Hydrologic Cycle and Genetic Classification of Water | Geology Optional Notes for UPSC PDF Download

The Hydrologic Cycle

• The hydrologic cycle refers to the continuous movement of water among the Earth's biosphere, atmosphere, lithosphere, and hydrosphere.
• Water is stored in various reservoirs on Earth, such as the atmosphere, oceans, lakes, rivers, soils, vegetation, swamps, glaciers, snowfields, and groundwater.
• This water moves between different reservoirs through processes like evaporation, transpiration, condensation, precipitation, runoff, infiltration, groundwater flow, sublimation, and melting.

Let's elaborate on each point:

Continuous Water Movement

• Water constantly moves between the Earth's biosphere, atmosphere, lithosphere, and hydrosphere.

Water Reservoirs

• Water is stored in various reservoirs on Earth, including the atmosphere, oceans, lakes, rivers, soils, vegetation, swamps, glaciers, snowfields, and groundwater.

Water Transfer Processes

• Several processes facilitate the movement of water between reservoirs, such as evaporation, transpiration, condensation, precipitation, runoff, infiltration, groundwater flow, sublimation, and melting.

For example, when water from oceans evaporates due to solar heat, it forms clouds in the atmosphere. Subsequently, this water falls back to the Earth as precipitation, replenishing lakes and rivers.

Distribution of Water

• The Earth's hydrosphere holds a vast amount of water, approximately 1.4 billion km³ in total.
• Most of this water, around 97%, is saline water, leaving only about 3% as freshwater.
• Within this 3% freshwater, the majority exists as ice, snow cover, and groundwater.
• Only a small fraction, about 0.3% of freshwater, is readily accessible in lakes, reservoirs, and river systems for human use.
• The amount of water present in the atmosphere is minimal, approximately 0.001%.
• However, there is a significant exchange of water between the atmosphere, land, and oceans through precipitation, with around 113 km³/yr. and 370 km³/yr. entering the land and ocean respectively.
• As a result, water cycles rapidly in the atmosphere, leading to a low residence time of water in this reservoir.

Water Cycling and Residence Times

• Water undergoes continuous cycling among various reservoirs on Earth.
• The residence times of water in different major reservoirs vary significantly.
• Water spends relatively short periods in the atmosphere and rivers, typically days to weeks.
• Conversely, water remains in large lakes, glaciers, oceans, and groundwater for much longer durations, ranging from tens to thousands of years.

Elements of the Hydrologic Cycle

• Evaporation

  • Evaporation is the process where liquid water changes into its gaseous form. It demands a significant amount of energy, around 2.4x10^6 J, to convert 1 kilogram of liquid water to vapor.
  • Air has a saturation point for moisture at a given temperature. This saturation humidity rises exponentially as the air temperature increases. This principle is known as the Clausius-Clapeyron relationship.
  • Relative humidity, expressed as a percentage, compares the measured humidity to the saturation humidity. Evaporation halts once 100% relative humidity is attained.
  • Solar radiation supplies the energy needed for the liquid-to-gas transition, while wind propels evaporation by maintaining a vapor pressure difference between the water surface and the air above it.
  • Measurement of evaporation typically involves the use of a land pan, a 4-foot-wide, 10-inch-deep, unpainted galvanized metal pan. Wind speed and precipitation data at the land pan site help calculate evaporation rates.
  • Due to differences in water depth between a pan and a natural water body, a pan coefficient corrects for any heat gain or loss disparities between the two.

Understanding Transpiration and Evapotranspiration

• Transpiration:
  • Plants absorb water from the soil and release it into the atmosphere through a process known as transpiration.
  • Transpiration rates are influenced by factors such as the size and density of vegetation, solar radiation, and soil moisture levels.
  • During the growing season, transpiration plays a crucial role in the water cycle of plants.
  • When soil moisture reaches a minimum level where even plants are unable to extract water, it is termed as the wilting point.
• Evapotranspiration:
  • Evapotranspiration refers to the combined loss of water to the atmosphere through both evaporation from free water or soil moisture and transpiration from plants.
  • It is a comprehensive term encompassing various processes that result in the transfer of water from land or ocean surfaces to the atmosphere.
  • Potential Evapotranspiration:
    • Potential evapotranspiration quantifies the maximum possible water loss under specific meteorological conditions assuming continuous soil moisture availability.
    • Under ideal conditions, evapotranspiration continues unabated, but in reality, it is limited by the availability of soil moisture.
  • Actual Evapotranspiration:
    • Actual evapotranspiration represents the actual amount of water lost under normal field conditions, which is always less than the potential evapotranspiration.
    • It reflects the practical limitation imposed by soil moisture on the evapotranspiration process.

