It’s another beautiful snowy day in the Mid-Atlantic and I have just cleared my sidewalks and driveway of eight inches of heavy snow and sleet, placing it on the adjacent lawn and flower beds. Unlike rain, which tends to run off rather quickly, these ice crystals will melt slowly and soak into the soil and provide a long drink for the lawn, garden, trees and shrubs when they re-waken in the spring.
This winter chore brings to mind the hydrologic cycle, nature’s grand scheme for recycling our planet’s most essential resource–H2O, which moves through solid, liquid and vapor phases during this process. Basically, “solar power” causes water to evaporate (and be purified by distillation) from bodies of water; it is then temporarily stored in the atmosphere as water vapor and subsequently precipitates as rain or snow. Upon contact it either washes over the earth, forming rivers, lakes and other bodies of fresh water; percolates into the earth, replenishing the groundwater; or accumulates in glaciers or polar ice caps. All of this is powered by the sun, gravity and geostrophic forces.
We humans, depending on age and sex, are composed of between 55 and 78 percent H2O. We have learned to intercept water wherever we can in the hydrologic cycle, store and distribute it, and when necessary, clean and disinfect it for life-sustaining uses.
Here‘s a brief profile of water on the Blue Planet.
- Approximately 71% of Earth’s surface is covered by water, which gives our planet its distinctive blue color.
- Earth is the only planet in the solar system that hosts flowing streams of water. Earth’s rivers and streams distribute fresh water and nutrients over the land and have historically been thoroughfares of travel and trade.
- The total amount of water on earth is estimated at 1.4 billion trillion liters, but most is too saline to be useful. The saline waters of the oceans and seas account for about 97.2% of earth’s water and polar ice about 2.15%. The fresh (non-saline) water that humans depend on account for approximately 0.65% of the water on Earth. Most fresh water resides under the earth’s surface, and this is known as groundwater.
- Annual precipitation on land amounts to about 41,000 trillion liters of H2O each year, or about one foot of water across the entire land mass. But because precipitation is unequally distributed, and in most cases arrives over relatively short period of time (e.g., monsoons, thunderstorms), humans have had to rise to the challenge of using innovative methods, such as damming rivers, managing groundwater, etc. to secure water supplies that are available when and where needed.
As a weather report notes a new winter storm path, I am mindful of water’s essential role in our survival and the many ways it touches our lives on the Blue Planet.
Fred Reiff, P.E., is retired from the Pan American Health Organization, and lives in the Washington, D.C., area.