Global Hydrological Cycle

By: Dr. Pranab Kr. Das, Assistant Professor, Department of Geography, Sree Chaitanya College, Habra

Concept of Global Hydrological Cycle:

    The cyclic movement of water within hydrosphere, lithosphere, atmosphere and biosphere is known as hydrological cycle. The hydrologic cycle is a conceptual model that describes the storage and movement of water of earth.      The hydrologic cycle begins with the evaporation  of water from the surface of the ocean. As moist air is lifted, it cools and water vapour 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.

Global Water Distribution:

     Water moves from one reservoir to another by way of processes like evaporation, condensation, precipitation, deposition,run-off, infiltration, sublimation, transpiration, melting and ground water flow. The oceans supply most of the evaporated water found in the atmosphere. Of this evaporated water, only 91% of it is returned to the ocean basins by way of precipitation. The remaining 9% is transported to areas over landmasses where climatological factors induce the formation of precipitation. The resulting imbalance between rates of evaporation and precipitation over land and ocean is corrected by runoff and groundwater flow to the oceans.

Reservoir	Volume (cubic km x 1,000,000)	Percent of Total
Oceans	1370	97.25
Ice Caps and Glaciers	29	2.05
Groundwater	9.5	0.68
Lakes	0.125	0.01
Soil Moisture	0.065	0.005
Atmosphere	0.013	0.001
Streams and Rivers	0.0017	0.0001
Biosphere	0.0006	0.00004

The planetary water supply is dominated by the oceans (Table: 1). approximately 97% of all the water on the Earth is in the oceans. The other 3% is held as freshwater in glaciers and icecaps, groundwater, lakes, soil, the atmosphere, and within life.Water is continually cycled between its various reservoirs. Water in the atmosphere is completely replaced once every 8 days. Slower rates of replacement occur in large lakes, glaciers, ocean bodies and groundwater. Replacement in these reservoirs can take from hundreds to thousands of years. Some of these resources (especially groundwater) are being used by humans at rates that far exceed their renewal times. This type of resource use is making this type of water effectively  non- renewable.

Controlling Forces of Hydrological Cycle:Evaporation: Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in our atmosphere via evaporation.

i. Evapo-transpiration: Evapo-transpiration is defined as the water lost to the atmosphere from the ground surface and the transpiration of groundwater by plants through their leaves. Plant transpiration is an invisible process—since the water is evaporating from the leaf surfaces, you don’t just go out and see the leaves “breathing”. During a growing season, a leaf will transpire many times more water than its own weight. A large oak tree can transpire 40,000 gallons (151,000 liters) per year.

ii. Condensation: Condensation is the process in which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth’s surface within the water cycle. Condensation is the opposite of evaporation.

iii. Precipitation: Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

iv. Surface runoff: When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers.

v. Infiltration: anywhere in the world, a portion of the water that falls as rain and snow infiltrates into the subsurface soil and rock. How much infiltrates depends greatly on a number of factors. Infiltration of precipitation falling on the ice cap of Greenland might be very small, whereas, as this picture of a stream disappearing into a cave in southern Georgia, USA shows, a stream can act as a direct funnel right into groundwater.

Different Phases of Hydrological Cycle:

Though hydrological cycle seems a single cycle, there are multiple cycles and phases such as;

1. Ocean-Atmosphere Cycle:

    The water cycle has no starting point, but we’ll begin in the oceans , since that is where most of Earth’s water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapour into the air; a relatively smaller amount of moisture is added as ice and snow submilate directly from the solid state into vapour. Rising air currents take the vapour up into the atmosphere. The vapour rises into the air where cooler temperatures cause it to condense into clouds. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. There is always water in the atmosphere. Clouds are, of course, the most visible manifestation of atmospheric water, but even clear air contains water—water in particles that are too small to be seen. One estimate of the volume of water in the atmosphere at any one time is about 3,100 cubic miles (mi³) or 12,900 cubic km (km³). That may sound like a lot, but it is only about 0.001 percent of the total Earth’s water volume. If all of the water in the atmosphere rained down at once, it would only cover the ground to a depth of 2.5 cm, about 1 inch. When rain downpour to oceans the ocean-atmosphere water cycle complete.  Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation. Once evaporated, a water molecule spends about 10 days in the air.

• Atmosphere-Terrestrial Cycle:

     Air currents move clouds around the globe, and cloud particles collide, grow, and fall out of the sky as precipitation . Some precipitation falls as snow and can accumulate as ice caps and glaciers , which can store frozen water for thousands of years. Snow packs in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snow-melt . Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with stream flow moving water towards the oceans. Runoff, and groundwater seepage, accumulate and are stored as freshwater in lakes. Although the water cycle sounds like it is describing the movement of water, in fact, much more water is in storage at any one time than is actually moving through the cycle. By storage, we mean water that is locked up in its present state for a relatively long period of time, such as in ice caps and glaciers. The world-wide scheme of the water cycle, runoff from snow-melt is a major component of the global movement of water. In the colder climates much of the springtime runoff and stream flow in rivers is attributable to melting snow and ice. The effect of snow-melt on potential flooding, mainly during the spring, is something that causes concern for many people around the world. Besides flooding, rapid snow-melt can trigger landslides and debris flows. Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. That is “overly simplified” because rivers also gain and lose water to the ground. Still, it is true that much of the water in rivers comes directly from runoff from the land surface, which is defined as surface runoff.

    When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers. This picture gives a graphic example of how surface runoff (here flowing off a road) enters a small creek. The runoff in this case is flowing over bare soil and is depositing sediment into the river (not good for water quality). The runoff entering this creek is beginning its journey back to the ocean

• Surface- Atmosphere Water Cycle:

  One part of the water cycle that is obviously essential to all life on Earth is the freshwater existing on the land surface. Surface water includes the streams (of all sizes, from large rivers to small creeks), ponds, lakes, reservoirs (man-made lakes), and freshwater wetlands. The definition of freshwater is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt. The amount of water in our rivers and lakes is always changing due to inflows and outflows. Inflows to these water bodies will be from precipitation, overland runoff, groundwater seepage, or tributary inflows. Outflows from lakes and rivers include evaporation and discharge to groundwater. When water evaporates from surface storages to atmosphere, surface-atmosphere water cycle finished.

• Surface- Ground Water Cycle:

    Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration . Some of the water infiltrates into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge , and some groundwater finds openings in the land surface and emerges as freshwater springs . Yet more groundwater is absorbed by plant roots to end up as evapo-transpiration from the leaves. Over time, though, all of this water keeps moving, some to re-enter the ocean and thus surface –ground water cycle complete.

Importance of Global Hydrological Cycle:

   The water cycle is an important process that recycles water and nutrients. In doing so, it brings freshwater to people, animals and plants all around the world. The water cycle begins with the ocean, lakes, ponds and other bodies of water on earth. Water evaporates from these bodies of water, and as the evaporated water lifts into the sky, it is cooled rapidly and condenses to form clouds. These clouds act as storage compartments for water. As they become filled with water, precipitation occurs. Clouds travel all around the world by wind currents and can bring precipitation to every part of the world. Once the water reaches the ground in the form of rain, snow, sleet or ice, some of the water may evaporate back into the air to form clouds, while other parts of the water may penetrate the soil and become groundwater. The groundwater can either return to the atmosphere and form clouds via transpiration, or it can flow into oceans, rivers, streams and other bodies of water. The cycle then begins again, with water evaporating from earth’s bodies of water.