Lecture 2 Flashcards
What is vapour
A vapour is a gas below it’s ‘critical temperature’ - it can easily be condensed or liquefied by a small change in temperature and pressure
There is a maximum amount of water that the…
Air can hold before become saturated - this increases with increasing temperature
Relative humidity (degree of saturation) =
Actual vapour pressure / saturation pressure
Dew point
Air becomes saturated and condensation may occur
Aerosol particles
Smoke, dust, pollen and salt act as condensation nuclei
Cloud formation occurs and their appearance reflects…
Air movements
Collision and coalescence of water droplets increases…
Droplet size
Precipitation is the main input of
Water on earth
When does precipitation occur?
When moist air is cooled below its dew point and the air becomes saturated
What variations are there in precipitation?
Stochastic variations
Periodic
Secular
Stochastic variations?
Arising from probabilistic or random nature of precipitation occurrences
Periodic (cyclic) variations ?
Astronomical cycles
Secular (long term) variations ?
Due to climate change
Measuring precipitation
Recording rain gauge
Automatically registers intensity or rate of accumulation
Rain gauge sources of error
Wind turbulence
Siting
Evaporation losses
Extreme rainfall
Density of rain gauge networks depend on
Accuracy of data requirements
Spatial variability of rainfall
Temporal nature of dataset required
I-rain: mobile app for precipitation data
Rain data from network of precipitation sensing satellites
Users enter own rainfall or snowfall observations from raingauges
Display top 50 extreme weather events around the world
Human impacts on precipitation
Land use / vegetation cover
Fossil fuel burning and global warming
Canopy interception
Is the precipitation retained by the vegetation as surface storage and subsequently evaporated
Interception by different vegetation types
Different storage capacity
Different aerodynamic roughness
Rainfall duration and intensity
Generally…
Interception losses will be greater for denser and taller vegetation in wetter climates
Coniferous forests v deciduous
D - bends more under weight and shaken more by wind
D - higher stem flow in deciduous plants
Factors influencing interception
Vegetation characteristics
Rainfall intensity and duration
Wind speed and turbulence
Energy balance
What is evaporation from open water
The process by which a liquid is turned into a gas and it’s transfer away from the surface to the atmosphere
Measuring evaporation
Evaporation pans and tanks
Easy and cheap to manufacture but prone to error
Estimating evaporation
Very difficult to measure directly so estimated with mathematical models
Modelling potential evaporation
Evaporation is often estimated using empirical models based entirely on data observations
This is because of the issues with directly measuring of evaporation
Models:
Predict potential evaporation
Modelling potential evaporation
Advantages
Modest data requirements so ease of use
Can work well over long time periods
Disadvantages
Does not work well over short time periods
Can severely underestimate evaporation in arid and semi arid regions
Evaporation from the soil depends on…
The amount of water in the soil
The ability of the sort to transmit water to the surface
Evaporation from different soils
Open water
size of water surface, water depth and mixing salinity
Bare soils
Size, density, orientation and connectivity of pours in the soil, depth to the water table, local heat budgets, pattern of rainfall
Vegetated
Vegetation high and windspeed, stomatal control, soil factors
Soil potential energy
Soil water moves so slowly that it’s connected energy is insignificant, and it is its potential energy that is important in determining water movement
Potential energy is defined as the work required to transport a quantity of water from a pool of pure water atmospheric pressure to a given soil water location
Soil water availability to plants
Build capacity is water retained by a soil once gravitational water has drained out, organic matter enhances field capacity because of its hydrophilic nature, clay soils had four times more water than sandy soils
Permanent melting point is the soil water potential at which most plants will it because they can no longer attend water from the soil
Plant available water is filled capacity minus permanent wilting point
Plant rooting depths
Plant rooting depth reflects a compromise between water and nutrients availability
Most roots near soil surface where nutrients are most available
Deep roots tap water from depth
Water movement through a plant
Takes place in soil-plant-atmosphere continuum inter-connected by a continuous thin film of liquid water
Driven entirely by physical process of evaporation from leaf surface
Transpiration
Evaporation of water that has passed through plants
Occurs at all exposed parts of the plant particularly the stomata
They open and close in response to environmental conditions such as light and temperature
Opening and closing affect the rate of transpiration and control the flow of water from the plant roots
What is water loss from leafs controlled by?
Evaporative potential of air
Water supply from soil
Regulation of water loss by leaves using stomata
Factors controlling transpiration?
Types of vegetation Season Time of day Stage of growth Meteorological factors Soil properties
