Water Resource Management (the Hydrologic Cycle)

Water Resource Management

The Hydrologic Cycle

Water is the source of all life on earth. The distribution of water, however, is quite varied; many locations have plenty of it while others have very little. Water exists on earth as a solid (ice), liquid or gas (water vapor). Oceans, rivers, clouds, and rain, all of which contain water, are in a frequent state of change (surface after evaporates, cloud water precipitates, rainfall infiltrates the ground, etc.). However, the total amount of the earth’s water does not change. The circulation and conservation of earth’s water is called the вЂ?hydrologic cycle”.

Introduction

The hydrologic cycle begins with the evaporation of water from the surface of the ocean. As moist air is lifted, it cools and water vapor condenses to form clouds. Moisture is transported around the globe until it returns to the surface as precipitation. Once the water reaches the ground, one of two processes may occur; 1) some of the water may evaporate back into the atmosphere or 2) the water may penetrate the surface and become groundwater. Groundwater either seeps its way to into the oceans, rivers, and streams, or is released back into the atmosphere through transpiration. The balance of water that remains on the earth’s surface is runoff, which empties into lakes, rivers and streams and is carried back to the oceans, where the cycle begins again.

Lake effect snowfall is good example of the hydrologic cycle at work. Below is a vertical cross-section summarizing the processes of the hydrologic cycle that contribute to the production of lake effect snow? The cycle begins as cold winds (horizontal blue arrows) blow across a large lake, a phenomenon that occurs frequently in the late fall and winter months around the Great Lakes.

Evaporation of warm surface water increases the amount of moisture in the colder, drier air flowing immediately above the lake surface. With continued evaporation, water vapor in the cold air condenses to form ice-crystal clouds, which are transported toward shore.

By the time these clouds reach the shoreline, they are filled with snowflakes too large to remain suspended in the air and consequently, they fall along the shoreline as precipitation. The intensity of lake effect snowfall can be enhanced by additional lifting due to the topographical features (hills) along the shoreline. Once the snow begins to melt, the water is either absorbed by the ground and becomes groundwater, or goes returns back to the lake as runoff.

Earth’s Water Budget

Water covers 70% of the earth’s surface, but it is difficult to comprehend the total amount of water when we only see a small portion of it. The following diagram displays the volumes of water contained on land, in oceans, and in the atmosphere. Arrows indicate the annual exchange of water between these storages.

The oceans contain 97.5% of the earth’s water, land 2.4%, and the atmosphere holds less than .001%, which may seem surprising because water plays such an important role in weather. The annual precipitation for the earth is more than 30 times the atmosphere’s total capacity to hold water. This fact indicates the rapid recycling of water that must occur between the earth’s surface and the atmosphere.

Evaporation

Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas.

Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world. For example, a typical hot and humid summer day in the Midwestern United States is caused by winds blowing tropical oceanic air northward from the Gulf of Mexico.

Condensation

Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air raises, cools and looses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets. The upward motions that generate clouds can be produced by convection in unstable air, convergence associated with cyclones, lifting of air by fronts and lifting over elevated topography such as mountains.

Transport

In the hydrologic cycle, transport is the movement of water through the atmosphere, specifically from over the oceans to over land. Some of the earth’s moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets. Clouds are propelled from one place to another by the jet stream, surface-based circulations like land and sea breezes, or other mechanisms. However, a typical 1 kilometer thick cloud contains only enough water for a millimeter of rainfall, whereas the amount of moisture in the atmosphere is usually 10-50 times greater.

Most water is transported in the form of water vapor, which is actually the third most abundant gas in the atmosphere. Water vapor may be invisible to us, but not to satellites, which are capable of collecting data about the moisture content of the atmosphere. From this data, visualizations like this water vapor image are generated, providing a visual picture of moisture transport in the atmosphere.

Precipitation

Precipitation is the primary mechanism for transporting water from the atmosphere to the surface of the earth. There are several forms of precipitation, the most common of which for the United States is rain. Other forms of precipitation include; hail, snow, sleet, and freezing rain. A well developed extra-tropical cyclone could be responsible for the generation of any or all of these forms of precipitation.

Amounts of precipitation can vary by location. For example, deserts like this one in Nevada, average less than an inch of total precipitation per year. The agricultural Midwest, however, receives approximately 15 inches per year, while tropical rain forests like this one in Hawaii, can receive more than 100 inches of precipitation per year.

Groundwater

Groundwater is all the water that has penetrated the earth’s surface