Water Engineering

Hydrology

occurrence, distribution, movement, and properties of the water of the earth

Hydrological cycle

the continuous movement of water on, above and below the surface of the Earth

Key Hydrological Processes

Precipitation

Runoff

Infiltration

Subsurface Flow

Evaporation & Transpiration

Precipitation

Condensation of atmospheric water vapour deposited on Earths surface. Mostly rain (snow, hail, fog drip, sleet etc)

Runoff

Water moves across land

includes surface runoff and channel runoff

may infiltrate into ground, evaporate into air, become stored in lakes/reservoir or extracted for human use

Infiltration

flow of water from ground surface into soil

becomes soil moisture or groundwater

Subsurface flow

flow of water beneath ground surface

eg in aquifers

Evaporation & Transpiration (Evapotranspiration)

Evaporation - vaporisation of liquid (to gas)

Transpiration - evaporation of water from aerial parts of plants

Catchment (drainage basin/river basin/watershed)

an extent of land where water from rain or snow melt drains downhill into a body of water

Water balance in a system

Q: input/output flow rate

S: storage

Catchment water balance

energy required to vaporise water

(specific latent heat for water vaporisation)

2.5 × 10^6 J/kg

(condensation energy release)

Precipitation events are recorded by (1) at (2).

Point precipitation data are used to estimate (3).

Rainfall data units (4)

1. gauges

2. specific locations

3. areal variability of rain/snow

4. mm/hour, mm/day etc

Methods of estimating areal rainfall

Arithmetic Mean

Thiessen Polygon Method

Arithmetic Mean

simple method when gauges are uniformly distributed

unpopular due to inaccurate result

Thiessen Polygon Method assumes…

(given on formula sheet but not expected to do calcs)

…at any point in catchment, rainfall is same at nearest rain gauge. so depth applied out to distance halfway to next gauge in any direction.

Double Mass Curve

Checks data quality of a specific rain gauge (not needed for exam?)

Flux

flux= flow rate/area

Radiation emission

radiation is continuously emitted from all bodies at rates linked with their surface temperature

Net radiation

When radiation strikes a surface it is partially reflected and partially absorbed.

Vapour Pressure, e

Saturated vapour pressure, e_s

e: the partial pressure contributed by water vapour

e_s: when pressure is in equilibrium

Relative humidity:

Sensible heat

responsible for liquid water temperature change

Latent Heat

used to vaporise liquid water into water vapour. Varies slightly with temp (vaporisation under higher temp needs less energy)

Evaporation from open water surface is influenced by 2 factors:

1. energy input

2. vapour transport

Energy Input provides…

latent heat for vaporisation

energy balance method used when transport is not limiting

Vapour Transport helps…

to move the vapour away from the surface

aerodynamic method is used when energy supply is not limiting

What weather variables are needed for calculating evaporation from open water surface with the combined method?

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What are the factors that influence actual evapotranspiration on land?

evapotranspiration on land - combination of evaporation from soil surface and transpiration from vegetation

factors - energy and water transport, availability of soil water

What are the potential evapotranspiration and reference transpiration? Describe their relationship with actual evapotranspiration.

Potential evapotranspiration - when water availability is not a limiting factor

Reference transpiration -