9 Hydrogeology and Hydrologic cycle

Asha Manjari,

1-   INTRODUCTION:

 

Air is a mixture of gases. The principal AMONG the gases are Nitrogen (78% by volume), Oxygen (21% by volume), Argon (0.95% by volume), Carbon dioxide (0.034% by volume) and a variable amount of water vapour. The amount of water vapour in the lower range of the atmosphere can vary over a wide range from about 0.05 % to almost 4% by volume.

Fig 1 Pie Diagram Showing Distribution Of Gases in Atmosphere

 

 

The water vapour present in the air is an important causative factor for the formation of mist, fog, clouds, rain or snow. The carbon dioxide plays a role in plant life and overall climate of the earth. Presence of ozone in the atmosphere in the altitude ranging between 20 to 30kms above the earth’s surface absorbs all the ultraviolet radiation from the sun, where otherwise it would have been detrimental to plant and animal life. The atmosphere, particularly with water vapour and carbon dioxide change the incoming solar radiation, reaches both the land and ocean at the earth’s surface. With the increase in carbon dioxide content the radiation would be more and the earth become warmer. P.R. Pisharoty, a pioneer in Indian Meteorology has in his book ‘Meteorology for the Indian Farmers’ (Special Publication, Indian Space Research Organisation, Bengaluru, September, 1986) has comprehensively dealt about the solar radiation, its interaction with the atmosphere, structure of the atmosphere and the variations of pressure and temperature with altitude. He has outlined how horizontal pressure differences on a rotating earth, primarily caused by temperature differences, generate winds. He also has described the physics of clouds and rain or snow. He has given an outline of different weather systems which has regional impact.

Fig 2 Conversion of Solar radiation into heat energy

 

Water in the planet earth, occurring in liquid, solid and gaseous status, displays varied degree of motion. The hydrological cycle, also known as ‘water cycle’ defines the movement of water and storage of water in earth. Agriculture is strongly dependent upon weather. Human survival is dependent on water availability. Seasons impart a rhythm to life of the earth. The sequence of events in ‘hydrologic cycle’ appears to be simplistic. But, in reality, the hydrologic cycle is a vast and complicated cycle which involves a large number of paths of varying time scale. It is a continuous recirculating cycle. It has neither a beginning nor an end (Subramanya, 2013).

 

2-   OBJECTIVES

 

  1. Explain stages of Hydrological cycle
  2. Describes the various factors effecting Hydrological cycle
  3. Narrating the importance of Hydrological cycle
  4. Describes the movement and circulation of water on earth

 

3-     CONCEPT MAP

 

The components of the hydrological cycle Evaporation — Condensation–Precipitation–Run-off

4-   DESCRIPTIONS

 

4-1 Hydrological cycle

 

The hydrologic Cycle begins with the evaporation of water from oceans and lakes. Evaporation is the change of water form from a liquid stage to a gaseous one. The largest movement of water in the ‘water cycle’ is the evaporation of water from the oceans. Evaporation of water from the warm surface of oceanic body results in increased amount of moisture in the colder/ drier air immediately above. As water vapour moves up, it gets cooled and condenses into clouds. Condensation is the change of water form from gaseous form into liquid form. Condensation occurs in atmosphere when warm air raises, cools and loses its capacity to hold water vapour. When, water vapour condenses it becomes ‘droplets’ in the cloud .The upward motion that generate clouds can be the result of ‘convection’ in unstable air. Convection is the result that happens when the land surface is hot; the overlying air  which warms up becomes less dense than the air in the surrounding parts and attains motion in a vertical direction. When surface winds are deflected by friction towards the centre of the low pressure surface, it would result in ‘extra tropical cyclones’. Surface convergence can generate rising motion, leading into condensation of water vapour.

