Precipitation and the Water Cycle

Water covers three-quarters of the Earth’s surface, and the oceans contain 97% of the water available at and near Earth’s surface. Evaporation and precipitation are the two processes that move water from the oceans to the other reservoirs. Evaporation from the ocean’s surface allows water vapor to form clouds and precipitation over land and oceans. Wind transports water vapor over land, creating precipitation that falls in liquid and solid forms. The liquid form flows in rivers and into the soil and rocks, eventually returning to the oceans. Snow and ice that last all year long on land store a sizeable amount of water from returning from the ocean for long periods of time.

The diagram from NASA JPL (right) illustrates how water moves between various reservoirs. How long a water molecule stays in a reservoir is its residence time. Water may be locked up in one reservoir for hundreds of thousands of years (there is ice in Antarctica over 400,000 years old). For example, humans living in dry regions are removing underground water thousands of years old, and it will take thousands of years to replenish by natural processes.

Matter Cycles

Earth systems involve the interconnection of energy flows, matter cycles, and life webs.  The water cycle is the first of the matter cycles to be explored since it plays such significant roles in Earth’s energy flows and life webs, as well as impacting many of the other matter cycles.

As matter moves through its reservoirs, it may change phase, which is a physical change, or it may change chemically.  In the water cycle, water molecules change phase only, whereas, in the carbon cycle, carbon goes through many chemical transformations, such as becoming carbon dioxide, fossil fuel (oil, gas, or coal), limestone, organic molecules in plants and animals, etc.

Note that the chemical reaction of water in photosynthesis has not been included in the water cycle. During photosynthesis, water’s hydrogen is incorporated into carbohydrates and its oxygen is released into the atmosphere. During plant and animal respiration, carbohydrates are broken down into water and carbon dioxide. These processes will be explored further in the carbon and oxygen cycles.

Water Cycle Dice Game

This seemingly simple dice game illustrates the many processes that move water between its primary reservoirs and how long it remains there. Residence time is vital to consider when processes are affecting matter cycles. Ice can store water for hundreds of thousands of years. When polar climates warm and ice melts, it takes a long time by a human’s perspective for it to form again when the climate cools again. The poles experience some of the lowest annual precipitation totals on Earth, so it takes centuries to create ice sheets.

Note: The website for the water cycle dice game above is a valuable resource for teachers on how to introduce and run the activity in a classroom.

Water Cycle Web App

The water cycle dice game provides an excellent introduction to how water moves through the various reservoirs. Still, one would need to play many, many rounds, keep track of the data manually, and then create graphs and statistics to see the connections between the rates and scales or the process moving water and the reservoirs.  To make this easier, use the Water Cycle web app.

You may use it as the dice game by selecting one step/run or change it to see the impact of averaging many steps per run. You may run the model up to 50,000 steps before needing to reset the simulation.

Explore the eight visualizations of the data (examples of each follow). This supports changing one’s perspective to see new patterns and relationships.  There is an introductory diagram of the water cycle, text describing the current location of the molecule and how it got there, diagrams based on the statistics of what has occurred to the water molecule up to that point, and a system view how the molecule has moved.

Click the button to simulate the water cycle processes.  The following images are screenshots from the various visualizations of how a water molecule moves through the nine major water reservoirs.

Text describing the current location of the water molecule in the water cycle, how it got there, and where it could go next.

Visualization of the frequency the water molecule was stored in each reservoir.

Visualization of the frequency of the processes moving water between reservoirs.

Visualization of the frequency the water molecule was stored in each reservoir and how often it remained once it arrived in the reservoir.

Visualization of the percentages of the processes that brought water into the reservoir.

Visualization of the percentages of the processes that removed water from the reservoir.

Water Cycle Model using Google Sheets

The water cycle dice game and app above is based on a random chain of dependencies, also called a Markov chain model. For example, a water molecule in the ocean has only two possible outcomes during the next cycle: remain in the ocean or evaporate into the atmosphere. Water molecules in the atmosphere have different possibilities: either remain in the air in the form of a cloud or precipitate onto glaciers, lakes, soil, or into the ocean.  Once we identify the probabilities of how a molecule would move between the nine reservoirs, we can create a Markov chain model to study how water moves through the water cycle involving the nine largest reservoirs.

We need to identify the probability that the molecule moves to one of the other eight reservoirs or stays put for each reservoir.  This means we create a 9×9 grid of probabilities, some of which indicate zero chance of water moving between two specific reservoirs.  Ocean water has only two non-zero probabilities, and the remaining seven are zero since water can’t leave the ocean and go directly to these reservoirs.

How Good is this Model?

When the 6-sided dice Markov chain model of the water cycle is run, the water distributes into a steady-state, so a good way to check the model’s validity is to compare to the amount of water in each of the reservoirs shown near the top of this page.  This version captures a rough ranking of the largest to smallest reservoirs, but it is quite different from the water distribution on Earth’s surface today.

What happens if you use dice with more sides?  Use an n-sided dice version (the current version uses 200 sides, but you may change it to any number you like).  Use the present-day water distribution in our reservoirs to find the best number of opportunities water may change between reservoirs.

Suggested Experiments

1. Does it make a difference how the water is initially distributed in the reservoirs?  The model available on the website has all of the water starting in the atmosphere, which may have been how the water was distributed on a very hot Earth shortly after it formed.
2. What was the water cycle like before plants and animals evolved?
3. How does the water cycle change when the Earth is too warm for glaciers?
4. How are humans impacting the global water cycle?

Open the n-sided dice model of the water cycle to see a version of experiments 1-3.  See if you can improve upon them!  Use the data that follows to begin exploring the human impact on the water cycle.

Human Impact on the Water Cycle

Like most living organisms, humans need water to support their cell’s many functions that keep them alive.  But humans also use water in food production, transportation, many of our technologies, recreation, and even a repository of our waste. Humans use water on such a large scale that scientists are now examining how the ‘human water cycle‘ impacts the natural water cycle.

To meet our growing demand for water for humans:

• Currently using nearly 50,000 dams to create hydroelectric power.  These disrupt rivers, create artificial and temporary lakes, and alter groundwater exchange.
• Altered landscapes to create homes, industrial buildings, and transportation infrastructure that alter runoff from the soil, groundwater recharge, and river and lake inflow.
• Removed forests and created agricultural fields that have changed transpiration rates.
• Polluted our oceans, rivers, lakes, soils, and groundwater, making it difficult for many to acquire clean water for drinking.
• Human-induced climate change is altering evaporation and precipitation rates globally, changing the natural distribution patterns on Earth.

Use the concept map above to predict how a change in any of the reservoirs or processes moving water between reservoirs would impact the water cycle.