Hydrologic Cycle | Biogeochemical Cycles | Geography

In this article we will discuss about the meaning and stages of hydrologic cycle.

Meaning of Hydrologic Cycle:

The hydrologic cycle refers to the process whereby water gets converted from its liquid or solid state into its vapor state. As a vapor the water has been capable of travelling considerable distances from its source prior to recondensing and returning to earth as precipitation.

Hence the hydrologic cycle has been a complex, interrelated system which is involving the movement of atmospheric, surface (marine and fresh), and groundwater, throughout various regions of the world. It has been the hydrologic cycle which has been solely responsible for the world’s precipitation, and it is this precipitation, falling on the terrestrial and surface freshwater environments, that has been the sole source of the earth’s supply of fresh water.

The hydrologic cycle may be having either a long or various short cycles. In the short cycles, water may evaporate from either marine or freshwater systems, condense almost immediately, and return as precipitation to the same system. Another varia­tion of a short cycle has been the precipitation and subsequent evaporation of water from land surfaces, which has been follo­wed by its condensation and return as precipitation to the land, followed by re-evaporation, and so on.

In the long cycle the major source of water vapor has been the world’s oceans, which are having 97.3% of the earth’s waters. In this cycle a portion of the water evaporates and forms clouds that move inland. The water vapor then cools and returns to earth as precipitation. It is estimated that only 0.007% of the oceanic water has been distributed to terrestrial areas annually. This water will ultimately return to the oceans through river and groundwater flow.

As precipitation may take place close to the source of initial evaporation or thousands of miles away, the water may remain in the vapor state for variable times (a few hours to a few weeks). The average residence time for water to remain in the atmosphere is regarded to be 10 days.

Stages of Hydrologic Cycle:

The various stages in the hydrologic cycle are as follows:

i. Evaporation and Condensation:

Water vapor gets evaporated primarily from the oceans and then tends to condense around minute particles, termed nuclei, which have been suspended in the atmosphere. The nuclei generally are having small particles of organic material (spores, pollen, etc.), fine mineral particles, volcanic ash, and the like.

Dust and smoke particles from industrial sources and auto­motive exhausts may be serving as nuclei and form a major factor in contributing to the contamination of rainwater. This is due to the fact that airborne particles (such as lead) readily dissolve in the newly condensed atmospheric water and these materials then gets returned to earth along with the rainwater.

Initially, the condensed liquid has been in the form of extremely small droplets [less than 0.04 millimeter (mm) in diameter]. Due to their small size, their rate of fall has been negligible, and they get retained in the atmosphere as clouds. Finally, however, this vapor forms precipitation in response to one of three major factors. The largest amount of precipitation would be formed when masses of warm, moist air move into regions of cold air. This causes a rapid condensation of the vapor and subsequent precipitation.

The second factor arises during periods of warm weather, when air, warmed at the earth’s surface, gets decreased in density and rises into overlying cold air, bringing about condensation of the water vapor. The third mechanism involved in condensation takes place with the cooling of air masses as they move over high mountains. In each case the cooling of warm, moist air has been responsible for the condensation and precipitation of rainwater.

ii. Runoff, Stream Flow, and Infiltration:

When rain is falling on a terrestrial area, a portion of the water would be caught by vegetation. This process is known as interception and this water readily get re-evaporated, as there is a large surface area that is exposed to wind action. The remainder of the water falls to earth, and a portion sinks into the soil surface by a process known as infiltration. The portion not infiltrated into the soil, called surface runoff, flows over the surface and gets discharged (in undeveloped areas) into streams.

The water entering a stream by surface runoff plus the water entering via groundwater flow is known as runoff. Thus the terms “runoff” and “surface runoff” have been different and distinct. Surface runoff equals precipitation minus the water lost by interception and infiltration. Runoff, on the other band, has been generally synonymous with stream flow and has been equal to the sum of the surface water plus the groundwater that enters a stream.