 

Fig 4 Hydrological cycle And distribution

 

 

The huge quantity of evaporated water, in one form or the other, moves around the globe until it returns to surface as ‘precipitation’. The precipitation  is the primary natural mechanism of water transportation from the atmosphere to the surface of the earth. It may include rain, hail, snow and freezing rain. While much of the clouds get back into the oceans and lakes as rain, some part of it when driven by wind on to the land, precipitate into rain, snow hail etc. A well-developed ‘extra tropical cyclone’ can generate any or all forms of precipitation. With continued evaporation, water  vapour in the cold part of atmosphere condenses to form ice crystals. It is said that the atmosphere keeps water for an average of 9 days before it releases it down as rain.

The boundary between air masses (front) can either be warm, cold or stationary. In case of ‘cold front’, a colder, denser air mass lifts the warm moist air ahead of it. As the air rises, it cools and the moisture in it condenses to produce clouds and precipitation. Due to steep slope of a cold front, vigorous rising motion leads to formation of showers and occasionally severe thunderstorm. In the case of ‘warm front’, the warm, less dense air lifts up the colder air ahead. Again, the air cools and its moisture condenses to form clouds. Precipitation that develops in advance of a surface warm front is typically steady and wide spread than precipitation associated with a cold front.

 

Inputs into the drainage basin include:

 

  • Energy from the sun for evaporation
  • Precipitation – rain & snow
  • Evaporation and transpiration from plants (collectively called evapotranspiration)
  • Runoff into the sea
  • Water percolating deep into underground stores where it can be effectively lost from the system

Stores of water

 

  • On the surface – glaciers, lakes, rivers, puddles
  • Vegetation stores water by interception and plants
  • The soil can hold water
  • Groundwater is stored in permeable rocks

Transfers and flows– moves water through the system and enable inputs of water to be processed from one store to another.

  • Transfers include through fall, stem flow, infiltration, through flow and groundwater flow.

 

Fig 7 Flowchart of water budget

 

While almost all the water is contained in oceans, only 3% is the fresh water available on land surface and of that 3 % almost 69% is trapped as glaciers. Glaciers can keep water trapped as ice for thousands of years. The ‘lake effect snow fall’ is caused when cold wind blows across a large lake. The intensity of snowfall can get enhanced due to additional lifting of clouds in mountainous terrain. Once the snow begins to melt, the water absorbed by the ground becomes groundwater or return to the lake / ocean as ‘run-off’. The water through precipitation when reaches the ground, some part of it evaporates back into the atmosphere, some part gets consumed by the vegetation and a part saturates the soil, infiltrate and percolate into ground as ‘groundwater’. The part of water consumed by vegetation reaches the atmosphere as ‘transpiration’. Groundwater may as ‘springs’ and or ‘base-flow’ reach the path of the stream. Rest of the precipitation, as over land flow largely finds its course into the streams and rivers as ‘run-off. The stream / river water flow ultimately gets emptied back into lakes and oceans.

Thus, as Subramanya (2013) puts ‘the total water resources of the earth are constant and the sun is the source of energy for the hydrological cycle ’.The main components of the hydrologic cycle can  broadly be classified as transportation component and storage component. The transportation components include precipitation, evaporation, transportation (flow), infiltration and runoff. Storage components are that of storage on the land surface (like ponds, lakes, reservoirs etc.), soil moisture storage and groundwater storage. The Horton’s representation of the hydrological cycle illustrates the storage and transportation components and their relative positions in the cycle.

 

The hydrological cycle has an influence on agriculture, forestry, geography, etc. Human interference in the hydrological cycle like through artificial rain, change of vegetal cover, change in land use, indiscriminate extraction of groundwater etc., can cause serious repercussion in the fields of agriculture, forestry, geography etc. It has its immense application in the design of projects about water supply, irrigation and drainage, flood control, coastal geomorphology and geohydrology.

 

4-2 El Nino: In the tropics, where weather is warm year round, rainy seasons known as monsoons alternate with dry period. But, the rhythm of the seasons cannot be relied upon. The tropical Pacific Ocean and global atmosphere disrupts normal patterns of climate and life. The alternation between the normal climate and a different recurrent set of climatic conditions in the Pacific region is called ‘El Nino’. The term ‘El Nino’ was originally used by fishermen along the coast of Ecuador and Peru to a warm ocean current appearing around the period of Christmas over several months.

