The Hydrologic Cycle

As you have demonstrated, an enormous volume of water falls on this area every year. What is the source of this water? Rain is produced by the condensation of water vapor in the atmosphere. Where does this vapor come from? Most of the water in the atmosphere originates from the evaporation of ocean water.

Picture yourself on the beach in southern Florida in August. You are sitting on the sand, reading a trashy novel, when due to the unpleasantly high temperatures, you decide to go for a swim. Ouch! As you walk across the beach, the sand is extremely hot, burning your feet. You run and dive into the water. Instead of being refreshingly cool, you discover that even the water is warm, about the temperature of bathwater. The sun pours enormous quantities of heat onto the earth. This solar energy warms up the ocean and evaporates (turns from liquid into gas) billions of gallons of water. Winds blow the water-laden atmosphere from over the ocean to the land, giving us the high humidities that help make Alabama summers unbearable. As this water vapor gains altitude and mingles with local weather systems, it will cool and condense (turn back into a liquid). This liquid water then falls as rain.

What happens to all of this rainwater? Much of this water evaporates again under our hot, southern sun and returns directly to the atmosphere as fresh water vapor. Water that is taken up through the roots of plants eventually transpirates back into the atmosphere through the leaves. Evaporation and transpiration, since they both involve turning liquid water back into water vapor, are often lumped together and called "evapotranspiration". Some rainwater infiltrates into the ground to later reappear in springs. Jacksonville gets most of its water supply from Big Springs, located just west of the Square, and Germania Springs, north of town on highway 21. What would happen to an area if every year it received slightly more water through rainfall than was drained away by rivers or removed by evapotranspiration or other means? Of course, the area would become flooded!

Most of the rainwater that falls on this area stays in liquid form and runs off the earth's surface. All of this surface water must eventually drain away back to the ocean through river systems. The hydrologic cycle can be represented schematically and is a central focus of physical geography. This process of water evaporating from the oceans, blowing over the land, falling as rain, and eventually returning to the ocean is called the hydrologic cycle.


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