You've turned on the evening news in the heat of summer and hear a news report of areas facing a drought. What causes this drought? Lack of rain? Overuse of water? If your part of the world experiences little rainfall, will you run out of water? Hundreds of questions run through your mind. Regular rain increases the water tables and leads to a steady water supply. Seems simple, right?
The hydrological cycle represents the storage system for water on earth. When we think of water storage systems, we think of rain barrels or lakes, rivers and streams. Surprisingly, the earth has many storage systems for its water supply. This storage mechanism is called the hydrological cycle or water cycle. This cycle generates renewable water sources for life on earth.
Our oceans hold 97 percent of the earth's water. Keep that in mind. The other 3 percent is the sum total of the fresh water supply for the earth that is stored in lakes, rivers, glaciers, groundwater and ice caps. So where do we get our water supply?
Let's start on the ocean surface, our main source of water storage. Water evaporates from the ocean and is absorbed into the air. Evaporation pulls moisture up into the air in a process that takes liquid and converts it into a gas. These minuscule water particles are captured in the air. Air becomes the second storage system of water. Our atmosphere holds the air until the right conditions exist to release the water to the next storage system.
Transportation of water from the ocean surface to the air requires conversion of the liquid to a gas. We can't drink a gas so the water must undergo another change to move to the next storage system. The evaporated water in the air must condense back into a liquid before reaching the next storage system. Condensation in the air happens when warm air rises and cools. As a result, the gas releases the water droplets into clouds. Clouds have the ability to hold water droplets until the right conditions allow the release of these particles. Thus, clouds become a liquid water storage system.
The release of the liquid in the clouds transfers the water to the next storage system, the earth itself. Water reaches the earth as snow, ice, hail, and rain in a process called precipitation. Once the liquid hits the earth, the next storage system comes into play. Water is stored in oceans, lakes, rivers, streams, and the ground. Surface water percolates into the earth's upper layers as a built in storage system. It's also stored on the surface for future evaporation.
Even trees and plants get into the water storage game. Transpiration occurs when the moisture collected in the leaves and stems of plants and trees transfer and evaporate into the air. Remember the water evaporating from the ocean? Trees and plants do the very same thing. Transpiration works in conjunction with evaporation of ocean water to release water vapor into the air. Water can also evaporate from the surface before being absorbed. Imagine a disappearing rain puddle. The correct temperature and moisture levels in the air allow evaporation of the liquid into the air.
Another process is working to move water into the storage system. Runoff occurs when water is held at higher elevations and literally, runs off to the lowest storage point. Melting snow from mountains collecting in streams that flow into rivers that eventually flow into oceans are a great example of runoff. Runoff is a major avenue of water replenishment for all bodies of water.
The hydrological cycle is continuous and seamless. Water is drawn into the air, condenses in clouds to release precipitation to the earth's surface. Transpiration and evaporation occurs to release the water vapor back into the air to continue the cycle.