified from the original from the US Environmental Protection Agency (EPA).

Even more immediately, the finitude of Earth’s freshwater resources is becoming more and more apparent. Groundwater can take thousands or millions of years to recharge naturally, and we are using these resources far faster than they are being replenished. The water table in the Ogallala Aquifer, which underlies 175,000 square miles of the US from Texas to South Dakota, is dropping at a rate of 10-60 cm per year due to extraction for irrigation. Surface waters around the world are largely contaminated by human and animal waste, most noticeably in countries like India and China, where untreated rivers provide the drinking and washing water for nearly 2 billion people. Although legislation like the Clean Water Act in the United States and water conservation practices such as the use of low-flow toilets and showerheads in parts of the world has begun to address these issues, the problems will only grow as world population increases. Every spring and well, every river and sea does indeed flow from the same source, and changes affect not just one river or lake, but the whole hydrologic cycle.

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