Introduction to Hydrology

Hydrology Definition

• Hydrology is the study of water on, above, and below the surface of the Earth.
• It encompasses the occurrence, circulation, and distribution of water.
• Hydrology is an interdisciplinary field that considers the interplay of water with living organisms, its properties, circulation, and distribution.

Origin and History

• History of hydrology dates back 5000-6000 years to civilizations along the Indus, Tigris, Euphrates, Hwang Ho, and Nile rivers, where early hydraulic engineers developed canals, dams, and wells.
• 18th century saw advancements with the Bernoulli piezometer, Pitot tube, and Bernoulli's equation.
• The 19th century saw development in groundwater hydrology, incorporating Darcy's law, Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.
• Three stages of development in the study of hydrology:
  • Geographical hydrology - Focused on the hydrological cycle and the concept of water balance.
  • Engineering hydrology - Focused on designing control structures and quantitatively analyzing hydrological phenomena.
  • Social development - Marked by increased water demand and large-scale water resource utilization, emphasizing resource analysis and management.

Branches of Hydrology

• Pure Hydrology: Focuses on the hydrological cycle, precipitation, runoff, the relationship between precipitation and runoff, hydrographs, and flood routing.
• Engineering Hydrology: Deals with planning, designing, and operating engineering projects related to water control and use.
• Chemical Hydrology: Studies the chemical characteristics of water.
• Ecohydrology: Examines the interaction between organisms and the hydrological cycle.
• Hydrogeology: Focuses on the presence and movement of groundwater, also known as geohydrology.
• Hydroinformatics: Applies information technology to hydrology and water resource applications.
• Surface water Hydrology: Studies hydrological processes occurring at or near the Earth's surface.
• Groundwater Hydrology: Focuses on underground water.
• Drainage Basin Management: Encompasses water storage, reservoir design, and flood protection measures.
• Water Quality: Analyzes water chemistry in rivers and lakes.
• Isotope Hydrology: Investigates the isotopic signatures of water to determine origin and age.

Hydrological Cycle

• Key processes in the hydrological cycle:
  • Evaporation: Conversion of water into vapor through heat.
  • Condensation: Transition of gas or vapor into a liquid or solid state.
  • Precipitation: Atmospheric moisture falling to the Earth's surface as rain, snow, hail, or other forms.
  • Interception: Storage of rain or snow on leaves and branches, leading to eventual evaporation back to the air.
  • Infiltration: Penetration of water into sub-surface soil from the ground surface.
  • Percolation: Downward and radial movement of water through subsurface soil layers, often reaching groundwater.
  • Transpiration: The process of water vapor loss from living plants to the atmosphere.
  • Runoff: Drainage or flood discharge leaving an area as surface flow or through pipelines.

Advantages of the Hydrological Cycle

• Recycles the Earth's water supply.
• Provides water for humans, animals, and plants.
• Evaporation and infiltration help remove impurities from water.
• Supports aquatic ecosystems and fish populations.

Disadvantages of the Hydrological Cycle

• No net gain or loss of water in the cycle.
• Acid rain can result from atmospheric pollution.
• Runoff can lead to flooding.

Global Water Availability

• Freshwater availability varies significantly worldwide.
• Freshwater scarcity is a growing concern, impacted by population growth and water use patterns.

Global Water Resources

• Global water resources are estimated at 105,000 km3, representing a small portion of the total water on Earth.

Water Availability vs Population

• Fresh water availability per capita has decreased over time due to population growth and increased water demand.

Application of Hydrology

• Determining the water balance of regions.
• Managing agricultural water balance.
• Mitigating and predicting flood, landslide, and drought risks.
• Flood forecasting and warnings.
• Designing irrigation schemes and agricultural productivity.
• Designing bridges, dams, sewers, urban drainage systems.

Sources of Data

• Hydrological studies require various data, including:
  • Weather records (temperature, humidity, wind velocity).
  • Precipitation data.
  • Streamflow records.
  • Evaporation and transpiration data.
  • Infiltration characteristics of the area.
  • Groundwater characteristics.
  • Physical and geological characteristics.