Water Flashcards
A few facts…
18% of the population do not have access to treated water
90% of urban dwellers have safe water compared to 62% of rural
Since 1950 the number of deaths due to floods = the number due to earthquakes and volcanoes combined
Input =
All forms of water hitting the earth’s surface
E.g. rain, snow, hail, fog drip
Precipitation =
When vapour cools to saturation point
Causes of precipitation
ADIABATIC DECOMPRESSION
- forced to rise e.g. over mountains
TWO AIR MASSES - common in the UK!!!
- warm, moist air forces over cold air/mix
COLD CONTACT
E.g. warm air from the sea blowing over cold land
Types of rain
OROGRAPHIC
- rainshadow on Lee side
- E vs W UK (Durham Pennines)
CONVECTIONAL
- rising warm air condenses
- short duration, high intensity
- common in continental interiors e.g. Alps/mid W US rather than UK
FRONTAL/CYCLONIC
- anti-clockwise rotating system in the northern hemisphere
- high duration, low intensity on the warm front
- low duration, high intensity on the cold front (showers)
- UK day of rain followed by a day of showers
UK rainfall patterns
Most air systems from the N Atlantic = high levels in the west due to…
1) cold contact
2) adiabatic e.g. Snowdonia
N.B. North York Moors high rainfall if weather system from the east
Measuring rainfall
1) point measurement
2) spatial measurement
Point measurement
- how many buckets does the UK require?
STORAGE RAIN GAUGES
- bucket with funnel
- daily/monthly data
RECORDING RAIN GAIGES
- measures intensity by measuring the no. of electric volts and capacity of buckets
- “seesaw mechanism”
The UK usually has 1 bucket per 60km2
Spatial measurement
Using a weather radar or satellite
Advantages;
- good for forecasting
Disadvantages;
- can’t calibrate against real rainfall parameters
- can’t tell how intense
Using rainfall data
1) areal rainfall
2) depth-area duration curves
3) probable maximum rainfall
4) rainfall statistics
Determination of areal rainfall
THIESSEN POLYGON
- area weighted rainfall
- lines drawn half way between gauges and joined up to form polygons
- can see AREAS where rainfall is highest
ISOHYETAL METHOD
- contouring
- isohyet = line joining two points of the same rainfall
HYPSOMETRIC/MULTIQUADRATIC METHODS
- area weighting like thiesson but ALSO adjusting for topography in 2/3D
Depth-area-duration curves
- what do they tell us?
How much?
What time period?
Where?
Taken as maximum values
BUT
- lots of data needed
How do we relate point estimates to depth-area-duration?
UK has an areal reduction factor designed for flood analysis
Rainfall data for the UK examples…
Sprinkling Tarn, Lake District 350mm/day
Füsson, Barvaria (Germany) 12cm/10 mins
East India - 13 m/yr
Probable maximum rainfall
How bad can rainfall be?
Based on real observations of rainfall of certain durations
Rainfall statistics
= frequency distribution at a site
Estimate likelihood and quantity of rain
Interception =
When precipitation lands on vegetation rather than land
Interception loss =
Amount of rain never reaching the ground
Throughfall =
Amount of rain that reaches the surface (including drip and stem flow)
Generally less than gross rainfall
Evaporation =
Loss from the earth’s surface as water vapour, including loss through transpiration of plants
Controls on evaporation
1) air/surface temp
2) humidity
3) solar radiation
4) wind speed
- otherwise stagnates and saturates
5) nature of evaporation surface
- rough = turbulent = increases
Reason for negative potential evaporation values
Water released from groundwater stores/fell as snowfall
- N.B. Actual and potential evaporation comparison works best on a long term basis when storage changes have less of an effect
Water table change
Recording details e.g. rainfall 11.55pm 31st Jan recorded as stream flow on 1st Feb
- water year 1st Oct (seasons)
Effective rainfall =
Amount of flow from the catchment (mm)
A few facts about the UK…
18,000 megalitres of water are supplied every day making £6.5billion turnover
1584 boreholes, 666 reservoirs, 662 river abstractions
Costs 62p/day/household