© J. Dirk Nies, Ph.D.
Water flows. It’s constantly on the move. Responding to gravity and heat, water circulates from land and sea to clouds and back again. Evaporation, condensation, precipitation along with the movement of water on and under the surface of Earth comprise the hydrological cycle–the continuous global processing of water powered by energy from the sun.
The world’s supply of fresh water depends upon this progression of water from earth to sky and back again. Evaporation purifies water. As water enters the atmosphere as a vapor, the gaseous molecules of H2O leave impurities behind. Of the world’s total supply of about 332.5 million cubic miles of water, 97.4 percent is saline. Fresh surface-water, such as rivers, streams and lakes, constitute a miniscule 1/150th of one percent of the total water on Earth. But it is these resources, along with fresh, shallow groundwater, all of which are continually fed by precipitation, from which we obtain life-sustaining water.
Weather and wildlife are affected profoundly by the great ocean currents that circumnavigate the globe. By delivering tropical warmth and moisture to northern regions, otherwise barren land becomes lush. Conversely, where there is upwelling of cold, nutrient-rich polar waters near the equator, such as occurs with the Humboldt current off the west coast of South America, tropical sunlight promotes phytoplankton growth, making the waters off the foggy coast of Peru one of the world’s greatest fishing grounds for anchovies and the tuna that feed upon them.
Whether as a vapor, a liquid, or as a solid, water always is on the go, although ever so slowly at times. To a great extent, this movement of water, and the water-borne heat, cold and nutrients it carries, shape the climate and influence the abundance and variety of life on Earth.
The major ice sheets found in Antarctica and Greenland can store ice for millennia. Ice from Antarctica has been dated to be 800,000 years old, though the average time frozen water spends in Antarctica is much shorter, about 20,000 years before it returns to the ocean.
As a liquid, water cycles through the environment much more rapidly than this. Soil moisture, especially in the absence of ground cover, quickly evaporates in a day or so under hot dry conditions. Water below the surface of lawns, fields and the forest floor can be held in place for weeks or a few months. Months elapse before the headwaters of rivers reach the sea. Waters in large lakes may reside there for 50 to 100 years before moving on. Groundwater in shallow aquifers on average takes 100 to 200 years to make its way through porous rock into streams and rivers.
In contrast, the waters of the ocean and those that are found deep underground are very slowly recycled to other parts of the environment. Water molecules that make their home within the bounding main reside there for more than 3,000 years on average before evaporating from the surface of the sea into the sky. And deep groundwater (fossil water) can spend more than 10,000 years beneath Earth’s surface before seeing again the light of day.
Unlike liquid and frozen water, water vapor always cycles very quickly. Once in the air, water vapor does not stay put for long. As moist air rises and cools, humidity in the air reaches its dew point and condenses on dust particles. When these initially small water droplets fuse into larger drops too heavy to be sustained in the air, they fall as precipitation from the clouds. Typically, after about nine days, moisture that has made its way into the atmosphere returns to earth again as rain, hail, sleet, snow or dew.
More than 100 quadrillion gallons of water fall from the sky back to earth each year. When this quantity is averaged across the entire surface of the planet, worldwide annual precipitation becomes a value we can more easily grasp; approximately 40 inches fall each year. But this is just the average, and real-life deviations from this are extreme. The amount of local precipitation that falls annually varies from less than 0.1 inches in Chilean deserts to more than 450 inches of rain on the slopes and mile-high summit of Mt. Waialeale in the Hawaiian Islands.
The conterminous states (the lower 48) receive about 30 inches during an average year, and here in Albemarle County we average about 46 inches of rain per annum. Higher amounts fall in the Shenandoah Mountains, owing to a phenomenon known as orographic uplift, as additional moisture is wrung out of the air when it rises to pass over them. Average annual precipitation at Big Meadows on Skyline Drive is 52 inches, for example.
The United States Geological Survey (USGS) estimates that 30 percent of the precipitation that falls in the U.S. fills streams, lakes and rivers through surface runoff, or replenishes the water table as it seeps deeper into the soil. The majority (70 percent) returns to the atmosphere, either by direct evaporation from land and bodies of water, or via transpiration from vegetation.
Through the process of transpiration—the drawing up of water from the soil by roots and evaporation of this water from aerial parts of plants, predominately from leaves but also from stems and flowers—vegetation plays a significant role in the movement of water from the ground into the air. Worldwide, vegetation provides about 10 percent of the moisture found in the atmosphere.
Consider these agricultural examples. During its lifetime, a corn plant will take up more than 50 gallons of water from the soil and release this water into the air. When taken together, an acre of corn emits 3,000-4,000 gallons of water each day to the atmosphere. For perspective, one inch of rain falling on an acre of ground is equal to 27,154 gallons. This means that a corn field transpires roughly an inch of moisture from the soil into the atmosphere each week at the height of the growing season (27,154 gallons divided by 4,000 gallons per day equals 6.8 days). Because much of the country west of the Mississippi does not reliably receive this much precipitation each week, many farmers in major corn-growing states such as Nebraska (the “Cornhusker State”) supplement natural rainfall by irrigating their fields with surface water and groundwater. Across the country on average each and every day, 74,900 million gallons of surface water and 53,500 million gallons of groundwater were used to irrigate fields and orchards in 2005, according to the USGS.
In summary, water is always in motion. Its movements in, on and over the land, air and sea shape the climate and promote the diversity of life on Earth. Agriculture and food production are absolutely dependent upon immense, reliable supplies of water. I try to bring this to mind whenever it rains. And as clouds shower refreshment upon the earth, I also take grateful pleasure in marveling at the travels and transformations of that fresh rainwater, some of which was in a salty ocean or a plant’s muddy roots a mere week or two ago.