The Hydrological Cycle


The hydrological cycle is the movement of water through Earth's hydrosphere in various states such as liquid, solid, and gaseous. 
•    The hydrological cycle is the cyclical movement of water in various forms. Water movement occurs independently within different realms in the water cycle.
•    Vertical and horizontal air movement in the atmosphere transferring moisture from one place to another, water movement through sea streams in the hydrosphere, and water movement through rivers and glaciers towards the sea in the lithosphere are all included. 
•    Similarly, infiltration allows vaporized water from the soil and evaporation of water from plants to reach the groundwater.
 
WATER IS STORED IN THE FOLLOWING RESERVOIRS:
•    Atmosphere,
•    Oceans,
•    Lakes
•    Rivers,
•    The glacier,
•    The soils,
•    Snowfields, and
•    Ground water.
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Evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, and groundwater flow are all processes that move water from one reservoir to another.
 
IN GENERAL, THE FOLLOWING FOUR FACTORS CONTROL THE AMOUNT OF WATER ENTERING THE ATMOSPHERE VIA THESE TWO PROCESSES:
1.    Availability of energy;
2.    Away from the evaporating surface, the humidity gradient
3.    The speed of the wind just above the surface
4.    The availability of water.
     
Hydrological Cycle
AGRICULTURAL SCIENTISTS SOMETIMES REFER TO TWO TYPES OF EVAPOTRANSPIRATION:
1.    Actual evapotranspiration
2.    Evapotranspiration potential.
•    Crop growth is influenced by water availability. Crop yields are reduced when there is a drought. Irrigation can provide additional water to crops. A farmer can calculate the irrigation water needs of their crops by calculating both actual and potential evapotranspiration.
•    The movement of water from precipitation into the soil layer is known as infiltration. Due to a variety of environmental factors, infiltration varies both spatially and temporally. Infiltration can result in a situation where the soil is completely saturated with water after a rain. This condition, however, is only temporary because a portion of the water quickly drains (gravitational water) due to gravity's force on the water.
•    The remaining portion is referred to as the field capacity. Field capacity is a film of water that covers all individual soil particles to a thickness of 0.06 mm in the soil.
•     Capillary action transports water from one part of the soil to another to compensate for losses (biggest losses tend to be at the surface because of plant consumption and evaporation). Water movement by capillary action generally results in a uniform water concentration throughout the soil profile.
•    Runoff is the flow of water from higher elevations to lower elevations. Runoff can be viewed as a series of interconnected events at the microscale. Runoff flows from landmasses to the oceans on a global scale. Because of the imbalance between precipitation and evaporation, runoff occurs on the Earth's continents.
•    The horizontal subsurface movement of water on continents is known as through flow. Through flow rates are affected by soil type, slope gradient, and water concentration in the soil. 
•    Stream flow or stream discharge refers to the flow of water through a stream channel. Humans measure stream flow on many streams because of the dangers that can arise from too little or too much flow. 
•    The majority of the Earth's surface is covered by oceans. The world's oceans have an average depth of 3.9 kilometers. Maximum depths, on the other hand, can exceed 11 kilometers. On the Earth, the distribution of land and ocean surfaces is not uniform. There is four times more ocean than land in the Southern Hemisphere. In the Northern Hemisphere, the land-to-ocean ratio is nearly equal.
•    The ocean's water is primarily a byproduct of lithospheric rock solidification that occurred early in the Earth's history. 
•    Volcanic eruptions are a second source of water. The dissolved constituents found in the ocean come from the leaching and stream runoff of terrestrial salts in weathered sediments. 
•    Seawater is made up of a combination of salts and water. 99 percent of the salts in seawater are chlorine, sodium, magnesium, calcium, potassium, and Sulphur. 
•    Seawater contains enough salt to allow ice to float on top of it. Small amounts of dissolved gases, such as carbon dioxide, oxygen, and nitrogen, are also present in seawater. 
•    These gases enter the ocean through a variety of organic processes as well as the atmosphere. Temperature, salinity, and ocean depth all affect the density of seawater. 
•    When seawater is frozen at the ocean's surface and contains no salts, it has the least density. The ocean floor has the highest density of seawater.
•    The oceans control the majority of the world's water supply. The oceans contain roughly 97 percent of all the water on the planet. The other 3 percent is held as freshwater in glaciers and icecaps, groundwater, lakes, soil, the atmosphere, and within life.
 
THE MULTIPLE CYCLES THAT MAKE UP THE EARTH’S WATER CYCLE INVOLVE FIVE MAIN PHYSICAL ACTIONS: 
1.    Evaporation 
2.    Precipitation 
3.    Infiltration 
4.    Runoff
5.    Subsurface flow. 
 
I. EVAPORATION: 
•    It occurs when the sun's radiant energy heats water molecules, causing some of them to become so active that they rise into the atmosphere as vapor. 
•    The transfer of water from bodies of surface water into the atmosphere is known as evaporation. 
•    The physical nature of water changes from liquid to gaseous during this transition. 
•    Plant transpiration can be counted alongside evaporation. As a result, evapotranspiration is a term used to describe this process. 
•    Evaporation accounts for about 90% of atmospheric water, with transpiration accounting for the remaining 10%.
•    Plants take in water through their roots and release it through their leaves, which is a process that can clean water by removing contaminants and pollution. 
•    Water evaporating from the ground and transpiration by plants are both examples of evapotranspiration. 
•    Water vapor re-enters the atmosphere through evapotranspiration.
 
