Hydrological Cycle Essay – This is one of the best essays on ‘Hydrological Cycle’ especially written for school and college students.
The movement of water on the earth’s surface and through the atmosphere is known as the hydrologic cycle. Water is taken up by the atmosphere from the earth’s surface in vapour form through evaporation. It may then be moved from place to place by the wind until it is condensed back to its liquid phase to form clouds. Water then returns to the surface of the earth in the form of either liquid (rain) or solid (snow, sleet, etc.) precipitation. Water transport can also take place on or below the earth’s surface by flow.
The hydrologic cycle is used to model the storage and movement of water between the biosphere, atmosphere, lithosphere and hydrosphere.
Water is stored in the following reservoirs:
vii. Snowfields, and
viii. Ground water.
It moves from one reservoir to another by processes like evaporation condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, and groundwater flow.
Water is stored in the atmosphere in all three states of matter. Water vapour in the atmosphere is commonly referred, to as humidity. If liquid, and solid forms of water can overcome atmospheric updrafts they can fall to the Earth’s surface as precipitation. The formation of ice crystals and water droplets occurs when the atmosphere is cooled to a temperature ‘that causes condensation or deposition. Four processes that can cause atmospheric cooling are orographic uplift; convectional uplift; air mass convergence; and radiative energy loss.
Precipitation can be defined as any aqueous deposit, in liquid or solid form, that develops in a saturated atmospheric environment and generally falls from clouds. A number of different precipitation types have been classified by meteorologists including rain, freezing rain, snow, ice pellets, snow pellets, and hail. Fog represents the saturation of air near the ground surface. Classification of fog types is accomplished by the identification of the mechanism that caused the air to become saturated.
The distribution of precipitation on the Earth’s surface is generally controlled by the absence or presence of mechanisms that lift air masses to cause saturation. It is also controlled by the amount of water vapour held in the air, which is a function of air temperature. In certain locations on the Earth, acid pollutants from the atmosphere are being deposited in dry and wet forms to the Earth’s surface. Scientists generally call this process acid deposition. If the deposit is wet it can also be called acid precipitation.
Normally, rain is slightly acidic. Acid precipitation, however, can have a pH as low as 2.3. Evaporation and transpiration are the two processes that move water from the Earth’s surface to its atmosphere. Evaporation is movement of free water to the atmosphere as a gas. It requires large amounts of energy. Transpiration is the movement of water through a plant to the atmosphere. Scientists use the term evapotranspiration to describe both processes.
In general, the following four factors control the amount of water entering the atmosphere via these two processes:
i. Energy availability;
ii. The humidity gradient away from the evaporating surface;
iii. The wind speed immediately above the surface; and
iv. Water availability.
Agricultural scientists sometimes refer to two types of evapotranspiration:
a. Actual evapotranspiration, and
b. Potential evapotranspiration.
The growth of crops is a function of water supply. If crops experience drought, yields are reduced. Irrigation can supply crops with supplemental water. By determining both actual evapotranspiration and potential evapotranspiration a farmer can calculate the irrigation water needs of their crops.
The distribution of precipitation falling on the ground surface can be modified by the presence of vegetation. Vegetation in general, changes this distribution because of the fact that it intercepts some the falling rain. How much is intercepted is a function of the branching structure and leaf density of the vegetation.
Some of the water that is intercepted never makes it to the ground surface. Instead, it evaporates from the vegetation surface directly back to the atmosphere. A portion of the intercepted water can travel from the leaves to the branches and then flow down to the ground via the plant’s stem. This phenomenon is called stem flow.
Another portion of the precipitation may flow along the edge of the plant canopy to cause canopy drip. Both of the processes described above can increase the concentration of the water added to the soil at the base of the stem and around the edge of the plant’s canopy. Rain that falls through the vegetation, without being intercepted, is called through fall.
