MEV 012: Unit 11 - Hydrology

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 UNIT 11: HYDROLOGY

11.0 Introduction

Hydrology is the scientific study of the movement, distribution, and quality of water on Earth. It encompasses surface water, groundwater, and the processes of the hydrological cycle. Hydrology is essential for managing water resources, understanding weather and climate systems, and planning sustainable development.

11.1 Objectives

After completing this unit, learners should be able to:

  • Understand the distribution of water on and below the Earth’s surface
  • Explain the hydrological cycle
  • Describe types and genetic origins of groundwater
  • Understand aquifers and spring classifications
  • Comprehend the residence time of water in various reservoirs

11.2 Distribution of Water in the Crust

Water is distributed in two primary forms:

11.2.1 Surface Distribution

  • Oceans and Seas – ~97.5% of Earth’s water is saline and found in oceans
  • Freshwater – ~2.5% of Earth's water; found in:
    • Ice caps and glaciers (~68.7%)
    • Groundwater (~30.1%)
    • Surface water (rivers, lakes, swamps) (~1.2%)

11.2.2 Subsurface Distribution

  • Groundwater – Water held below the Earth's surface in soil pore spaces and fractures in rock formations
  • Soil Moisture – Water held in the unsaturated zone above the water table
  • Aquifers – Permeable geological formations that can store and transmit water

11.3 Hydrological Cycle

The hydrological cycle is the continuous movement of water through the atmosphere, lithosphere, and biosphere. Major processes include:

  • Evaporation: Liquid water to vapor (from oceans, lakes, soil)
  • Transpiration: Water loss from plants
  • Condensation: Vapor turning into clouds
  • Precipitation: Rain, snow, sleet, or hail
  • Infiltration: Water entering the ground
  • Runoff: Surface flow of water into streams and rivers
  • Percolation: Downward movement through soil and rock to form groundwater

11.4 Genetic Types of Groundwater

Groundwater is classified based on its origin:

  • Meteoric Water: Derived from precipitation; most common type
  • Connate Water: Trapped in rock pores at the time of formation (ancient)
  • Juvenile Water: Originates from deep within the Earth’s crust; not part of the current hydrologic cycle

11.5 Residence Time of Water

Residence time refers to the average duration water stays in a particular part of the hydrological cycle.

Reservoir

Residence Time

Atmosphere

~10 days

Rivers

~2 weeks

Lakes

~100 years

Groundwater

Up to thousands of years

Glaciers/Ice caps

Up to hundreds of thousands of years

Oceans

~3,000 years

11.6 Types of Aquifers

An aquifer is a geological formation that stores and transmits water. Aquifers are classified into:

  • Unconfined Aquifer: Water table is exposed to atmospheric pressure
  • Confined Aquifer: Bound above and below by impermeable layers; water is under pressure
  • Perched Aquifer: Isolated by an impermeable layer above the main water table
  • Artesian Aquifer: Pressurized confined aquifer where water rises on its own when tapped

11.7 Springs and Their Classification

A spring is a natural discharge of groundwater to the Earth's surface. Springs occur when the water table intersects the ground surface.

11.7.1 Classifications of Springs

11.7.1.1 Based on Temperature

  • Cold Springs: Temperature similar to ambient air
  • Hot Springs: Water emerges at higher temperature due to geothermal heating
  • Thermal Springs: Heated by deep circulation near geothermal zones

11.7.1.2 Based on Flow

  • Perennial Springs: Continuous year-round flow
  • Intermittent Springs: Flow only during specific seasons
  • Periodic Springs: Discharge at regular intervals (due to siphon-like mechanisms)

11.7.1.3 Based on Geomorphologic Control

  • Fault Springs: Occur along fractures/fault zones
  • Karst Springs: Found in limestone regions with underground channels
  • Contact Springs: At the boundary between permeable and impermeable strata

11.7.1.4 Based on Source of Water

  • Artesian Springs: From pressurized confined aquifers
  • Gravity Springs: Water flows due to gravity from elevated recharge zones
  • Seepage Springs: Slow flow from porous soil or rock

11.8 Let Us Sum Up

Hydrology deals with the distribution, movement, and quality of water. Understanding surface and subsurface water systems, the hydrologic cycle, aquifers, and spring formations is vital for effective water resource management. Key concepts such as groundwater origin, residence time, and aquifer types provide insight into sustainable hydrological practices.

11.9 Keywords

·         Hydrology-Study of water's movement, distribution, and properties on Earth

·         Hydrological Cycle-Natural cycle involving movement of water through different Earth systems

·         Aquifer-Underground layer of permeable rock that holds groundwater

·         Groundwater-Water located beneath Earth's surface in soil and rock formations

·         Spring-Point where groundwater flows to the surface naturally

·         Meteoric Water-Water from recent precipitation

·         Connate Water-Ancient water trapped in sediments during formation

·         Juvenile Water-Water originating deep within the Earth's interior

·         Residence Time-Average time water stays in a specific reservoir

·         Unconfined Aquifer-Aquifer where water is exposed to atmospheric pressure

·         Confined Aquifer-Aquifer bounded by impermeable layers under pressure

·         Perched Aquifer-Small, localized aquifer above the main water table

·         Artesian Spring-Spring formed by pressure in a confined aquifer

·         Thermal Spring-Spring with water heated by geothermal activity

·         Karst Spring-Spring from limestone terrain, often with cave-like features

 

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