Water Flashcards
How does rain form?
- Cools to saturation point
- Adiabatic decompression – no temp change, no energy change, was below saturation point, forced upwards and decompresses, but there is no energy change so it rains
- Meeting of two air masses – warm moist air forced to move over cold air and decompresses
- Contact with cold object – warm sea to cool land
Orographic and Conventional Rain?
- Orographic – Moist air rising over mountains – more rain when down wind – creates a rainshadow effect on the leeward side of mountain ranges, e.g. East vs. Western UK.
- Convectional rain – Short duration high intensity rainfall – UK only during summer - warmth of the ground causes moist air to rise, the air cools and condenses as rain, leads to short duration, intense rain typical of continental interior (not especially in the UK)
Rain typical of UK?
• Frontal or cyclonic rain - Typical of UK - Rotating weather systems contain warm and cold air – anticlockwise rotating systems in the Northern Hemisphere contain warm and cold air bodies
o Warm front where warm air rises over cold - Long duration, low intensity rain – followed by high intensity, short duration rain - In UK a day of rain is followed by day of showers.
• Most rain on Western side of UK – First contact of North Atlantic – Where highest points are – higher topography
• North York Mores is exception – they jut out – cycling systems more likely to hit them – similarly in North Norfolk – wetter than they should be – These parts of East Coast more likely to have snow due to the systems come from the North Sea and bring colder air from continental Europe
Types of rain measurement?
• Point measurement - storage raingauges - typically daily or monthly recording - recording raingauges - measures intensity by registering change of water level or tipping bucket.
o About 400 rain gages in the UK
o Tipping bucket – two buckets on see-saw mechanism – creates voltage which produces intensity readings
= how many point measurements are needed?
- UK density is 1 per 60 km2
• Spatial measurement - weather radar or satellite - difficult to calibrate radar signal against real rainfall parameters.
o Difficult to calibrate weather radar – no intensity
Use of areal rainfall?
o Arithmetic mean – mean of data
o Theissen Polygon – area-weighted rainfall??
o Isohyetal method – a line joining two points of the same rainfall – contouring method
o Preferred method – Hypsometric/multiquadratic methods – Area-weighting and adjusting for topography (altitude) in 2 or 3D
Interception and types?
rainfall falling on vegetation rather than on land surface
- interception loss – the amount of rain never reaching the ground - throughfall – the amount of rain that gets through to the surface - throughfall is generally less than gross rainfall
What is evaporation?
The loss of water from the Earth surface as water vapour, this definition includes water loss through transpiration of plants (evaporation and evapotranspiration are synonymous)
What controls evaporation?
- temperature – temperature of the air and the surface
- saturation deficit – how much water vapour is in the air
- solar radiation
- wind speed
- nature of the evaporating surface – (rougher surface = more evaportation)
Potential vs actual evapotranspiration?
- meteorological factors give rise to a potential evaporation but this rate will only occur if there no supply limitation
- actual evaporation ≤ potential evaporation
- the difference between actual and potential evaporation is controlled by soil moisture.
• Why do negative values of actual evaporation occur?
o Groundwater storage is supplying the river flow
• Rainfall from winter, released from storage in summer
• Comparison between total potential evaporation and annual actual evaportation works best on a yearly basis where changes in storage are minimised
What is runoff?
the gravitational movement of water in surface channels of whatever size
Hyrdrograph explained
- the Hydrograph –
o when it rains the rivers go up
o sometimes referred to as the storm hydrograph - the hydrograph is sourced from a range of places
o direct precipitation – precipitation directly enters the stream
o quick flow – the rapid response to rainfall– new water - slow flow – the flow in the river between storm events, often referred to as base flow – water does not stop flowing during dry periods – old water
- the quick flow component divides between old and new water
o old water – water in the catchment/soil/ground prior to the event that is pushed out by the rain
o new water – water coming in with the storm
Horton theory?
• Slow flow: Base flow Horton:
o Soil surface partitions rainfall into a surface flow and groundwater flow
o Partitioning is controlled by infiltration capacity of soil surface
o Infiltration capacity declines during a rainstorm generating more infiltration-excess runoff
o Therefore, hydrograph dominated by “new” water from overland flow
• Surface runoff (overland flow) – hydrograph dominated by “new water” – infiltration capacity exceeded due to crusting over of surfaces and water flows to rivers – infiltration excess
• Experimented in Northern Texas – large rainstorms in bare soils– observed overland flow – not relatable
Hewlett theory?
o Overland flow is very rare, most rainfall infiltrates the soil
o Soils saturate leaving infiltration capacity is zero, i.e. saturation excess runoff
o Runoff generated only in saturated areas of catchment but that these are variable and would be expected to increase during a storm.
o Often referred to as a variable-source model
• Soil saturates but not to the surface – rainfall builds up in the soil – water that reaches river will be “old water”
• Still thought that most water was “new” – wrong
What runoff theory actually occurs?
- Hewlett
- Very rare overland flow
- Most water is “old” – oxygen isotopes
- Horton - Works best for bare soils in arid regions, even here source areas are variable, i.e. Hewlett is a better model
Unit hydrograph?
- Time based relationship – gives a unit
- Need to be adjusted for location
- How much runoff does a unit of rain produced?
- Unit hydrographs can be seasonally adjusted but requires a lot of information and difficult to calculate
Stage relationship?
- During flood prediction depth is more important than volume
- Stage = river height
- Relationship between river height and flow at a given location
Frequency analysis?
• Flow duration curve: no use for predicting floods or droughts – used to predict dilution – cumulative frequency plot
• Annual maximum series – good for flood prediction – highest flow period
• Annual minimum series – good for drought prediction – lowest flow period
• Peaks over threshold (Partial duration series) – an improvement on the annual maximum series as it allows for more than one flood per year and other flood data is not thrown out – can find river thresholds
o Calculation of return period
• The return period is the probability of an event expressed in terms of years
• This is the Gringorten formula
How do we measure flow?
- gauging stations – a structure with a fixed cross-section in which stage and/or velocity can be measured.
Groundwater profile?
- Unsaturated zone – rocks, little water in pores
- Capillary zone/fringe – pores full of water
- Water table at bottom of the capillary fringe
- Saturated zone – pore zone full of water
Water is more attracted to surfaces than to itself – will spread across a surface = wetting
Capillary fringe – water attracted upwards by empty pore spaces
Water table = point where water pressure = atmospheric pressure – When water stops being held in place by capillary forces
Water table – the point to which a bore hole will fill to