Infiltration is the movement of water from precipitation into the soil layer. Infiltration varies both spatially and temporally due to a number of environmental factors. After a rain, infiltration can create a condition where the soil is completely full of water. This condition is, however, only short-lived as a portion of this water quickly drains (gravitational water) via the force exerted on the water by gravity.
The portion that remains is called the field capacity. In the soil, field capacity represents a film of water coating all individual soil particles to a thickness of 0.06 mm. The soil water from 0.0002 to 0.06 mm (known as capillary water) can be removed from the soil through the processes of evaporation and transpiration. Both of these processes operate at the surface.
Capillary action moves water from one area in the soil to replace losses in another area (biggest losses tend to be at the surface because of plant consumption and evaporation). This movement of water by capillary action generally creates a homogeneous concentration of water throughout the soil profile.
Losses of water stop when the film of water around soil particles reaches 0.0002 mm. Water held from the surface of the soil particles to 0.0002 mm is essentially immobile and can only be completely removed with high temperatures (greater than 100 degrees Celsius). Within the soil system, several different forces influence the storage of water.
Runoff is the surface flow of water to areas of lower elevation. On the microscale, runoff can be seen as a series of related events. At the global scale runoff flows from the landmasses to the oceans. The Earth’s continents experience runoff because of the imbalance between precipitation and evaporation.
Through flow is the horizontal subsurface movement of water on continents. Rates of through flow vary with soil type, slope gradient, and the concentration of water in the soil. Groundwater is the zone in the ground that is permanently saturated with water. The top of groundwater is known as the water table. Groundwater also flows because of gravity to surface basins of water (oceans) located at lower elevations.
The flow of water through a stream channel is commonly called stream flow or stream discharge. On many streams humans gauge stream flow because of the hazards that can result from too little or too much flow. Mechanical gauging devices record this information on a graph known as a hydrograph. In the online notes there is a representation of a hydro graph showing some of its typical features.
Oceans cover most of the Earth’s surface. On average, the depth of the world’s oceans is about 3.9 kilometers. However, maximum depths can be greater than 11 kilometers. The distribution of land and ocean surfaces on the Earth is not homogeneous. In the Southern Hemisphere there is 4 times more ocean than land. Ratio between land and ocean is almost equal in the Northern Hemisphere.
Water is continually cycled between its various reservoirs. This cycling occurs through the processes of evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, melting, and groundwater flow. Table 1.2 describes the typical residence times of water in the major reservoirs.
The water found in the ocean is primarily a byproduct of the lithospheric solidification of rock that occurred early in the Earth’s history. A second source of water is volcanic eruptions. The dissolved constituents found in the ocean come from the transport of terrestrial salts in weathered sediments by leaching and stream runoff. Seawater is a mixture of water and various salts. Chlorine, sodium, magnesium, calcium, potassium, and sulfur account for 99 % of the salts in seawater.
The presence of salt in seawater allows ice to float on top of it. Seawater also contains small quantities of dissolved gases including carbon dioxide, oxygen, and nitrogen. These gases enter the ocean from the atmosphere and from a variety of organic processes. Seawater changes its density with variations in temperature, salinity, and ocean depth. Seawater is least dense when it is frozen at the ocean surface and contains no salts. Highest seawater densities occur at the ocean floor.
Atmospheric circulation drives the movement of ocean currents. Within each of the ocean, the patterns of these currents are very similar. In each basin, the ocean currents form several closed circulation patterns known as gyres. A large gyre develops at the subtropics centered at about 30 degrees of latitude in the Southern and Northern Hemisphere.
In the Northern Hemisphere, several smaller gyres develop with a center of rotation at 50 degrees. Similar patterns do not develop in the middle latitudes of the Southern Hemisphere. In this area, ocean currents are not bound by continental masses. Ocean currents differ from each other by direction of flow, by speed of flow, and by relative temperature.
The planetary water supply is dominated by the oceans (Table 1.1). 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.
