Core Concepts
This article dives into how the water cycle works. It explores the chemical and physical processes that drive the phase changes we experience everyday.
Introduction
Water is so important for the entire world. We need water for survival. In fact, you may have already drank some today. Beyond just our health, water is essential for many things we rely on such as energy production, climate regulation, agriculture, and ecosystem health. Water connects everything from the earth to the sky. Despite being abundant, freshwater is actually limited in some regions. Its availability is determined by a balance of processes that help replenish the surface water and groundwater sources. Some of these you may already be familiar with such as evaporation, precipitation, and condensation.
The Water Molecule
The structure of water is simple but powerful. It is comprised of one oxygen and two hydrogen molecules in a bent shape. Water is also a polar molecule because of the electronegativity difference between the oxygen and the hydrogen. One end of the molecule has a positive charge (hydrogen) and one end has a negative charge (oxygen). Water’s polarity enables it to dissolve many substances. It is known as the “universal solvent”. This polarity also allows it to form hydrogen bonds where the hydrogen of one molecule bonds to the oxygen of another.
These hydrogen bonds are responsible for the cohesion and adhesion forces water exhibits. Cohesion is the attraction of water molecules to other water molecules. Adhesion is the attraction of water molecules to other surfaces. Adhesive forces are a big reason why water travels up plant stems.
The Thermodynamics of the Water Cycle
Water cycles through solid, liquid, and gas phases based on energy transfer. This is based on the absorption or release of heat. For example, evaporation is an endothermic reaction because the water absorbs a significant amount of heat to turn into vapor. Let’s walk through exactly how this happens, water’s specific heat capacity determines how much energy is needed to raise the temperature of one gram of water by one degree. Water has a fairly high specific heat capacity of 4.184 Joules per gram per degree Celsius. It can absorb a lot of heat when compared to other substances. This property of water is often why oceans and lakes heat up and cool down faster than the outside air. However, this just represents water changing temperature. To represent water changing phases in evaporation and condensation, we must look at the latent heat of vaporization. The latent heat of vaporization of water is 2257 Joules per gram. It represents the amount of energy that is absorbed by water during evaporation without a change in temperature and is much higher than the specific heat capacity. The same amount of energy is released during condensation. Condensation is exothermic because the water releases considerable heat to turn from a vapor to liquid. On the other hand, for melting and freezing we must look at water’s latent heat of fusion which is 334 Joules per gram. So, how much heat would it require to evaporate 1 liter of water? Well, 1 liter of water is 1000 grams. So, if we multiply 1000 grams by 2257 Joules we need 2.26 million Joules! This is enough energy to power a standard 60 watt lightbulb for over 10 hours!
The temperature where these phase changes occur not only depends on energy input but it also on atmospheric pressure. While on earth we typically experience stable pressures around 1 atm, even settle changes in pressure affect how and when condensation or freezing occur. The thermodynamics behind the water cycle help shape the weather, mitigate heat, change landscapes, and sustain life. Let’s take a deeper look at how these pressure and energy balances affect the weather we experience daily.
The Phases of the Water Cycle
There are nine processes of the water cycle this article will discuss: evaporation, transpiration, condensation, freezing, melting, precipitation, infiltration, runoff, sublimation, and deposition. These processes are often driven by changes in the physical state (also known as the phase) of water. Water can exist as a solid, liquid, or gas. So, many of these processes represents a phase change of the water. Phase changes of water are mainly caused by two factors: pressure and energy. The energy is in the form of heat energy (thermal energy).
Evaporation and Transpiration
Evaporation is when water undergoes a phase change from a liquid to a gas (water vapor). It is an endothermic process that requires an input of energy. Evaporation cools surfaces of lakes, soils, and even our skin. Water can evaporate at any temperature between its freezing point (0 degrees Celsius) to its boiling point (100 degrees Celsius). It happens when something loses moisture. Evaporation can happen from natural surfaces like soil and water bodies. It can also happen in urban environments such as off of roads and sidewalks. Transpiration is evaporation specifically from plant surfaces such as leaves, stems, and flowers. Often times, evaporation and transpiration are combined to represent a single process: evapotranspiration. Evapotranspiration represents the combined process of water moving from the earth’s surface to the atmosphere.