Infiltration has been somewhat also distinct from ground­water when the water percolates into the earth’s surface, different portions would be following three distinct pathways. A portion of the water will function as interflow, since the presence of impermeable lower sediments will disallow deep penetration of this water.

Consequently, this segment will flow just below the soil surface and discharge into streams. Another portion would be remaining above the water table in an area called the zone of unsaturated flow. Both of these portions of water are regarded to be in the zone of aeration. The third portion will percolate down into the groundwater table and will, eventually, get discharged into streams.

iii. Evapotranspiration:

A large amount of the precipitated water would be converted to vapor by evaporation and/or transpiration. Evaporation refers to the process whereby molecules of liquid water (at the surface of a water body or in moist soil) absorb sufficient energy to leave the liquid state and enter the vapor state. The energy absorbed by the water in this process gets stored within the vapor, hence somewhat large amount of heat contained in the moist air.

Transpiration refers to the process whereby terrestrial and emergent aquatic vegetation release water vapor to the atmos­phere. In the process of photosynthesis all plants have been taking in liquid water and carbon dioxide and, by a complex series of reactions, convert these materials to carbohydrate, oxygen (gas), and water vapor.

Submergent aquatic vegetation (plants growing completely beneath the water surface) also liberates water and oxygen as vapors but, in these cases, the oxygen and water vapour produced and released into the surrounding liquid water would immediately form hydrogen bonds with the liquid water. The water vapour will get converted to its liquid form, whereas the oxygen remains in the water column as dissolved oxygen.

As the leaves of both terrestrial and aquatic emergent vegetation have been in air, the water vapour and oxygen, when released by these plants in photosynthesis, will remain in the vapour state as there are insufficient water molecules in the immediate vicinity to form hydrogen bonds and keep the water vapour and oxygen in solution.

The water converted from liquid to vapor in transpiration has been quite considerable, and the ability of plants to remove water from both soil and aquatic systems cannot be underestimated. Transpiration by emergent vegetation has been a major factor in the “drying up” of lakes.

Groundwater, unless it has been within a few feet of the surface, could not be evaporated. Rather, the portion that gets converted to vapour gets transpired by plants. In most regions the water lost by evaporation cannot be measured separately from the water lost transpiration. Consequently, the two are regarded together as evapotranspiration.

iv. Ground Water:

The lengthy portion of the hydrologic cycle would get completed when groundwater gets returned to the earth’s surface. The return may take place by springs, transpiration, or by artificial means.

Any natural surface discharge of sufficient water that will flow as a small rivulet is known as a spring, while a smaller discharge is known as surface seepage. Groundwater may also be discharged as subaqueous springs below the surface of lakes, rivers, and marine systems. It is possible to classify springs generally on the basis of their magnitude of discharge, as summarized in Table 2.1.

Three major factors affecting the magnitude of a spring have been the land area accepting the rainfall and thus contributing water to the sub-surface system, the permeability of the soil and subsoil, and the quantity of water entering the system (the amount of recharge). The use of water, in developed areas, prior to recharge has been known to alter the quality and quantity of the water entering the sub­surface system.

Discussion:

The water used by man is primarily fresh surface water and ground water. In arid regions a small fraction of man’s water supply comes from the ocean, a source which is likely to become more important as the world’s supply of fresh water dwindles relative to demand. Saline or brackish ground waters may also be used in some areas.

Ground water and surface water are having appreciably different characteristics. Many substances either dissolve in surface water or get suspended in it on its path to the ocean. Surface water collected in a lake or reservoir, and having the mineral nutrients essential for algae growth, may support a heavy growth of algae. Surface water having a high level of biodegradable organic material normally contains a high population of bacteria. All these factors are having a profound effect upon the quality of surface water.

Ground water is able to dissolve minerals from the forma­tions through which it passes. Most microorganisms originally present in ground water are slowly filtered out as it seeps through the ground. Occasionally, the content of undesirable salts may become excessively high in ground water, although generally it has been superior to surface water for use by man as a domestic water source.