Hydrology is the study of the movement and distribution of water throughout the Earth, and thus addresses both the hydrologic cycle and water resources. Many processes work together to keep Earth’s water moving in a cycle. There are five processes at work in the hydrologic cycle condensation, precipitation, infiltration, runoff, and evapotranspiration. These occur simultaneously and, except for precipitation, continuously.
Water vapour condenses to form clouds, which result in precipitation when the conditions are suitable. Precipitation falls to the surface and infiltrates the soil or flows to the ocean as runoff. Surface water (e.g., lakes, streams, oceans, etc.), evaporates, returning moisture to the atmosphere, while plants return water to the atmosphere by transpiration.
The water cycle technically known as the hydrologic cycle is the circulation of water within the Earth’s hydrosphere, involving changes in the physical state of water between liquid, solid, and gas phases. The hydrologic cycle refers to the continuous exchange of water between atmosphere, land, surface and subsurface waters, and organisms. In addition to storage in various compartments (the ocean is one such compartment).
The multiple cycles that make up the Earth’s water cycle involve five main physical actions:
iv. Runoff, and
v. Subsurface flow.
It occurs when radiant energy from the sun heats water, causing the water molecules to become so active that some of them rise into the atmosphere as vapour. It is the transfer of water from bodies of surface water into the atmosphere. This transfer entails a change in the physical nature of water from liquid to gaseous phases. Along with evaporation can be counted transpiration from plants.
Thus, this transfer is sometimes referred to as evapotranspiration. About 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration. Transpiration occurs when plants take in water through the roots and release it through the leaves, a process that can clean water by removing contaminants and pollution. Evapotranspiration is water evaporating from the ground and transpiration by plants. Evapotranspiration is also the way water vapour re-enters the atmosphere (Fig. 1.2).
In cold air way up in the sky, rain clouds will often form. Rising warm air carries water vapour high into the sky where it cools, forming water droplets around tiny bits of dust in the air. Some vapour freezes into tiny ice crystals which attract cooled water drops. The drops freeze to the ice crystals, forming larger crystals we call snowflakes.
When the snowflakes become heavy, they fall. When the snowflakes meet warmer air on the way down, they melt into raindrops. In tropical climates, cloud droplets combine together around dust or sea salt particles. They bang together and grow in size until they’re heavy enough to fall.
Sometimes there is a layer of air in the clouds that is above freezing, or 32° F. Then closer to the ground the air temperature is once again below freezing. Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. This kind of precipitation is called sleet. It bounces when it hits the ground.
If snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets. This is called freezing rain, and a significant freezing rain is called an ice storm. Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents. Ice can also build up on tree branches and power lines, causing them to break and our lights to go out. There is another kind of precipitation that comes from thunderstorms called hail.
Under some circumstances precipitation actually evaporates before it reaches the surface. More often, though, precipitation reaches the Earth’s surface, adding to the surface water in streams and lakes, or infiltrating the A portion of the precipitation that reaches the Earth’s surface seeps into the ground through the process called infiltration.
Infiltration into the ground is the transition from surface water to groundwater. The infiltration rate will depend upon soil or rock permeability as well as other factors. Infiltrated water may reach another compartment known as groundwater (i.e., an aquifer). Groundwater tend to move slowly, so the water may return as surface water after storage within an aquifer for a period of time that can amount to thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures.
The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water. The more openings in the surface (cracks, pores, joints), the more infiltration occurs. Water that doesn’t infiltrate the soil flows on the surface as runoff.
Precipitation that reaches the surface of the Earth but does not infiltrate the soil is called runoff. Runoff can also come from melted snow and ice. Also it includes the variety of ways by which land surface water moves down slope to the oceans. Water flowing in streams and rivers may be delayed for a time in lakes. Not all precipitated water returns to the sea as runoff; much of it evaporates before reaching the ocean or reaching an aquifer.
v. Sub-Surface Flow:
Surface flow incorporates movement of water within the earth, either within the recharge zone or aquifers. After infiltrating, subsurface water may return to the surface or eventually seep into the ocean.
