By the end of this lecture, you should be able to:

• Describe the structure and properties of water relevant to hydrology
• Understand the concept and components of the hydrological cycle
• Explain the global distribution of water and its limited availability
• Define the water balance equation and interpret each term
• Be able to recognise a surface catchment
• Identify challenges in applying hydrological concepts at different scales

• Hydrology comes from Greek hydor (water) and Latin logia (study).
• Despite the name, it focuses on fresh water, not all water.
• Oceanography is the science of saline water.
• Hydrology examines:
  • Distribution and movement of water
  • Interactions with land, atmosphere, and ecosystems

The oldest known description of water cycle:

[...] precipitation falling in the mountains infiltrated the Earth’s surface and led to streams and springs in the lowlands. [...]

Vitruvius, Roman Architect, 1st century BC

“The total quantity of fresh water on earth could satisfy all the needs of the human population if it were evenly distributed and accessible.” — Stumm (1986)

• Hydrology explains uneven water distribution
• Serves as:
  • A pure science – understanding natural processes
  • An applied science – solving water challenges

• Two main traditions:
  • Engineering hydrology – design-focused, quantitative
  • Earth science hydrology – process-focused, explanatory
• This course adopts a quantitative Earth science perspective
• Related fields:
  • Geohydrology – groundwater systems
  • Ecohydrology – water–ecosystem interactions

ETH-Zurich

• Covers more than 70% of Earth’s surface
• Essential to:
  • Human survival
  • Agricultural production
  • Ecological functioning
• For Indigenous Australians, water is integral to Country, ceremony, and responsibility

Physical properties of water

• Water: H_2O two hydrogen atoms covalently bonded to oxygen

• It is a bipolar molecule:

  • Positive hydrogen, negative oxygen

• Bonds:

  • Covalent within molecules
  • Hydrogen between molecules

• Leads to:
  • High surface tension
  • Strong cohesion
  • Exceptional solvent capability

Density

• Water is most dense at 4°C
• Ice is less dense → floats on water
• Implications: - Lakes freeze from top-down - Aquatic ecosystems persist in cold conditions

Specific Heat

• Water has high specific heat capacity (4.2 kJ/kg/K)
• Slows temperature change
• Buffers climate and daily temperature swings
• Comparison with other materials:

• The high energy required to break hydrogen bonds makes water an ideal climate stabiliser.
• Water changes state:
  • Solid ↔ Liquid ↔ Gas
• Processes include:
  • Melting, freezing
  • Evaporation, condensation
  • Sublimation, deposition
• Involves latent heat
• Crucial for energy transfer in climate systems

Let’s switch from micro to macro! Where do we look?

A spatial unit to study the water cycle?

• A catchment is a land area where surface water drains to a common point
• Defined by topographic divides
• Used to define hydrological boundaries
• Contains nested sub-catchments

• Surface water divides ≠ groundwater divides
• Groundwater can flow across topographic boundaries
• Important for:
  • Integrated water resource management
  • Understanding flow connectivity

1. Get a topographic map

   • A contour map with visible elevation lines.
   • Mark known streams (especially the outlet point).

2. Identify the outlet (pour point)

   • The point you want to delineate the catchment (where stream exits).

3. Mark all flow lines (ridges to valleys) - Draw approximate flow paths down the steepest slope.

4. Trace ridgelines (water divides)

   • Trace along high points (ridges) that separate your catchment from neighbouring ones.
   • Follow the highest elevation between two drainage areas.
   • Cross contour lines at right angles (steepest ascent).
   • Close the boundary loop at the other side of the outlet.

5. Check connectivity

   • Ensure all water within your boundary flows to the outlet.
   • Everything outside drains elsewhere.

• The hydrological cycle describes water movement between Earth and atmosphere as gas, liquid, or solid.
• It is a conceptual model — useful but simplified.
• Hydrology begins with the global scale, then zooms into catchments.

• Most of Earth’s water is stored in:
  • Oceans/seas: 96.5%
  • Ice/glaciers: 1.74%
  • Groundwater: 1.69%
• Rivers and lakes make up tiny fractions of total volume.

• Key global processes:
  • Evaporation from oceans/lakes
  • Precipitation over land and sea
  • Run-off moves water back to oceans
• Oceans evaporate more than they receive.
• Continents receive more precipitation than they lose.
• Ironically the terrestial component of the cycle is by far the smallest.

• Precipitation is partitioned into:
  • Evaporation
  • Run-off
  • Groundwater recharge
• Distribution varies by climate:
  • Humid temperate: balanced
  • Arid zones: dominated by evaporation

• Per capita water availability is misleading if population and use patterns are ignored.
• Example: Australia is water-rich by volume but highly variable in space and time.
• Effective management depends on abstraction, storage, and equity.

• OECD countries vary widely in water abstraction.
• USA: ~1,730 m³/person/year
• Often driven by:
  • Agriculture
  • Industry
  • Irrigation infrastructure
• Water use ≠ water availability

approximately eleven per cent of non-renewable groundwater use for irrigation is embedded in international food trade, of which two-thirds are exported by Pakistan, the USA and India alone.

Dalin et al (2017), Nature

• Focuses on processes at basin scale:
  • Evaporation
  • Precipitation
  • Run-off
• Includes sub-processes:
  • Interception
  • Transpiration
  • Infiltration and through-flow

• The water balance equation represents the continuity of water mass in a system.
• It quantifies water input, output, and storage over time.

• Often rearranged to estimate streamflow (Q)Q = P - E - \Delta S

Where:

• P: Precipitation
• E: Evaporation
• \Delta S: Change in storage
• Q: Run-off (or river discharge)

• The equation includes both fluxes and stores:
  • Fluxes: P, E, Q
  • Store: \Delta S (soil, groundwater, snow)
• Each term may be:
  • Positive (gain) — e.g. precipitation
  • Negative (loss) — e.g. evaporation or outflow
• Storage can increase or decrease depending on the balance.

• Used widely in:
  • Catchment hydrology
  • Water resource models
  • Climate impact studies
• Knowing three of the four terms allows you to estimate the fourth.
• Example: If P = 100, E = 40, \Delta S = 10, then Q = 100 - 40 - 10 = 50

• Difficulties in application include:
  • Spatial variability in rainfall
  • Temporal mismatch in data resolution
  • Estimating \Delta S is often hard
• Hydrological models often use the water balance to simulate run-off.
• Model example: Input daily rainfall and evaporation to calculate daily discharge using: Q = P - E - \Delta S

• Hydrological events vary by:

  • Magnitude (e.g. rainfall depth)
  • Frequency (how often)
  • Duration (how long)

• Frequency histograms help visualize event rarity

• Important for flood design and infrastructure planning

• Probability: p = \frac{n}{N}

• Recurrence interval = ( 1/p )

• A 1% chance event = 1-in-100-year event

• Recurrence ≠ prediction of exact timing

• Used for rainfall design curves and flood risk estimation

• Magnitude–frequency–duration (MFD) curves relate:
  • Rainfall depth
  • Storm duration
  • Event rarity
• Longer durations require more rainfall to be “rare”
• Used to develop intensity–duration–frequency (IDF) charts

In this lecture, we covered:

• The definition and scope of hydrology as a science of fresh water
• The physical and molecular properties of water that influence climate and flow
• The structure and function of the hydrological cycle, from global to catchment scale
• Key concepts such as catchments, water availability, and flux vs storage
• The water balance equation as a tool for understanding and modelling water movement
• Challenges in quantifying water fluxes and interpreting hydrological variability