Evapotranspiration Overview

• Evapotranspiration encompasses:
• 
  
  • Evaporation from various open water bodies such as oceans, lakes, rivers, and ponds.
  • Evaporation occurring from bare soil.
  • Transpiration from different types of vegetation including aquatic, terrestrial, and riparian plants.

Significance of Evapotranspiration

• Evapotranspiration plays a crucial role in the hydrologic cycle by contributing significantly to the amount of rainfall retained in a particular region.
• In regions characterized by semiarid and arid climates, the potential for evapotranspiration is high. Consequently, a substantial portion of water loss from watersheds, which may equal or even exceed the amount of rainfall received, can be attributed to evapotranspiration.

Precipitation

• When an air mass cools, its saturation humidity decreases, causing the relative humidity to increase. If the relative humidity surpasses 100%, condensation (precipitation) occurs, typically triggered by the cooling of the air mass.
• The cooling of an air mass happens as it ascends in altitude since the temperature in the troposphere decreases with height. Precipitation is influenced, in part, by the condensation of moisture in the atmosphere.
• As a parcel of air rises, it experiences temperature variations due to the surrounding temperature distribution, leading to changes in pressure and volume within the parcel.
• For rainfall to happen at significant rates, four essential processes must take place:
  • Cooling of a moist air parcel to the dew point.
  • Condensation, which involves the phase change of water vapor into liquid water droplets.
  • Growth of droplets through the accumulation of water vapor.
  • Importation of additional water vapor to sustain the precipitation process.

Condensation and Cloud Formation

• Moist Air Cooling and Condensation:
  • When moist air rises due to various reasons like frontal convergence orographic uplift, or convective uplift, it cools as it ascends, leading to condensation.
• Cloud Condensation Nuclei (CCN):
  • Cloud Condensation Nuclei are crucial for cloud formation as they provide a surface for condensation to take place.
  • CCN typically consist of tiny particles known as aerosols that are suspended in the atmosphere.
• Sources of Aerosols:
  • Aerosols can originate from both natural sources and human activities, with natural sources being more prevalent on average.
  • Industrial activities and agricultural practices such as burning crops and forests can significantly increase local aerosol concentrations.
• Requirements for Cloud Formation:
  • Apart from CCN, cloud formation requires water droplets to grow in size significantly so they can overcome air upliftment and evaporation effects.
  • These droplets need to reach a size where they fall through the cloud due to their terminal velocity.

Estimating Water Budgets

• Effective Uniform Depth (EUD) of Precipitation
  • To estimate water budgets accurately, determining the EUD of precipitation over a specific area is crucial.
  • If the rain-gauge network is evenly distributed, a simple arithmetic average suffices for calculating the EUD.
• Adjustments for Non-Uniform Gauge Networks
  • In reality, rain-gauge networks are often unevenly spread, necessitating adjustments for accurate estimations.
• Precipitation Contour Mapping
  • The most precise method involves creating a precipitation contour map with isohyets (lines of equal rainfall).
  • Isohyets consider factors like topography's impact on precipitation distribution.
• Measurement of Precipitation Depth
  • By delineating the area between adjacent isohyets using a planimeter, the average precipitation depth is calculated as the mean of the bounding isohyets.
• Properties of Isohyets
  • Isohyets adhere to rules similar to topographic contours; they do not split, intersect, or cross.

Calculation of Area-Average Precipitation Methods

• Theissen Polygon Method:
  • The Theissen polygon method is a technique used to calculate area-average precipitation by assigning a weighting factor to each rain gauge based on the area it represents within a drainage basin.
  • To implement this method, the closest rain gauge stations are connected by lines to create triangles.
  • Perpendicular lines are drawn at the midpoint of each line between two stations, creating bisectors that are extended to form polygons around each station.
  • The area of each polygon is then measured to determine the weighted average precipitation for each station.
• Theoretical Background:
  • The concept behind the Theissen polygon method lies in creating polygons around each rain gauge station to define the area of influence for that particular station.
  • By calculating the area of these polygons, the method estimates the contribution of each station to the overall average precipitation in the region.
  • For example, if Station A has a larger polygon area compared to Station B, Station A's precipitation data will have a greater impact on the final calculated average.
• Comparison with Isohyetal Method:
  • In contrast to the Theissen polygon method, the Isohyetal Method calculates area-average precipitation by drawing lines of equal precipitation (isohyets) on a map.
  • These lines connect points of equal precipitation intensity, providing a different approach to estimating average precipitation across a region.
  • For instance, the Isohyetal Method is particularly useful when dealing with irregularly distributed rain gauge stations or complex terrain.

