The Hydrologic Cycle

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An argument can be made that water is the single most important molecule on our planet. First and foremost, it is crucial for sustaining life – you and I literally can’t live without it! It’s also a critical part of our planet’s climate system. The role of water is evident in extreme weather events like seasonal floods and droughts, and it is vital for the health of natural ecosystems and human societies. It serves as a connecting medium among the primary components of the climate system. And it’s incredibly unique in that it’s one of only a handful of substances that exist in solid, liquid, and gaseous states within the Earth’s atmosphere (and on its surface) at any one time.

While you have certainly talked about the states of water somewhere before, let’s make sure we are all on the same page. It's important to understand that water can exist in three primary phases: solid, liquid, and gas. These phases refer to the physical state that water takes under different temperature and pressure conditions. Let's explore the three phases below.

Water, with its distinctive characteristics, is abundant on Earth. It's interesting to note that over two-thirds of our planet's surface is enveloped by water, predominantly over the oceans. In fact, oceans hold approximately 97% of Earth's water, amounting to an astonishing volume – over a billion cubic kilometers. The rest, a mere three percent, is distributed among the polar ice caps, various lakes, rivers, streams, and as groundwater, which is water contained within soil, sand, or rock crevices.

You’ll notice that I only mentioned water on the surface – notoriously absent was any discussion of the atmosphere. In fact, only a very tiny amount of water (by volume) exists in the atmosphere (about 0.03 percent), and nearly all of it exists as water vapor. Still, the small fraction that exists as water vapor in the atmosphere is enough to fuel all the extreme weather we observe on Earth, including tornadoes, hurricanes, and blizzards. What little water vapor exists in the atmosphere at any given moment doesn't last for long because water is regularly changing phases, and being exchanged between the surface and the atmosphere. Remember, water vapor is a variable gas, meaning that its concentration changes in time and space, from near zero to four percent of atmospheric gases (by volume). The possible paths that water can take as it changes phases and gets transported between the surface and the atmosphere make up the hydrologic cycle (or "water cycle"), a simplified version of which is shown in the graphic below.

A simplified hydrologic cycle diagram. Water enters the atmosphere primarily through evaporation and transpiration, and is returned to the surface through precipitation.

Credit: The Water Cycle. Global Precipitation Measurment (NASA) (Public Domain).

Before we analyze the hydrologic cycle's components, we need to formally define some important processes. Water exists in these three states, and an individual water molecule frequently bounces between them -- in other words, it exists in different phases during its lifetime as a water molecule. The process of moving between these phases is known as a "phase change." For example, think about what happens when you take an ice cube out of the freezer and leave it on the counter. Over time, the ice melts into liquid water—that’s a phase change from solid to liquid. These transitions are not only fundamental to understanding how water behaves in different environments but also essential for grasping its role in the climate system. They affect things from weather patterns to life on Earth, influencing a wide range of natural processes.

I will assume you are already familiar with melting (where a solid changes to a liquid) and freezing (where a liquid changes to a solid). But the other transformations of interest are:

• Evaporation: The process by which liquid water changes to water vapor, as bonds between neighboring liquid water molecules break, and molecules escape to the air as water vapor. Watch vapor rise off asphalt after a mid-summer thunderstorm, and you are witnessing evaporation.
• Condensation: The process by which water vapor changes to liquid (the reverse of evaporation). Ever grab an ice-cold beverage out of the fridge and notice that water droplets grow on the outside of the bottle? That is water vapor from the air condensing on the bottle’s cold surface!
• Sublimation: the process by which ice changes directly to water vapor without becoming liquid first. This can be observed if you leave an ice cube tray in the freezer too long—eventually, there’s nothing left!
• Deposition: the process by which water vapor is deposited directly as ice. If you’ve ever seen cool (no pun intended!) frost patterns on a window first thing in the morning, you’ve observed deposition.

Water evaporating off a field after a summer rainstorm

Credit: n.a. “Evaporation.” National Geographic. October 19, 2023.

Water that has deposited (i.e., gone from vapor directly to solid) on a window, forming something known as "fern frost" where the deposition patterns look like little ferns!

Credit: Schnobby, CC BY-SA 3.0, via Wikimedia Commons

Remember, these are all “phase changes” where water in one phase is transformed into another through either the release or uptake of energy (more on that later). There is one more term we need to define, which isn’t technically a phase change, but is important for understanding how water on Earth’s surface can get moved into the atmosphere.

• Transpiration: The process by which plants release water vapor into the air (plants transport water from their roots to the leaves, where they "sweat," and the water evaporates into the air).

Movement of Water Through the Earth-atmosphere Cycle

Now that we understand these concepts, we can explore the movement of water through the Earth-atmosphere system. Water in its liquid form, found in lakes, streams, rivers, and oceans, evaporates into the atmosphere. This is supplemented by transpiration from plants and the evaporation of groundwater from soil, among other sources. As air ascends, some of the water vapor forms cloud droplets. When clouds become sufficiently dense, water returns to Earth as precipitation. Some of this water replenishes groundwater, while the rest flows into lakes, streams, rivers, and eventually back to the oceans, where it can evaporate again, continuing the cycle.

Evaporation is the primary way water vapor enters the atmosphere, with transpiration and sublimation contributing to a lesser extent. In the hydrologic cycle, the largest movements of water occur through evaporation and precipitation. The water quantity near the Earth's surface remains fairly stable over short time spans (like a year), indicating that global precipitation is approximately equal to global evaporation. We'll cover this in more detail in a little while.

Interestingly, once water vapor enters the atmosphere, it doesn't linger there for long. On average, it takes about 11 days for a water molecule to evaporate (or enter via transpiration or sublimation), condense into a cloud, and return to Earth as precipitation. However, water stays much longer in its liquid or solid state on Earth. A water molecule in the ocean typically remains for about 2,800 years before evaporating, and one in a glacier might stay frozen for tens of thousands of years.

This explains why a relatively small amount of water in the Earth-atmosphere system exists as water vapor: it spends a brief period in the atmosphere before returning to the Earth. Most of the water is found in oceans or ice sheets, as water molecules reside there for extended periods before evaporating. Despite this, the small fraction of water that cycles through the atmosphere as water vapor and then as precipitation significantly influences the weather. Therefore, understanding the phase changes of water, especially evaporation and condensation, is crucial in the hydrologic cycle. Let's delve deeper into these phase changes in the following sections.