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

The Hydrologic Cycle

Overview
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.

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
• Melting

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.
• 97%: Saline water.
• 3%: Freshwater, with most of it as ice, snow cover, and groundwater.
• 0.3%: Readily accessible freshwater in lakes, reservoirs, and river systems.
• Atmospheric water comprises about 0.001% of Earth's water.
• Significant water exchange occurs between the atmosphere, land, and oceans through precipitation.
• 113 km³/yr enters land.
• 370 km³/yr enters oceans.

Water Cycling and Residence Times

• Water undergoes continuous cycling among various reservoirs.

Residence Times:

• Atmosphere and rivers: Days to weeks.
• Large lakes, glaciers, oceans, and groundwater: Tens to thousands of years.

Elements of the Hydrologic Cycle

Evaporation

Process where liquid water changes into gaseous form, requiring significant energy (~2.4x10⁶ J/kg).

Governed by:

• Clausius-Clapeyron relationship: Saturation humidity rises with temperature.
• Relative Humidity: Compares measured humidity to saturation humidity.

Key Influences:

• Solar radiation: Provides energy.
• Wind: Propels evaporation by maintaining a vapor pressure difference.

Measurement:

• Using a land pan (4-foot-wide, 10-inch-deep galvanized metal pan).
• Adjustments using pan coefficients for heat gain/loss differences.

Transpiration and Evapotranspiration

Plants absorb water from soil and release it into the atmosphere.
Influenced by:

• Vegetation size and density.
• Solar radiation.
• Soil moisture.
• Wilting Point: Minimum soil moisture level where plants cannot extract water.

• Combined water loss via evaporation and transpiration.
• Potential Evapotranspiration: Maximum water loss under optimal conditions.
• Actual Evapotranspiration: Real-world water loss limited by soil moisture.

Precipitation

• Occurs when an air mass cools, decreasing its saturation humidity and causing condensation.

Processes for Significant Rainfall:

• Cooling air to the dew point.
• Condensation of water vapor into droplets.
• Droplet growth.
• Sustained water vapor supply.

Condensation and Cloud Formation

• Cloud Condensation Nuclei (CCN)

  • Tiny particles (aerosols) essential for condensation.

  • Sources: Natural (dominant) and human activities.

• Droplet Growth: Droplets must overcome uplift and evaporation to precipitate.

Estimating Water Budgets

Effective Uniform Depth (EUD) of Precipitation

• Even Rain-Gauge Network: Simple arithmetic average suffices.
• Non-Uniform Network: Adjustments required.
• Isohyets: Contour maps of equal rainfall.

Theissen Polygon Method

• Technique to calculate area-average precipitation.

Steps:

• Connect rain gauge stations to form triangles.
• Draw perpendicular bisectors to create polygons.
• Measure polygon areas to determine weighted averages.

• Comparison:

  • Isohyetal Method: Connects points of equal precipitation intensity, suitable for irregular terrain.

Runoff and Water Flow

• Runoff: Total streamflow, including overland flow, interflow, and baseflow.
• Infiltration: Downward movement of rain into soil.
• Overland Flow: Surface flow when precipitation exceeds infiltration.
• Baseflow: Groundwater contribution to streams, influenced by hydraulic gradient.

Streamflow Measurement Techniques

Discharge Estimation

Common methods:

• Measuring stream velocity at various points.
• Gauging river stage (water surface height).
• Discharge: Volume of water flow (m³/s).
• Rating Curve: Relates river stage to discharge.

The Hydrologic Budget Equation

Principles

• Conserving water mass within a control volume.
• Equation: Change in Storage = Inflow - Outflow.

Control Volume and Watershed

• Watershed: Natural hydrologic unit drained by a stream network.
• Inflows and outflows vary by location and volume.

Types of Inflows and Outflows

• Inflows: Precipitation, surface water inflow, groundwater inflow, anthropogenic inputs.
• Outflows: Evapotranspiration, surface water outflow, groundwater outflow, anthropogenic outputs.

Genetic Classification of Groundwater

Introduction

• Early Earth water released via degassing formed the hydrosphere.

Classification

• Exogenic Water: Flows downward into the crust.
• Endogenic Water: Moves upward from the crust.

Types of Water

• Infiltrogenic Water: From precipitation.
• Sedimentogenic Water: Trapped during sedimentation.
• Metamorphogenic Water: Released during metamorphic processes.
• Magmatogenic Water: Linked to magmatic activities.