Runoff and Water Flow

• Definition of Runoff: Runoff refers to the total flow in a stream, comprising overland flow, interflow, and baseflow.
• Infiltration: Infiltration is the process where rainfall moves downwards into the soil layer. This movement varies based on factors like soil type, precipitation rate, and soil moisture.
• Overland Flow: When the rate of precipitation exceeds the infiltration rate, water drains across the land surface, forming overland flow. This is like a thin film of water moving over the ground.
• Interflow: Some infiltrated water moves horizontally through layers of soil with low permeability in the unsaturated zone. This horizontal movement within the soil is termed interflow.
• Groundwater Flow: Water flows in saturated subsurface areas due to gravity and hydraulic gradient. The part of this flow that discharges into streams is known as baseflow.
• Baseflow: Baseflow represents the groundwater contribution to streamflow. It increases when groundwater levels rise due to infiltration into the ground.
• Factors Affecting Baseflow: The amount of baseflow to a stream is directly related to the hydraulic gradient towards the stream.

Groundwater Contribution to Stream Flow

• Factors Affecting Groundwater Contribution

  • Groundwater contributes to stream flow based on the hydraulic gradient.
  • In periods of no precipitation, streamflow consists of groundwater contributions and tributary flow.

• Gaining Streams vs. Losing Streams

  • In humid regions, streams receive groundwater discharge, leading to increased stream discharge downstream (gaining stream).
  • Conversely, in arid regions, streams rely on overland flow, interflow, and baseflow, resulting in water draining away from the stream at lower elevations (losing stream).

• Hydraulic Gradient and Stream Behavior

  • The hydraulic gradient of the surrounding aquifer determines whether a stream is gaining or losing water.
  • In gaining streams, the gradient is toward the stream, while in losing streams, water drains away from the stream into the ground.

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Streamflow Measurement Techniques

• Measuring streamflow involves determining the volume of water flowing in a river or stream.
• Common technique: measuring stream velocity at various points across the stream width.
• Velocity measurements represent cross-sectional areas of the stream.
• Water velocity peaks towards the surface; 60% depth measurement provides average velocity.

Estimating Discharge

• Discharge estimation methods: measuring stream velocity or water surface height.
• Discharge commonly expressed in cubic meters per second (m³/s).
• Discharge is a function of cross-sectional area, river stage, and flow velocity.

Gauging Station Operations

• Discharge measurement at gauging stations is costly and labor-intensive.
• River stage measurements are more common due to ease of measurement.
• Rating curve: relationship between river stage and discharge over time.
• Rating curve development involves concurrent stage and discharge data.

Streamflow Measurement Process

• Continuous recording of river stage above a reference point.
• Establishing the stage-discharge relationship is crucial.
• Converting stage records into discharge records.

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The Hydrologic Budget Equation

• A hydrologic budget is based on the principle of conserving water mass within a control volume, which can vary in size and time scale.
• The equation for a hydrologic budget is: Change in Storage (with respect to time) = Inflow - Outflow.

Control Volume and Watershed

• A control volume is a selected region where inflow and outflow components are calculated.
• A watershed, a natural hydrologic unit, is often chosen as a control volume and is separated by divides from other watersheds.
• Watersheds have three-dimensional topography drained by a stream network.

Variability of Inflows and Outflows

• Inflows and outflows vary based on the size and location of the control volume.
• For instance, precipitation is a dominant inflow in watersheds, but not in groundwater aquifers.

Types of Inflows and Outflows

• Sources of inflow: Precipitation, Overland flow, Surface water inflow, Groundwater inflow, Anthropogenic inputs (e.g., pipes).
• Sources of outflow: Evapotranspiration, Evaporation of surface water, Surface water outflow, Groundwater outflow, Anthropogenic outputs.

