Introduction to Hydrology

Hydrology Definition

• Hydrology is the science that describes and predicts spatial/temporal characteristics of water in all phases.
• It focuses on terrestrial, atmospheric, and oceanic reservoirs of the global system.
• Hydrology covers fluxes of water between these reservoirs and the physical processes affecting its movement.

Motivation for Studying Hydrology

• Water is essential for human life, making sustainable water management a key concern.
• Water is a key component in the Earth system regarding climate, weather, landscape evolution, and biogeochemical processes.
• Engineering requires water resource systems for water supply and hazard mitigation.
• Managing and allocating water resources sustainably is important.

Sciences Embodied by Hydrology

• Potamology: Science of surface streams
• Limnology: Science of surface lakes
• Cryology: Science of surface snow and ice
• Glaciology: Science of surface glaciers
• Oceanology: Science of surface oceans
• Hydrometry: Science of measurement of water
• Hydrography: Science that describes physical features of all waters on the Earth's surface

Quantity of Water on Earth

• The total estimated quantity of water on Earth is 1348.25305 x 10^6 km^3.
• This quantity would cover the Earth to an average depth of 2.73 km, assuming Earth to be a uniform sphere with a diameter of 12,800 km.

Global Water Budget

• Salt Water accounts for 96.9781% of all water.
• Oceans constitute the largest portion of salt water at 1307.410 x 10^6 km^3.
• Freshwater accounts for 3.0219% of the Earth's water resources
• Glaciers and polar ice caps make up the largest portion of freshwater at 30.4300 x 10^6 km^3.
• Total water resources of the earth account for 1348.25305 x 10^6 km^3.

Early Hydrology Assumptions

• Early thinkers assumed rainfall was inadequate to supply surface water in rivers and springs.
• They thought that Earth was impervious below a certain depth.
• Homer, Thales, and Plato theorized subterranean flow from sea to land and, that the flow supplied water to springs.
• Aristotle assumed the existence of an atmosphere below ground.
• Vitruvius stated that the source of rivers and springs was rain or ice.

Key Figures in Hydrology

• Pierre Perrault (1608–1680) observed river discharge, noting it was more than precipitation.
• Edme Mariotte (1620–1684) supported Perrault's observations.
• Edmund Halley (1656–1742) concluded the Mediterranean Sea evaporation could supply rivers.
• Pitot (1732) proposed the flow velocity measurement tube.
• Chezy (1775) proposed the flow formula.
• Dalton's evaporation measurements (1801) and infiltration measurements (1802) helped progress the field.

19th Century Hydrology Contributions

• Francis formula (1856)
• Venturimeter by C. Herschel (1886)
• Currentmeter by Ellis
• Ganguillet and Kutter formula to determine Chezy's coefficient (1869)
• Contribution by Darcy, Dupuit, Gabriel, Daubree, Abbe Parramelle, Prinz Adolph Thiem, Gunther, Theiss Forcheimer, Slitcher and Hazen in groundwater flow
• Manual of Hydrology was published in 1850 by civil engineer Nathaneil Bardmore.

Later Hydrogology Contributions

• Mead published Notes on Hydrology (1904) and Hydrology (1909).
• Meyer published Elements of Hydrology in 1927.
• L. K. Sherman proposed the unit hydrograph theory in 1932, and introduced the instantaneous unit hydrograph
• UNESCO started the International Hydrological Decade in 1965.

Properties of Water

• Water consists of two hydrogen and one oxygen molecules bonded together.
• It is an inorganic liquid that naturally exists as a solid, liquid, and gas.
• Water has the highest surface tension compared to other liquids besides mercury.
• It is the universal solvent.
• Pure water boils at 100 °C and freezes at 0 °C.
• Dissolved ions like Ca, Mg, Na, and Cl enable water to conduct electricity.
• Water has the greatest thermal conductivity except for mercury.
• Mass density is ρ = 1000 kg/m³.
• Weight density is γ = 9.81 kN/m³.

Hydrologic Reservoirs

• Key reservoirs include the atmosphere, soil water, groundwater reservoirs, surface water, cryosphere and biosphere.

Hydrologic Fluxes

• Key fluxes between reservoirs are precipitation, evapotranspiration, infiltration, recharge and runoff.
• Precipitation and evapotranspiration are the main fluxes between the atmosphere and surface.
• Percolation recharges groundwater aquifers and feeds surface water bodies.
• Evaporation from the soil, open water, and transpiration from vegetation replenish atmospheric water.

Driving Force of the Hydrologic Cycle

• The hydrologic cycle is driven by energy inputs.
• At a global scale, the system is closed with negligible water entering or leaving.
• The solar radiative energy input, averaging 342 W m-2, drives the hydrologic cycle.
• The hydrological cycle dissipates energy, it includes energy storage and movement.

Quantitative Analysis of the Hydrological Cycle

• Global precipitation is 872 mm which equates to 445,000 km^3 of water.
• Global precipitation on ocean is 346,000 km^3 at 961 mm.
• Global precipitation on land is 99,000 km^3 at 660 mm.
• Global evaporation is 872 mm which equates to 445,000 km^3 of water.
• Global evaporation from oceans is 383,000 km^3 at 1063 mm.
• Global evaporation from land is 62,000 km^3 at 413 mm.
• Runoff is 37,000 km^3 at 247 mm.
• The atmospheric moisture average is 14,000 km^3 and is replaced 32 times a year so atmosphere moisture is 10 days.

Processes in the Hydrological Cycle

• Evaporation: Water turns into vapor and diffuses into the atmosphere.
• Transpiration: Water passes from liquid to vapor state through plant metabolism.
• Precipitation: Moisture falls from clouds to the ground.
• Interception: Precipitation is received and retained by vegetation then evaporates later.
• Infiltration: Precipitation enters the ground and flows downwards.
• Vapour diffusion: Water retained by soil flows as vapour towards the surface.
• Surface detention: Temporary water stored in a river channel.
• Depression storage: Water stored in ditches and ponds before river flow starts.
• Surface runoff: Water flows in a stream after meeting all requirements.
• Interflow: Water moves laterally towards a stream and appears on the surface.
• Groundwater flow: Infiltrated water reaches and moves through the saturated zone.

Liquid-Transport in Hydrological Cycle

• Precipitation on the land surface and vegetation
• Through fall from vegetation to land surface
• Melting of snow and ice onto land surface
• Surface runoff
• Infiltration from land surface to subsoil
• Ex-filtration from subsoil to land surface
• Interflow from subsoil to streams
• Percolation from subsoil to aquifers
• Capillary rise from aquifers to soil
• Groundwater flow from aquifer to streams and from streams to aquifer

Vapour-Transport Phases of the Hydrological Cycle

• Evaporation from land, stream, lakes, oceans and atmosphere
• Evapotranspiration from vegetation to atmosphere.
• Sublimation from snow-packs and ice-caps to atmosphere
• Vapour diffusion from subsoil to land surface

Hydrological Process Types

• Deterministic: Based on exact law, ignores chance occurrences.

The Hydrological Equation

• Inflow - Outflow = Change in storage

Modern Techniques in Hydrology

• Remote sensing
• Artificial neural network
• Geographical information system
• Genetic algorithm
• Fuzzy logic

Mass Balance, Fluxes and Units

• Analysis involves application of mass balance.
• A mass balance equation can be derived using Reynolds transport theorem.
• Theorem starts with control volume in space.
• A mass Balance application is a pre-specified control volume.