Quantitative Analysis of the Hydrological Cycle:
The quantitative analysis of the hydrological cycle on the earth is given in Table 1.3.
The average global precipitation is 872 mm which is equivalent to 445,000 km3 of water. The average atmospheric moisture is 14,000 km2. This means that the atmospheric moisture is replaced 32 times in a year, or the residence time of atmosphere moisture is 10 days.
Water Budget Equation:
The complete water cycle is global in nature. Many sub-cycles exist in hydrologic cycle. That is why water resources are a global problem with local roots. Total supply of fresh water available to the earth is limited. Catchment area affords a logical and convenient unit to study various aspects related to the hydrology and water resources of a region.
For a given problem area or catchment in an interval of time Δt., the continuity equation for water in its various phases is:
Mass inflow – Mass outflow = Change in storage
P – R – G – E – T = Δ S
where P- precipitation, primary input and the starting point in the analysis; R = runoff; G-groundwater; E – evaporation; T – transpiration. All terms in equation have dimensions of volume but can be expressed as depth over the catchment. Infiltration does not appear explicitly in the water budget equation, because it is loss to runoff but a gain to groundwater system.
Because the total quantity of water available to the earth is finite and indestructible the global hydrologic system may be looked upon as closed system. However, for a specific area on earth, the hydrologic subsystem for that area is open, meaning that the total amount of water in that area changes from time to time. Thus need to know availability of water within that area, a water budget analysis is conducted for this purpose.
Though the equation is simple but determining each term is exceedingly complex due to:
(a) Paths taken by particles of water are numerous and varied,
(b) The system and variables are changing continuously,
(c) Some terms are difficult to measure, and
(d) E, T and G are highly heterogeneous.
Hydrologic phenomenon are complex may never be fully understood. However in the absence of knowledge they may be represented in a simplified way by means of the system concept. A system is a set of connected parts that form a whole. The hydrologic cycle may be treated as a system whose components are precipitation evaporation, etc. These components can be grouped into subsystems of the overall cycle; to analyse the total system, the simpler subsystem can be treated separately and results combined according to the interrelations between the subsystems.
A hydrologic system is defined as a structure or volume in space, surrounded by a boundary, that accepts water and other inputs, operates on them internally, and produces them as outputs.
The structure or volume in space is the totality of the flow paths through which the water may pass as through put from the point it enters the system to the point it leaves. The boundary is a continuous surface defined in three dimensions enclosing the volume or structure.
A working medium enters the system as input, interacts with the structure and other media and leaves as output. Physical, chemical and biological processes operates on the working media within the system; the most common working media involved in hydrologic analysis are water, air and heat energy.
The objective of hydrology system analysis is to study the system operation and predict its output. A hydrologic system model is an approximation of the actual system; its inputs and outputs are measurable hydrologic variables and its structure is a set of equations linking the inputs and outputs.
Central to the model structure is the concept of a system transformation is a transformation equation e.g. I(t) → O(t). If the surface and soil of watershed are examined in great detail the number of possible flow paths becomes enormous. Along any path, the shape, slope and boundary roughness may also vary in time as soil becomes wet. Also precipitation varies randomly in space and time.
Because of these great complications it is not possible to describe some hydrologic processes with exact physical laws. By using the system concept, effort is directed to the construction of a model relating inputs and outputs rather than to the extremely difficult task of exact representation of the system details, which may not be significant from a practical point of view or may not be known. Nevertheless, knowledge of the physical system helps in developing a good model and verifying its accuracy.
The procedure of developing working equations and models of hydrologic phenomenon are similar to that in fluid mechanics. In hydrology however there is generally a greater degree of approximation in applying physical laws because the system are larger and more complex and may involve several working media. Also most hydrologic systems are inherently random because their major input is precipitation a highly variable and unpredictable phenomenon. Consequently statistical analysis plays a larger role in hydrologic analysis.