Steady State vs. Transient State

• In a steady state, inflow equals outflow, resulting in zero accumulation.
• A transient state occurs when there is a difference between inflow and outflow, leading to accumulation or depletion.
• Positive or negative changes in storage indicate accumulation or depletion, respectively.

Changes in Land Water Storage

• Components of Land Water Storage:
  • Surface water
  • Soil moisture
  • Ice and snow
  • Plant moisture
  • Groundwater
• Understanding Storage Changes:

  The hydrologic budget equation helps in calculating changes in water storage mass. However, it doesn't pinpoint the exact component where the change occurs.

• Dominant Components and Influences:

  The main components of land water storage vary based on subsurface geology and climate. For instance:

  • In high-latitude areas, snow and ice can significantly impact total storage changes.
  • In tropical regions, groundwater and soil moisture changes play a major role in storage variations.
• Equation and Components:

  The equation for storage changes involves:

  • Inputs: Precipitation, surface water inflow, groundwater inflow, injection
  • Outputs: Evapotranspiration, surface water outflow, groundwater outflow, pumping
  • Accumulation: Changes in storage in various components like surface water, soil moisture, ice, snow, plant moisture, and groundwater

Understanding the Hydrologic Budget Equation

• Basin-Wide Scale Recharge Determination:
  • Estimating groundwater recharge on a basin-wide scale involves using the hydrologic budget equation for the aquifer.
• Factors Involved:
  • Groundwater inflows, outflows, and storage processes need to be comprehended to calculate recharge accurately.
  • It is crucial to balance all known estimates to determine the amount of recharge effectively.
• Practical Applications and Assumptions:
  • Practical applications of the hydrologic budget equation often necessitate making certain assumptions.
  • Quantifying groundwater inflows and outflows along a watershed's boundary is challenging and is often disregarded for practical purposes.
• Comparison of Fluxes:
  • It is essential to compare the magnitudes of fluxes on the right side of the equation.
  • Flows with significantly smaller magnitudes can be ignored if they make negligible contributions to storage change.
• Hydrologic Budget for an Open System:
  • An open system's hydrologic budget involves both surface and groundwater considerations.
  • This budget provides a comprehensive picture of the system's water dynamics.
• Surface Water System:
  • Surface Water Inflow: This refers to the input of water into the surface water system, primarily through processes like precipitation (P).
  • Surface Water Outflow: This represents the output of water from the surface water system, including surface runoff (Q) and other forms of water movement.
  • Net Change in Surface Water: Calculated as the difference between inflow and outflow, represented by the symbol ∆H.
• Groundwater System:
  • Groundwater Inflow: Involves water entering the groundwater system, typically through processes like subsurface runoff (G).
  • Groundwater Outflow: Refers to the water leaving the groundwater system, which includes processes such as discharge and other forms of water movement out of the system.
  • Net Change in Groundwater: Represented by the symbol ∆G, this value reflects the overall change in groundwater storage within a defined area over a specified period.
• Total System Balance:
  • Overall Water Balance: This accounts for all water inputs and outputs in the entire system, including surface water, groundwater, and other components like evapotranspiration (ET) and storage (S).
  • Net Change in Total System: Calculated as the difference between total inflows and total outflows, denoted by the symbol ∆T.
• Hydrologic Budget:

  The hydrologic budget simplifies the overall water balance equation by considering the net mass exchanges.

  The key components of the hydrologic budget include precipitation (P), surface runoff (Q), subsurface runoff (G), evapotranspiration (ET), and storage in the control volume (S).

  Ensuring a balanced water budget involves measuring or estimating all these components accurately to reflect the true state of the system.

• Challenges and Uncertainties:
  • Defining Control Volume Boundaries: One of the key challenges lies in accurately determining the boundaries of the control volume, which can significantly impact water balance calculations.
  • Estimating Fluxes at Boundaries: It is crucial to estimate the water fluxes at boundaries accurately over both time and space to ensure a comprehensive understanding of the system.
  • Knowledge of Storage Capacity: Understanding the storage capacity within the system is essential for accurate water balance assessments, but this can be challenging due to various factors.
  • Internal Redistribution: The internal redistribution of water within the control volume can introduce complexities that may affect the accuracy of water balance computations.

Genetic Classification of Groundwater

Introduction

• We understand that during the early cooling stages of the earth, water was released through degassing, leading to the formation of the hydrosphere across the planet.

