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The Hydrologic Cycle

The circulation and conservation of earth’s water is called hydrologic cycle

It describes the continuous movement of water above or below the earth surface.
Many processes work together to keep Earth's water moving in a cycle.It is a complex combination of following eight processes that make up the hydrologic cycle:
1.  Evaporation              
2. Condensation                                
3. Transportation                            
4.  Precipitation             
5.  Infiltration (percolation),          
 6. Transpiration                                                                                           
7.  Runoff.                      
8.  Evapotranspiration              
These occur simultaneously and, except for precipitation, continuously.  75 % of the earth is covered with water. 97.25 % of earth’s water is in the oceans.
Factors affecting the hydrologic cycle 

Temperature plays direct affect and caused evaporation.

Infiltration rate.
If infiltration rate is high then hydrologic cycle will slows down.

Utilization of water.
Utilization of water by plants and animals also slows down the hydrologic cycle.


“Evaporation is the transfer of water from a liquid to a gas from the surface to the atmosphere”. Evaporation is when the sun heats up water in rivers or lakes or the ocean and turns it into vapour or steam which rises in to the air.

Approximately 80 percent of all evaporation is from the oceans, with the remaining 20 percent coming from inland water. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world.

Factors affecting the rate of evaporation

Concentration of substances in air.
If the concentration of other substances (salt crystals, water vapours) increases in the air, the evaporation rate will decrease and the substances decreases the evaporation increases.

Intermolecular forces.
If the inter molecular forces are strong and then rate of evaporation will low due to strong bonding in water molecules and if these forces are weak the evaporation will be high.

Surface area.
If the surface area is large the rate of evaporation will high and if area is small the rate of evaporation will be low.

Temperature affect
If the temperature is high the evaporation will be high and if the temperature is low the rate of evaporation will be low.

 2. Condensation (the opposite of Evaporation)

“Condensation is the change of water from its gaseous form (water vapor) into liquid water.”
Water vapour in the air gets cold and changes back into liquid, forming clouds. This is called condensation.

Condensation generally occurs in the atmosphere when warm air rises, cools, and loses 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.

3. Transportation

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 either 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 to 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.

4. Precipitation

“Water falls to the earth in the form of rain, hail, sleet or snow it is called as presipitation”. Precipitation occurs when so much water has condensed that the air cannot hold it anymore.  Precipitation is the primary mechanism for transporting water from the atmosphere to the surface of the Earth. There are several forms of precipitation.

Factors affecting the rate of precipitation

·         Precipitation is influenced by geographic location, elevation and aspect. Precipitation increases in areas of high humidity, as well as areas with high elevation.

·         Since lifting of air masses is the cause of almost all precipitation, amount and frequency of rain is generally greater on windward side of the mountain. As downslope motion of air results in decease in humidity, thus the opposite sides of barriers  usually experience relatively light precipitation. High amount of presipitation is reported at higher elevations.

5. Infiltration

“This refers to water that penetrates into the surface of soil”. Infiltration is controlled by soil texture, vegetation, and soil moisture status etc.

Factors affecting the rate of Infiltration

Soil Texture
Coarse-textured soils with large well-connected pore spaces tend to have higher infiltration rates than fine textured soils. However, coarse-textured soils fill more quickly than fine-textured soils due to a smaller amount of total pore space in a unit volume of soil. Runoff is then generated quicker than one might have with a finer-textured soil.

Soil moisture status
High infiltration rates occur in dry soils, with infiltration slowing as the soil becomes wet.

Soil characteristics: 
Some soils, such as clays, absorb less water at a slower rate than sandy soils. Soils absorbing less water result in more runoff overland into streams.

Vegetation also affects infiltration. For instance, infiltration is higher for soils under forest vegetation than bare soils. Tree roots loosen and provide conduits through which water can enter the soil.

The greatest factor controlling infiltration is the amount and characteristics (intensity, duration, etc.) of precipitation that falls as rain or snow. Precipitation that infiltrates into the ground often seeps into streambeds over an extended period of time, thus a stream will often continue to flow when it hasn't rained for a long time and where there is no direct runoff from recent precipitation.

Land cover: 
Some land covers have a great impact on infiltration and rainfall runoff. Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground. Impervious surfaces, such as parking lots, roads, and developments, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. Agriculture and the tillage of land also changes the infiltration patterns of a landscape. Water that, in natural conditions, infiltrated directly into soil now runs off into streams.

Slope of the land:
Water falling on steeply-sloped land runs off more quickly and infiltrates less than water falling on flat land.

6. Transpiration

“Transpiration is the transfer of water to the atmosphere by plants and vegetation from the leaves and stems of plants”.

It is a process by which plants lose water from their leaves.  The water rises in to the air.  Plants absorb water through their roots and this water can originate from deep in the soil. (For example, corn plants have roots that are 2.5 meters deep, while some desert plants have roots that extend 20 meters into the ground). Plants pump the water up from the soil to deliver nutrients to their leaves. This pumping is driven by the evaporation of water through small pores called "stomates," which are found on the undersides of leaves. Transpiration accounts for approximately 10 percent of all evaporating water.

Factors affecting the rate of transpiration 

Humidity in air. 
The transpiration rate decreases with rise in vapors pressure in outer atmosphere and vice versa.
High temperature increase transpiration and also open stomata.

Light influences transpiration indirectly by increasing the temperature.
Wind mostly increases the rate of transpiration but high winds and mechanical shocks causes stomatal closure and reduces the transpiration.

7. Runoff

“It is the movement  of water on the surface”.
Rivers, lakes, and streams transport water from land to the oceans. Runoff consists of precipitation that neither evaporates, transpires, nor penetrates the surface to become groundwater. Even the smallest streams are connected to larger rivers that carry billions of gallons of water into oceans worldwide. Too much rainfall can cause excess runoff, or flooding.

Factors affecting the surface runoff

  • Soil type
  • Soil moisture
  • Grade (steepness of ground)
  • Vegetation
  • Rate of rain fall.
8. Evapotranspiration

Evapotranspiration is the term used for area where evaporation through water and transpiration through plant’s leaves and/or stems occurs collectively and not measured separately.

About the Author: Kashif Ali Bhatti is a B.S in Geology from University of Sargodha, Pakistan

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