 

During 1920s, Walker who was on assignment to India, trying to find a way to predict the Asian Monsoon, discovered a remarkable connection between barometer readings at eastern and western sides of the Pacific. He notified when pressure rises to the east, it usually falls low in the west and vice versa. Walker coined the term ‘southern oscillation’ to the ups and downs in that east-west seesaw in Southern Pacific Barometer pressures.

 

Fig 9 Typical winter time pattern (El Nino)

 

Fig 10 Diagram showing formation of El Nino

 

 

When the seesaw is in its ‘high index’ (strongly tilted) state, pressure is high on the eastern side of the Pacific and low on the western side. Along the equator, the east-west pressure contrast drives easterly (east to west) surface winds from the Galapago’s islands nearly all the way to Indonesia. When seesaw switches into its ‘low index’ (weakly tilted) state, the easterly surface winds weaken. The biggest changes in the slope of the seesaw and in the strength of the easterlies occur over the western Pacific. To understand how El Nino affects the ocean, it is necessary to learn how surface winds move water during normal years and how the resulting motions affect water temperature. The easterly winds that blow along the equator and the south easterly winds that blow along Peru and Ecuador coasts both tend to drag the surface water along with them. The earth’s rotation then deflects the surface currents northward in the northern hemisphere and southward in the southern hemisphere. The surface waters are therefore deflected away from the equator in both directions and away from the coastline. Where the surface water moves away, colder nutrient water comes up from below to replace it and this phenomenon is called ‘upwelling’. Both the equatorial upwelling and the coastal upwelling are concentrated in narrow regions of around 150km wide. The winds that blow along the  equator also affect the properties of up welled water. In the absence of the wind, the dividing layer between the warm surface and the deep cold water called ‘ thermo cline’ would be nearly flat; but, the winds drag the surface westward, raising ‘thermo cline’ up to the surface in the east and depressing it in the west. The winds control the upwelling. The winds are also responsible for the cold tongue in the sea surface temperature pattern. But, the resulting changes in the sea –surface temperature will in turn have effects on the winds. When the easterlies are blowing at full strength, the upwelling of cold water along the equatorial Pacific chills the air above it making it too dense to rise high enough for water vapour to condense to form clouds and rain-drops.As a result, this strip of the ocean stays conspicuously free of clouds during normal years and the rainin the equatorial belt is confined to the extreme western Pacific near Indonesia. But, when the easterlies weaken and retreat eastward during the early stages of an El Nino event, the upwelling slows and the Ocean warms. The moist air above the ocean also warms. It forms deep cloud resulting in heavy rain along the equator. The change in ocean temperature thus cause major rain zone over the western Pacific to shift eastwards. Related adjustments in the atmosphere cause barometer pressure to fall over the central and eastern Pacific and rise over India, Indonesia and Australia-thereby resulting in eastward retreat of the easterlies.

 

Thus, it is difficult to identify the subtle change in the ocean –atmosphere system that initiates a transition into or out of El Nino condition. The twists between ocean and atmosphere can have ripple effect on climatic conditions in the far regions of the globe. Dense tropical clouds distort the air flow aloft(8 to 16km above sea level), but on a horizontal scale of thousands of kilometre, the waves in the air flow in turn determine the positions of the monsoons. The impact of El Nino upon climate in high temperature latitudes show up more clearly during winter time.

 

5-   HYDROGEOLOGY

 

Hydrogeology is the subject that deals with distribution and movement of groundwater in the soil and rocks of the earth’s crust. Merriam Webster defines ‘hydrogeology’ more appropriately as a branch of geology concerned with occurrence, use and functions of surface water and groundwater. Hydrogeology is the study of water contained in materials of earth’s crust, its origin, evolution, physical and chemical characteristics and ultimate destination.. The terms groundwater hydrology, geohydrology and hydrogeology are used interchangeably. While ‘hydrogeology’ is the term commonly used by geologists and hydro geologists, the term ‘geohydrology’ is most often used by  the civil engineer. Whether it is called hydrogeology or geohydrology, the meaning remains almost equivalent. Hydrogeology is an interdisciplinary subject and deals on chemical, physical, biological and legal aspects of surface and ground waters. The subject of hydrogeology, apart from various science faculties like of agriculture, horticulture, soil, civil engineering etc., has also a larger bearing on urban, forest and coastal hydrology.

