05011 The Hydrological Cycle
My fake plants died because I did not pretend to water them. — Mitch Hedberg
While a person doesn’t have to become a scientist to practice permaculture design, it helps if folks understand some basic scientific concepts. This document gives a basic lay person’s discussion of the hydrological cycle.
Seventy percent of the surface of planet Earth is water, but 97.5% of those waters are salty. Only 2.5% of the planet’s water is fresh water. Most of the planet’s fresh water — 67.8% — is ice and permanent snow in the Arctic, Antarctic, and mountainous regions. This is one of the reasons why the melting of ice and permanent snow in these regions is a critical problem.
About 30% of our fresh water is ground water, located under the surface of the Earth.
Only 1/4th of 1% of the planet’s fresh water is in lakes, streams, ponds, and rivers. That is the water we can most easily access for human purposes. Note that this is not 1/4 of 1% of 100% of the water endowment of the earth. It is 1/4 of 1% of the 2.5% of the planet’s water that is fresh water.
Despite its apparent abundance, from the viewpoint of “water available for human uses,” water is a scarce resource.
Water exists in three forms —
- liquid,
- solid (ice, snow, sleet, hail),
- vapor.
While the total amount of water in, on, and around the planet (this is known as the hydrosphere) remains the same, the water is constantly in motion. It changes phases between liquid, solid, and vapor. It moves by winds through the air, currents in bodies of water and rivers/creeks/streams, and by human intervention.
The hydrological cycle is the process that moves water around the planet and among its various phases.
It is powered by solar energy.
As the sun shines upon water, it warms up and changes phase from liquid to vapor via evaporation. About 80% of evaporation is from oceans, and 20% from lakes, rivers, creeks, streams, etc.
Water can go directly from the solid state (ice) to vapor. We refer to this as sublimation.
Plants evaporate water directly into the atmosphere. We know this as transpiration.
The water vapor rises, carried upward by air currents. As it rises, the temperature cools, and the water vapor begins to condense back into droplets of solid water. These droplets transport around in the atmosphere by the winds. We often see this process in the form of clouds. Most of the water vapor, however, is invisible to us. Water vapor is the third most abundant gas in the atmosphere.
The droplets collide with each other, grow bigger, and eventually fall back to the Earth’s surface as precipitation. This can be rain, snow, sleet, or hail, depending on local conditions. Some precipitation evaporates as it falls and never makes it to the ground.
When the water hits the ground, one of two things happens:
Some of the water is run-off. It does not soak into the ground. It instead flows into lakes, streams, rivers, creeks, wetlands, etc. This water eventually finds its way into the ocean, some of it evaporating or soaking into the ground as it flows.
Precipitation that soaks into the ground, if not taken up and used by plants, becomes groundwater. Groundwater is the primary source for surface waters like lakes and springs. Water will soak into the ground until it reaches an impermeable layer of rock or clay. It will travel laterally along the surface of the barrier. We refer to these areas as aquifers. Aquifers feed lakes and ponds on the surface and come to the surface as natural springs and wells drilled by humans.
Underground aquifers have a recharge rate. This refers to the balance between water being withdrawn into lakes and springs by natural processes, water withdrawn by pumping for human uses, and precipitation soaking into the ground and refilling the aquifers. In many areas of the planet, we deplete underground aquifers much faster than they are being recharged. In effect, we mine ancient waters that cannot be replenished as fast as we use them.
The hydrological cycle is important for purifying water and thus making it available to life. Water evaporates as pure water with a neutral pH (pH is a measure of acidity or alkalinity of a substance). Once into the atmosphere, it reacts with carbon dioxide in the atmosphere and develops a slightly acid pH of about 5.7. Lightening discharges may react with water vapor to form nitric acid. These are natural processes and the Earth’s biosphere evolved in conjunction with these processes. In fact, the nitric acid that results from lightening strikes in water vapor is an important source of nitrogen for plant life on the ground.
With the beginning of the Industrial Revolution, additional pollutants began to be released in huge quantities into the atmosphere. People reported the effects of acid rain on the stone of buildings and outdoor statuary in the British Isles as early as the 17th century. In the 1850s, scientists established the connection between acid rain and atmospheric pollution in England. In the 1960s, as the effects of acid rain became more widely reported in the press, more science was brought to bear on the subject. Dead lakes and forests killed by acid rain were the subjects of extensive research.
The major contributor to acid rain is sulfur dioxide emissions from coal-burning power plants. The United States began controlling such emissions in the 1990s.
As a result of these controls, sulfur dioxide emissions fell by 40%, and acid rain decreased 65% in the United States.
But it has not been eliminated.
Part of the problem is that sulfur dioxide comes natural sources, not controllable by humans, such as volcanoes. Emissions driven by human activity, however, make the problem much worse.
Acid rain causes major problems for aquatic and plant life. At a pH of less than 5, most fish eggs will not hatch. Even lower pH levels can kill adult fish, thus greatly reducing biodiversity. As biodiversity reduces, resilience declines.
Acid rain harms forests and plant life by changing the pH of soil, which hurts the micro fauna and micro flora in the soils upon which life depends.
Understanding the hydrological cycle is basic to an understanding of the importance of water for all life on the planet. Humanity has been entirely too reckless with the hydrological cycle in the past. Going forward, we must better learn how to care for the planet so that we can care for people.
Scientific literacy is important for those of us who plan to care for people, care for the planet, and have a care for the future.
Going forward into the future, water issues will become as critical and problematic as energy is in the present.
The catastrophic effects on Earth’s biosphere due to human activities since the inception of the industrial era now imperil all life. Conservation of resources and environmentalism alone are not adequate to the task.
A transformation of fundamental aspects of consciousness that will drive actual changes in our behaviors and ways and manners of living is urgently required to halt and reverse this destructive process.