II. PRECIPITATION:
•    Rain clouds frequently form in cold air high in the sky. 
•    Water vapor is carried high into the sky by rising warm air, where it cools and forms water droplets around dust particles in the air. 
•    Some of the vapor condenses into tiny ice crystals that attract cooled water droplets. The drops condense on the ice crystals, forming larger crystals that we refer to as snowflakes.
•    Snowflakes fall when they become too heavy. Snowflakes melt into raindrops when they come into contact with warmer air on their way down. 
•    Cloud droplets clump together around dust or sea salt particles in tropical climates. 
•    They collide and grow in size until they're too heavy to fall. In the clouds, there is occasionally a layer of air that is above freezing, or 32° F. 
•    The air temperature drops below freezing as you get closer to the ground. 
•    Snowflakes melt partially in the warmer air layer before freezing in the cold air near the ground. Sleet is the name for this type of precipitation. When it hits the ground, it bounces.
•    Rain may freeze on contact with the ground or streets if snowflakes completely melt in the warmer air but temperatures near the ground are below freezing. This is referred to as freezing rain, and a significant amount of freezing rain is referred to as an ice storm. 
•    Ice storms are extremely hazardous because the ice layer on the roads can result in traffic accidents. 
•    Thunderstorms can also produce hail, which is a different type of precipitation.
 
III. INFILTRATION:
•    Precipitation can actually evaporate before it reaches the surface in some cases. A portion of the precipitation that reaches the Earth's surface seeps into the ground through a process called infiltration, adding to the surface water in streams and lakes. The transition from surface water to groundwater is called infiltration.
•    The rate of infiltration is determined by the permeability of the soil or rock, as well as other factors.
•    Infiltrated water may reach groundwater, which is a separate compartment (i.e., an aquifer). Because groundwater moves slowly, it may reappear as surface water after being stored in an aquifer for a period of time that can be thousands of years in some cases. 
•    Under the force of gravity or gravity-induced pressures, water returns to the land surface at a lower elevation than where it infiltrated.
 
IV. RUNOFF:
•    The amount of water that infiltrates the soil is affected by the degree of land slope, the amount and type of vegetation, the soil type and rock type, and whether or not the soil is already saturated.
•    The more cracks, pores, and joints there are in the surface, the more infiltration occurs. Runoff is the result of water that does not penetrate the soil and flows to the surface.
•    Runoff is precipitation that reaches the Earth's surface but does not penetrate the soil. 
•    Melted snow and ice can also cause runoff. It also encompasses the various methods by which land surface water flows down slope to the oceans. 
•    The flow of water in streams and rivers may be delayed in lakes for a period of time. Much of the precipitated water evaporates before reaching the ocean or an aquifer, so not all of it returns to the sea as runoff.
 
V. SUB-SURFACE FLOW:
•    Water movement within the earth's surface, whether in the recharge zone or aquifers, is referred to as surface flow. Subsurface water may eventually seep into the ocean or return to the surface after infiltrating.
•    The amount of moisture in the atmosphere is 14,000 km2. This means that the atmospheric moisture is replaced 32 times per year, or that the atmospheric moisture has a 10-day residence time.
 
WATER BUDGET EQUATION
•    In nature, the entire water cycle is global. In the hydrologic cycle, there are numerous sub-cycles.
•    As a result, water resources are a worldwide issue with local roots. 
•    The total amount of fresh water available on the planet is finite. 
 
Hydrological Cycle
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 stands for precipitation, the primary input and the starting point for the analysis;
•    R stands for runoff; G stands for groundwater; E stands for evaporation; 
•    T stands for transpiration. 
The global hydrologic system can be considered a closed system because the total amount of water available to the earth is finite and indestructible. The hydrologic system for a specific area on earth, on the other hand, is open, which means that the total amount of water in that area changes over time. As a result, it is necessary to know the availability of water in that area; a water budget analysis is carried out for this purpose.
 
THOUGH THE EQUATION IS SIMPLE BUT DETERMINING EACH TERM IS EXCEEDINGLY COMPLEX DUE TO:
a.    The paths taken by water particles are numerous and varied.
b.    The system and variables are constantly changing.
c.    Here are some terms that are difficult to quantify, and
d.    E, T, and G are extremely diverse.
 
SYSTEM CONCEPT
•    Hydrologic phenomena are intricate and may never be fully comprehended. They can, however, be represented in a simplified way using the system concept in the absence of knowledge. 
•    A system is a collection of interconnected parts that work together to form a whole. Precipitation, evaporation, and other components of the hydrologic cycle can be treated as a system. 
•    These elements can be divided into subsystems of the overall cycle; in order to analyze the entire system, the simple subsystems can be treated separately and the results combined based on the interrelationships between them.
•    The totality of the flow paths through which water may pass as a throughput from the point it enters the system to the point it leaves is referred to as the structure or volume in space. The boundary is a three-dimensional continuous surface that encloses the volume or structure.
•    A working medium is introduced into the system as an input, interacts with the structure and other media, and then exits as an output. The working media in a system are used by physical, chemical, and biological processes; the most common working media in hydrologic analysis are water, air, and heat energy.
•    The goal of hydrology system analysis is to examine the operation of the system and predict its output. The inputs and outputs of a hydrologic system model are measurable hydrologic variables, and the structure is a set of equations linking the inputs and outputs.
•    The concept of a system transformation is a transformation equation, such as I(t) O, which is central to the model structure (t). When the surface and soil of a watershed are thoroughly examined, the number of possible flow paths grows exponentially. As the soil becomes wetter, the shape, slope, and roughness of any path may change over time. Precipitation also varies erratically in space and time.
•    Because of these significant complexities, it is impossible to use exact physical laws to describe some hydrologic processes. 
•    Using the system concept, effort is focused on building a model that connects inputs and outputs rather than the extremely difficult task of accurately representing system details that may or may not be important from a practical standpoint or are unknown. 
•    Nonetheless, understanding the physical system aids in the development of a good model and its verification.

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