 

Fig 11 Interaction of hydrogeology with other geo components

 

 

Hydro geological features mainly dependent upon geology, geomorphology and climatology of an area. The rainfall distribution both in space and time, intensity, frequency and variability is of considerable importance for understanding hydrological aspects of any given area. In a virgin unaltered landscape, after a dry rainless period when rainfall occurs, a good part of it will be lost through evaporation. Subsequent rains form part of the surface run-off .A rapid run-off would mean minimum percolation into the earth even if the rains are normal. Saturation run-off occurs after soil  –water zone is fully saturated. Infiltration of water is the downward movement of water from the ground surface. The maximum infiltration rate is the ‘infiltration capacity’ or ‘f- capacity’. Surface flow in a natural landscape appears when the water availability exceeds the f- capacity. Infiltration of water is distinct from percolation. The water percolation is the transit of infiltrated water by gravity through the zone of aeration to the zone of saturation. In case of hard rocks, the water percolates through weathered mantle (that constitutes zone of aeration) and / or fractures. The water contained in materials of earth’s crust is ‘groundwater’ and this quite often spelled by many as ‘ground water ‘. When groundwater is not near to the earth’s surface, there forms a zone in where majority of open spaces are with air. This is called ‘zone of aeration’ or ‘vadose zone’.

 

‘Vadose zone’ which is the zone of aeration is the part of earth between the zone of saturation and the ground surface. Vadose zone has a pressure head less than atmospheric pressure. The vadose zone does not include the area that is with water content above the water table and this is referred to as ‘capillary fringe’. The capillary fringe is the result of the process of capillary action. Movement of water within the vadose zone involves study of soil, hydrology, and hydrogeology. Groundwater recharge under natural process occurs through vadose zone. To put it in brief the ‘vadose zone’ is the subsurface un-saturated or under- saturated part of the geological rock formation that lies above the groundwater table. In marshy zone, the vadose zone may be absent. Rate of water percolation and groundwater recharge is dependent upon the rock characteristics in the vadose zone. When there is impermeable rock in the zone of aeration, down ward movement of infiltrated water gets blocked or restricted resulting in the saturation of the interstices of the rock material above the impermeable barrier. Water in such a zone is called ‘perched groundwater’. The upper surface of the zone of saturation is the’ water table’ or ‘phreatic surface’. In other words, the area separating the vadose zone and the phreatic zone is ‘water table’. The water table fluctuates between seasons. Some use the term ‘phreatic zone’ in preference to ‘zone of saturation ‘. The part of the geological rock type, be it alluvium or hard rock when the pore spaces / fracture zones in them are filled with water that is infiltrated and or percolated, forms the’ zone of saturation’ and this zone of saturation is the one between the water table and the depth at which the rock type can no more hold and transmit water because of impermeable characteristics. In other words, the lower limit of zone of saturation extends to such depths at which the interconnected openings remain absent or negligible.

 

An aquifer is the rock formation or part of rock formation which yields appreciable quantity of groundwater. Every aquifer has a capacity to store water which is expressed as ‘coefficient’. The ‘storage coefficient’ which is otherwise called ‘ storability’ is defined as the volume of water that can be released or taken into storage per unit surface area of the aquifer per unit change in head. In the case of water table aquifer otherwise called ‘unconfined aquifer’ which is bound at the top by the water table, coefficient of storage is analogous to ‘specific yield’. The specific yield is the saturated part of the rock to drain water under the force of gravity. It is expressed as ‘percentage volume of water yield by draining a saturated rock to its total volume’. The specific yield of rock is complimentary to the ‘specific retention’. In case of hard rocks, the inter-granular space (rock matrix) and the openings like joints / fractures largely are the factors that govern the specific yield. The rock matrix when porous and permeable and the joints / fractures together form double porosity characteristics. If the rock matrix is hard, massive, non- porous and not permeable, in such cases fractures/ joints when are present will act as water conduits. Rock will not be an aquifer When with its extensive spread are massive, hard non –porous, not permeable and fractures / joints are absent. In case of clay formations, though they are highly porous are not permeable. The semi-confined aquifer condition prevails when the confining layer is semi- permeable. A’ confined aquifer’ is the one which will be overlaid and under laid by impervious and impermeable rock formation