Evaporation and transpiration are a source of water vapor that fuels cloud formation and even storms. The rate of evaporation increases with increasing land temperatures, this can create a feedback loop that intensifies droughts and storm patterns.
Condensation and Cloud Formation
The opposite of evaporation is condensation which is the phase change of water from a gas to a liquid. This is an exothermic process that releases the same heat absorbed during evaporation. You may have noticed this on the outside of a glass with ice or on a cold window. Condensation allows for the formation of clouds. This heat release warms the surrounding air which can lead to instability. Condensation forms the basis for hurricanes and thunderstorms through its production of warm air. This positive feedback loop creates strong updrafts and can lead to more intense storms. Hurricanes can produce up to 6.0 x 1014 watts per day. This is 200 times the world’s electricity generating capacity!
Freezing and Melting
During the winter, water often undergoes a phase change called freezing. It goes from a liquid to a solid. It is an exothermic process that releases heat. This freezing produces what we all know as snow. Freezing also happens in our freezers to produce ice cubes. Water freezes at the freezing point (0 degrees Celsius). The freezing point is where the solid and liquid phases are in equilibrium. After water freezes, it changes back into a liquid through melting. Melting is an endothermic process that absorbs heat. Freezing helps snow formation while melting events influence runoff and flood risks.
Precipitation and Runoff
While not considered phase changes, precipitation (rainfall) and runoff are key processes that happen as a result of phase changes. Condensation allows for water droplets to form in the atmosphere. When these droplets get large enough, they fall to the surface. The exact type of precipitation you get depends on the vertical temperature and pressure gradient of the atmosphere. For example, sometimes during winter you will get freezing rain that turns into snow or sleet. As precipitation falls, it can come into contact with air of different temperatures or pressures causing it to melt, refreeze, or stay frozen the entire time!
After the clouds start to precipitate, the rainwater reaches the earth’s surface. Some of it will begin entering the ground through a process called infiltration. However, when the ground becomes saturated or if the precipitation rate exceeds the infiltration capacity of the surface, excess water will start to flow over the surface. This is called runoff. Certain landscapes like steep slopes are more prone to runoff than others. A good example of this are steep mountain slopes.
Sublimation and Deposition
Sublimation and deposition are unique because they are direct phase jumps. Sublimation is an endothermic process and needs large amounts of energy to “skip” the liquid phase. It is when a solid goes to a gas. Sublimation is more common in high altitude settings such as mountain tops covered in snow and ice or glaciers. Snowy mountain tops can lose snow or ice in the form of water vapor. This water vapor gets carried away by the wind. Deposition is just the opposite, it is when a gas turns directly into a solid. The process of deposition is highly exothermic. Frost formation on a cold morning can be an example of deposition when humid air comes into contact with a cold surface.
How Humans have Changed the Water Cycle
Humans have greatly altered the movement of water and the energy balance of the water cycle. Land use changes, fossil fuel burning, groundwater pumping, and pollution as a whole have influenced clean water availability and accessibility. Developing land into urban areas changes the water cycle because it increases the amount of impervious surfaces like pavement and concrete. Water has a hard time infiltrating these surfaces, so a lot of it ends up as runoff. Furthermore, these impervious surfaces can get really hot, increasing the water temperature. Additionally, converting previously forested lands into agricultural lands changes the way water infiltrates that land. Sometimes, runoff can actually increase after converting land to agriculture because of the reduced ground cover and the sowing of crops that may not be well-suited for that specific land. Agricultural lands often need to be irrigated, so there is an increasing amount of water being added to that land. This can increase evapotranspiration rates and lower water levels of nearby surface water and groundwater bodies. Now that we understand how the water cycle is an energy balance, it also makes sense on why certain practices that raise temperature can affect water phase changes. Climate change and the burning of fossil fuels are a big influencer of the water cycle. With rising global temperatures, evaporation can increase and change precipitation patterns. As a result of rising global temperatures, glaciers have also started melting at faster rates. By understanding how energy drives these phase changes, and how water cycles through the environment, we can learn how to manage water more sustainably.