Historical Background

• Hydrologists like V.I. Vernadiski initially considered water to be a mineral, sparking the introduction of genetic classification in hydrology.
• In 1947, G.M. Karmenshki became the pioneer hydrologist to categorize waters based on their origins.

Sources of Groundwater

• There are two primary sources of water in the earth's crust: precipitation from the atmosphere that percolates downwards and water released through deep-seated processes at the crust-mantle boundary, known as juvenile water.
• Juvenile water, although a small contributor, has been supplying water to the earth for billions of years, playing a crucial role in the water cycle.

Genetic Classification

• Water is classified into exogenic and endogenic categories based on its movement: exogenic water flows downward into the crust, while endogenic water moves upwards.

Types of Waters

• Infiltrogenic Water: Originates through infiltration processes.
• Sedimentogenic Water: Formed in sedimentary environments.
• Metamorphogenic Water: Arises from metamorphic processes.
• Magmatogenic Water: Generated through magmatic activities.

Classification Details

• Infiltrogenic and Sedimentogenic waters belong to the exogenic category, while Metamorphogenic and Magmatogenic waters fall under the endogenic classification.

Infiltrogenic Water Summary

• Overview of Infiltrogenic Water

  Infiltrogenic water primarily comes from precipitation and moves within the Earth's crust in various forms.

• Types of Infiltrogenic Water

  • Meteoric Water

    Meteoric water originates in the atmosphere with very low salinity.

    • Atmogenic Water: Water contributed from the atmosphere, mainly through rain and snow.
    • Biogenic Water: Water that undergoes composition changes when it falls on plants or humus-rich soil.
    • Lithogenic Water: Water that undergoes chemical modifications and dissolution of carbonate rocks, affecting permeability.
    • Evaporation Stage: Common in arid regions, leading to increased salt content in water and soil acidification.

  • Thalassogenic Water

    Another category of infiltrogenic water based on source, supply mode, and salinity.

Thalassogenic Water

• Definition: Thalassogenic water refers to water of marine origin that becomes trapped in sediments during the process of sedimentation. It typically has a high saline content.
• Classification: Some hydrologists categorize thalassogenic water as sedimentogenic water due to its origin and characteristics.
• Characteristics: Thalassogenic water is characterized by its saltiness, deriving from its marine source.
• Examples: An example of thalassogenic water is the seawater that seeps into sediment layers near coastlines, becoming trapped over time.

Sedimentogenic Water

Introduction

• Water entering the Earth's crust through burial along with sediments in basins or lakes is known as sedimentogenic water, also referred to as fossil or relict water.

Classification of Sedimentogenic Water

• Syngenetic Water:
• 
  
  • In this scenario, the age of the water and the formation are similar, or there is no contribution of water after the sedimentary formation's hydrology formation.
  • Found in relatively young sedimentary basins.
• Epigenetic Water:
• 
  
  • In this case, the age of the water differs from that of the formation, with a significant amount of water contributed laterally or vertically over time.
  • Water moves laterally and vertically within the crust.

Water Movement in the Crust

• Water moves within the crust due to exogenic or endogenic processes.
• Formation water in deep crustal regions is typically a mixture of infiltrated, sedimentogenic, and other types of water.

Metamorphogenic Waters

• Metamorphogenic waters are located deep within the Earth's crust.
• Geology students understand that when sediments are buried in the crust, metamorphic rocks are formed.
• During metamorphosis, rocks go through chemical changes.
• An example of this process is the conversion of kaolinite into sillimanite, releasing water as shown in the reaction: Al₄Si₄O₁₀(OH)₈ = Al₂O₃ 3SiO₂ SiO₂ H₂O
• This water is associated with the burial of sedimentary formations.
• Metamorphogenic waters also form due to mineral hydration when in contact with lava.
• Water is released into the crust during the crystallization of minerals.
• Dehydration of various minerals results in the release of water, such as in gypsum-bearing rocks.

Magmatogenic Water

• Generated in the deeper regions of the Earth's crust and also formed in the mantle through the interaction of hydrogen and oxygen molecules.
• Creation of the first water molecule on Earth is attributed to this process.
• Linked to magma due to its origin, which can be plutonic or a result of volcanic activities.
• Mainly sourced from the cooling of the Earth and the solidification of molten magma.
• Water released during volcanic eruptions, in the form of water vapor, falls under this category.
• Categorized as volcanic or transmagmatic based on its release in the crust or mantle.

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