Depending upon the capacity of the aquifer to accept, store and or transmit water, the rainwater and or water from any other source that infiltrates, when reaches the upper surface of the vast massive rock zone or impermeable bed, finding no scope to further percolate gets into the process of gradually saturating all the fractures, joints and any other voids or inter granular pore space in an ascending manner. With this process on, depending upon the rate and quantity of water that infiltrate, the interconnected fractures, joints and any voids in the semi- confined, semi-unconfined and unconfined aquifer zones gradually get saturated. Groundwater recharge is dependent upon the porosity and permeability of the soil and underlying geological formation that prevails in the area. Porosity is the pore space between unconsolidated soil and other earth material / weathered and or fractured part of the rock. Permeability is the expression of the connectivity of pores to permit the water movement

.This process of saturation from semi-confined to unconfined aquifer zone is possible only when the net annual draft component from the aquifer is less than the net annual groundwater recharge. Thus the unconfined aquifer, semi-unconfined aquifer and semi-unconfined aquifers which are transitionally interconnected form a ‘composite aquifer system’; more so in a hard rock terrain. Recharge to the composite aquifer system will be through the unconfined aquifer.

 

When bore well taps unconfined / semi-confined aquifer, the water in the well rises to a particular level and the water level attained under such a condition is called ‘piezometric surface’. The piezometric surface or piezometric level will be dependent upon the hydro -static pressure that sustains in the aquifer. The fluctuation in the piezometric surface is dependent on the variation in the hydro-static pressure of groundwater within the aquifer. When the storage co-efficient value (S) and fluctuation in piezometric surface (h) is known, the quantity of water added or released from the aquifer can be calculated.

 

.Darcy’s experiment in 1850s led to determine the rate of flow of groundwater. Darcy’s Law states that the amount of groundwater discharge through a given portion of aquifer is proportional to the cross sectional area of flow, hydraulic gradient and hydraulic conductivity. It is expressed as: K= Q/ AI

 

Where Q = quantity of water discharged in unit time

 

A = total cross sectional area through which water percolates I= hydraulic gradient

 

Permeability of a rock is its capacity to transmit water under differential pressure and is measured by the rate at which it will transmit water through a unit cross section under unit pressure differential per unit distance. Rocks that lack porosity do not transmit water and are hence referred to as impermeable.

 

Porosity thereby is not a measure of permeability. Permeability was then understood to be a field co-efficient at the prevailing temperature of water. It is later the term ‘hydraulic conductivity’ used in preference to ‘coefficient of permeability’.

 

The capacity of a saturated rock to retain water after the drainage is ‘specific retention ‘. The quantity of water that will drain from a rock depends on the duration of drainage, temperature and chemical capacity of water. The ‘specific yield’ is the capacity of a saturated rock to drain water under the force of gravity. It is expressed as percentage volume of water yielded by draining a saturated rock to its total volume.

 

The groundwater resources can be categorised as ‘dynamic reserve’ and ‘static reserve’. The dynamic reserve represents long term average annual recharge under the condition of maximum use. Static reserve is the groundwater contained within the permanently saturated zone of the aquifer and that represents the total groundwater reserve minus the dynamic reserve. When the groundwater draft exceeds the normal annual replenishment and static resources are exploited in any manner it amounts to ‘mining of groundwater’ and a time may come where it no longer is a sustainable resource. Once groundwater is contaminated, it becomes a costly issue to clean up.

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