Fresh Water is
Essential to LifeOne part of the water cycle that is obviously essential to all life on Earth is the freshwater existing on the land surface. Just ask your neighbor, a tomato plant, a trout, or that pesky mosquito. Surface water includes streams of all sizes, from large rivers to small creeks, ponds, lakes, reservoirs (man-made lakes like Canyon Lake), and freshwater wetlands. The definition of freshwater is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt. The amount of water in our rivers and lakes is always changing due to inflows and outflows. Inflows to these water bodies will be from precipitation, overland runoff , ground-water seepage, or tributary inflows. Outflows from lakes and rivers include evaporation and discharge to ground water. Humans get into the act also, as people are quick to use diverted surface water for their wants and needs. So, the amount and location of surface water changes over time and space, whether naturally or with human help. Certainly during the last ice age when glaciers and snowpacks covered much more land surface than today, life on Earth had to adapt to different hydrologic conditions than those which took place both before and after. And the layout of the landscape certainly was different before and after the last ice age, which influenced the topographical layout of many surface-water bodies today. Glaciers are what made the Great Lakes not only "great," but also such a huge storehouse of freshwater. Surface Water Keeps Life GoingLife can even bloom in the desert if there is a supply of surface water (or ground water) available. Water on the land surface really does sustain life, and this is as true today as it was millions of years ago. I'm sure dinosaurs held their meetings at the local watering hole 100 million years ago, just as the whitetail deer and raccoons in the Texas Hill Country do today. And, since ground water is supplied by the downward percolation of surface water, even aquifers are happy for water on the Earth's surface. You might think that fish living in the saline oceans aren't affected by freshwater, but, without freshwater to replenish the oceans they would eventually evaporate and become too saline for even the fish to survive. Usable Fresh Water is Relatively Scarce Notice how of the world's total water supply of about 332.5 million cubic miles of water, 97 percent is saline. And, of the total freshwater, almost 69 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles, which is about 1/700th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use everyday. Fresh water represents only about three percent of all water on Earth, and freshwater lakes and swamps account for a mere 0.29 percent of the Earth's freshwater. Twenty percent of all freshwater is in one lake, Lake Baikal in Asia. Another twenty percent is stored in the Great Lakes (Huron, Michigan, and Superior). Rivers hold only about 0.006 percent of total freshwater reserves. You can see that life on Earth survives on what is essentially only a "drop in the bucket" of Earth's total water supply! Back to Water Cycle
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