Hydrologic Cycle: Water Movement
Discover How Precipitation and Runoff Can Determine Flooding Extent
Water moves throughout and above the Earth as part of the hydrologic cycle. As water vapor condenses, precipitation falls, infiltrating the soil or runoff to the ocean.
The Hydrologic Cycle
Water is constantly moving through different stages of the hydrologic cycle. Although water covers over two thirds of the Earth, only a small percentage of it is seen on land. The University of Illinois Department of Atmospheric Sciences provides the percentages of water throughout the Earth and atmosphere:
- Oceans contain 97.5 percent of all water on Earth
- Earth surface 2.4 percent contained on land
- Atmosphere contains less than .001 percent
Evaporation and Transpiration
Evaporation is the conversion of liquid water to a gas. This process returns moisture to the atmosphere. As water droplets rise into the air, water is converted to gas. Most of the evaporation on Earth occurs over the oceans. A small percent of evaporation comes from surface water such as lakes, rivers and vegetation. The hydrologic cycle begins with evaporation.
Transpiration is the evaporation of water from plants to the atmosphere. Water evaporates from the leaves and stems of plants, as they pump water from the soil. Transpiration is responsible for 10 percent of all evaporated water.
Condensation
Condensation occurs in the atmosphere as warm air rises, then cools and water vapor is unable to be held in the air. Condensation is the conversion of water vapor or gas to liquid. Clouds consist of excess water vapor, as clouds contain tiny water droplets formed as the gas condenses. As air becomes less dense, it begins to rise vertically. This vertical motion is referred to as convection.
Effect of Topography on Hydrologic Cycle
As air meets topography such as mountain ranges, it is forced to rise. As air rises up and over mountains, water vapor can condense to form orographic clouds. The American Meteorological Society defines orographic clouds as "mountain clouds produced by orographic lifting of moist air to saturation." In the United States, these clouds form on the western side of mountain ranges, such as the Rockies or Cascade Mountains. Orographic clouds begin on the windward side of mountains and end near the summit of the mountain range. The clouds evaporate as they begin to sink, moving over the mountains.
Transport of Water Vapor
Water moves around the Earth in the atmosphere. The most visible transport of water is by clouds. Clouds, containing small water droplets and ice crystals, are moved by winds and the jet stream. Although clouds are the most visible form of water transport, most water is transported invisibly in the form of water vapor in the atmosphere. Invisible to the humans, satellites are able to detect water vapor amounts by measuring the moisture content of the air.
Precipitation
The primary way in which water is transported from the atmosphere to the Earth’s surface is through precipitation. Precipitation can be rain, snow, hail, sleet or freezing rain. Precipitation amounts vary in geographical locations, due to influences of topography, as explained earlier.
Surface Water Removal by Groundwater and Runoff
Water moves down into the soil through infiltration or percolation. Groundwater is water found in soil layers, after penetrating the Earth’s surface. Hydrologists monitor the surface permeability rate at which water flows through the soil. Differences in absorption rates of soil are important to observe. When the water table rises, flooding can take place, as this forces precipitation to remain on the surface. Flooding happens in the winter because frozen ground is not permeable, causing the precipitation to pool or runoff.
Runoff is the movement of water to the ocean by lakes, rivers and streams. Precipitation that does not evaporate, transpire or become groundwater, returns to the ocean. Excess runoff can also cause flooding, when too much precipitation occurs.
