Hydrologic Cycle Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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Also known as the water cycle, the hydrologic cycle is a process of circulation of the earth’s water and its storage in reservoirs as a continuous flux, powered by solar energy. In some cases, the term hydrologic cycle implies a change of state of water. The water cycle involves considerable exchanges of energy between the atmosphere and the oceans, and significantly contributes to other processes like the alteration and breakdown of minerals and rocks, known as erosion, and the transportation of weathered particles as solids or ions in solution from land surface to the oceans.

Water exists on Earth in the three states of matter: solid (ice), liquid, and gas (water vapor). The processes by which water changes from one state to another are known as evaporation (liquid changing to gas), condensation (gas changing to liquid), freezing (liquid changing to solid), and sublimation (the direct change of ice to water vapor, without first becoming a liquid).

The origin of the earth’s water remains a controversial matter over whether it was released during a prolonged volcanic activity during the early stages of Earth, released through an outgassing process about 4 billion years ago, or has an extraterrestrial origin.

More than 70 percent of the earth’s surface is covered with water; however, water only represents 0.025 percent of the planet’s mass, which indicates the superficial nature of the hydrologic process. The total water circulating in the cycle is 332.5 million cubic miles (1,386 million cubic kilometers), although only 1 percent is in constant movement. Water is held in various reservoirs distributed all over the earth as follows: oceans and seas (96.5 percent), ice caps, glaciers, and permanent snow (1.74 percent), groundwater (1.7 percent), ground ice and permafrost (.022 percent), lakes (.013 percent), atmosphere (.001 percent), soil moisture (.001 percent), rivers (.0002 percent), and biological water (.0001 percent). The percentages of water stored in rivers and the atmosphere are very low and seem to be marginal, yet the relevance derives from the quantity that passes through those reservoirs.

The flux of water among the containers is continuous but highly differs in rate. The residence time measures the average time a molecule of water remains in a reservoir: 3,176 years in the ocean with respect to the atmosphere, 33,750 years in the ocean with respect to rivers, 1 year as soil water with respect to precipitation or evapotranspiration, and 1,377 years as groundwater with respect to rivers. Balances between the different reservoirs have varied as a result of climate change, particularly during glacial ages. During the last peak glaciation (20,000-18,000 years ago), 10.08 million cubic miles (42 million cubic kilometers) of water was trapped in polar ice caps, which lowered sea level by about 393.7 feet (120 meters) relative to the present day. Climate change produced other great modifications of the water cycle, like lower rates of evaporation and less precipitation.

In the present day, important regional differences in the hydrologic cycle are also observed in regard to evaporation and precipitation, and the balance between the two processes differs at this scale. Evaporation is higher than precipitation in the subtropical areas, while the opposite is true at the Equator and the higher latitudes. Water moves from or reservoir to another by way of evaporation, condensation, precipitation, deposition, run-off, infiltration, sublimation, transpiration, melting, and groundwater flow processes.

Water goes into the atmosphere from the evaporation of water on the ocean surface (86 percent), the evaporation of falling rain and snow before the water droplets reach the ground-a phenomenon termed rain fog-sublimation in the ice covers, and as a by product from the metabolic processes of fauna and flora (transpiration by plants and respiration by animals). In general, evaporation exceeds precipitation over the oceans while precipitation exceeds evaporation over land areas. While in the atmosphere, water is stored in the form of vapor and small water droplets that form clouds.

Nevertheless, moisture is redistributed all over the world by a process of advection-the horizontal transport of water vapor by moving air masses-that modifies the moisture content of the air. On average, air contains 2-3 percent water vapor, although the variation between regions is broad. Humidity in the tropics is 30 times the humidity at the poles. Aside from air temperature, other factors such as vapor pressure and atmospheric pressure affect the rate and amount of evaporation that takes place. Air becomes saturated at a certain point when it contains the maximum possible amount of water vapor without starting to condense, so warm air can hold higher concentrations of water vapor than cooler air.

The change from a gaseous to a liquid phase is termed condensation. Water vapor condenses in the atmosphere on the aerosols-small airborne nuclei of dust, salt particles, or ions-yielding the formation of water droplets, visible as clouds and fog. As the air containing water vapor rises, it cools and reaches the saturation point, and excess water vapor is released in one of several forms of precipitation: rain, snow, or hail, depending on air temperature and atmospheric processes of crystal formation or coalescence. Air lifting is produced by convection when unstable, less dense warm air that is heated by the earth’s surface ascends. Other processes that cause air to rise are convergence in cyclones, topographic elevation, and warm and cold fronts.

Evaporation-Condensation Cycle

The evaporation-condensation cycle is a transferring mechanism of heat energy horizontally from region to region and vertically between the earth’s surface and the atmosphere. The movement of water vapor entails the transfer of energy in the form of latent heat-the amount of energy released or absorbed during a change of state-which is released into the atmosphere when condensation occurs. The opposite process happens when heat energy is used in the process of evaporation. When liquid water is evaporated, 600 calories of heat are absorbed and later released in the process of condensation.

Similarly, when ice melts, 80 calories of heat are captured and the same amount is released in freezing. The high heat capacity of the oceans, as opposed to the atmosphere, is an energy storage facility that helps to keep the global temperature relatively stable, shaping the earth’s climate. This heat is constantly transported by the global ocean currents. Thus, changes in the hydrologic cycle as a result of climate change would cause critical alterations of the fluxes between reservoirs and energy transfer, with direct effects on water availability, weathering rates, nutrient transport, plant development, and indirect effects on agriculture production and economic development.

Rainwater is intercepted in its advance to the ground by canopy leaves and branches, leaf litter, small land formations, or as snow cover, reducing water availability and buffering the surface against erosion. The thin water layer deposited on the vegetation gradually descends to the ground by drippage or stemflow down stems and trunks, but stays exposed to evaporation for a certain length of time.

Infiltration is the process of vertical movement of water into the soil layer, which depends on various factors, such as soil properties, vegetation cover, and topographical properties, besides gravity and capillary action. Flat and rough surfaces with dense vegetation facilitate a prolonged retention of water for infiltration to take place. Next, migration is controlled by several soil features such as moisture content, the amount of open spaces (porosity), the texture, structure, and organic matter content. The rate of infiltration, or soil permeability, decays with saturation as the result of pore filling, expansion of clay particles and packing with small particles.

Soil water is not stationary; it continues moving downward under the pull of gravity and capillary forces. This process called percolation follows further the reach of the plant roots toward the bedrock and through the fissures and grains. It reaches the unsaturated zone, where still some air is present, and then forms the saturated zone when it encounters impermeable rock. Aquifers have a larger distribution than surface water, for they are in both dry and humid regions. Eventually, groundwater leaves the aquifer and flows back to the surface through springs or seeps, and discharges into terrestrial waters or the sea.

While at the surface, water is used by plants and eventually returned to the atmosphere by transpiration as water vapor. Plants pull water from the ground through the roots to transport nutrients from the soil, transfer sugars, and transpire water by means of the stomata to cool the leaves.

When the precipitation rate exceeds the infiltration rate of the soil, water accumulates on the surface, and subsequently starts to flow downslope, over land and is eventually channeled in a process termed surface runoff. Stream channels are organized hierarchically in networks that carry land water to the oceans or inland seas, a return flow that adds to the groundwater flow. Rivers and minor streams are located in areas of regular precipitation flow, while they exist only intermittently in areas of irregular precipitation.

Bibliography:

Elizabeth A. Berner and Robert A. Berner, The Global Water Cycle (Prentice Hall, 1987);
Rafael L. Bras, Hydrology, An introduction to Hydrologic Science (Addison-Wesley, 1990);
Tim Davie, Fundamentals of Hydrology (Routledge, 2002);
Lawrence Dingman, Physical Hydrology, 2nd ed. (Prentice-Hall, 2002);
M. Hornberger, J.P. Raffensperger, P.L. Wiberg, and K.N. Eshleman, Elements of Physical Hydrology (Johns Hopkins University Press, 1998);
David R. Maidment, , Handbook of Hydrology (McGraw Hill, 1993).