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
Runoff =
Gravitational movement of water in surface channels (of any size)
Why are we interested in runoff?
It is the process of getting water to us
Floods
Droughts
Transport/pollution dilution
Direct precipitation =
Directly enters stream
Quick flow =
Rapid response to rainfall, rainfall goes up
N.B. Can be old water or new water
Old water =
In catchment/soil/ground prior to event and pushed out by rain
New water =
water coming in with the storm
What is Horton’s hypothesis?
The soil surface partitions water into surface flow and groundwater flow
Controlled by the infiltration capacity of the soil surface which decreases during a storm bc the soil surface “crusts over”
= INFILTRATION EXCESS RUNOFF
- hydrograph dominated by NEW water from overland flow
What is Hewlett’s hypothesis?
Most water infiltrates and soils become saturated, decreasing the infiltration capacity
Runoff generated only in saturated areas of catchment, varying in size
Contact rivers join up to form hydrograph
= “SATURATION EXCESS RUNOFF”
- hydrograph dominated by OLD water
When does Horton’s hypothesis work?
Bare soils in arid regions
Although source areas are variable so Hewlett better
Why does Hewlett work?
1) doesn’t rely on overland flow which is actually v rare
2) heavy water/oxygen isotopes due to evaporation prove that they’ve been in the soil for a while
3) contrasting chemistry of incoming rain storm and water coming out = prove old water
When does Hewlett not work?
Source areas may not necessarily be attached by streams, may be isolated patches that grow towards streams
Thought some of the water was still new when actually old
In some cases can use Horton as a special case of Hewlett
Types of graphs
Simple rainfall/runoff
Unit hydrograph
- how much runoff per unit of rainfall
- requires lots of info
- has to be adjusted for each location since each location has a different response
- difficult to calculate
Stage (river height) relationship
- how much increase in height for a given flow
- useful for flood prediction
- has a maximum (beyond = flood)
- COMPOUND CRUMP WEIR
Frequency analysis*
Compound crump weir
V notch for a fixed cross section that responds to changes in height
Sensitive for lowest flows
- jumps correspond to step changes in the weir profile
Frequency analysis
A) FLOW DURATION CURVE
B) ANNUAL MAXIMUM SERIES
C) ANNUAL MINIMUM SERIES
D) PEAKS OVER THRESHOLD (partial duration series)
Flow duration curve
Flow vs %exceedance
%exceedance = how much of the time flow is greater than this value
:) likely resource/dilution (plot in terms of exceedance, like a cumulative frequency type plot)
:( not good for drought/flood prediction
Annual maximum and minimum series
Good for flood prediction
Looks at the return period
:( only one data point per year - wasted data???
Peaks over threshold
Allows for more than one data point per year if an important flow level is known
Return period =
Probability in years of the same event occurring
Gauging station =
Structure with a fixed cross section on which stage and or velocity can be measured
Groundwater =
How much of the earth’s water supply comes from groundwater?
How much of London/Denmark’s?
A component of the hydrological cycle in the subsurface
50% earth’s water supply
75%
95%
Recharge =
Water underground following into the unsaturated zone
- prevented by interflow
Groundwater profile
Vadose zone
Capillary fringe
Water table
Phreatic zone
Vadose zone =
Unsaturated
- pore spaces are water or air
Capillary fringe =
Start of unsaturated zone
Area where water is attracted upwards to
Water table =
Where water pressure = atmospheric pressure
NOT THE TRANSITION BETWEEN SATURATED/UNSATURATED ZONES
Phreatic zone
Saturated
- pore spaces = water
Capillary force =
Force of attraction due to:
SURFACE TENSION
- attraction between water molecules
WETTING EFFECT
- balance between
1) attraction between liquid molecules
2) adsorption on the surface
Wetting surfaces
Water is attracted to a surface causing capillary rise (adsorption of fluid up a surface) outweighing gravity and its own surface tension
Example: :) water
:( mercury
Specific yield =
Effective porosity =
Amount of water drained/drop of water in the water table
Sometimes called effective porosity = proportion of void space capable of transmitting a fluid
Due to capillary forces
ALWAYS LESS THAN POROSITY
Porosity =
Proportion that is void space, a fraction of the rock volume
Varies 0-60%
Can have “blind pores”
Permeability =
How easily fluid can move through a rock
Aquifer =
A rock or sediment with sufficient permeability to supply water in useful quantities
Aquitard =
An aquifer with very slow movement
A rock/sediment with insufficient permeability to supply water in useful quantities
Aquiclude =
An aquifer which excludes water I.e. water cannot get in or out
A rock/sediment with insufficient permeability to supply water in useful quantities
UK AQUIFERS
The NE has abundant surface resources so aquifers are not necessary BUT they are important in the S/E
CHALK:
Limestone = fracture porosity
London
UK’s major aquifer
PERMO TRIASSIC SANDSTONE:
Intergranular porosity
Birmingham/Nottingham
TERTIARY:
Large number of smaller aquifers
Local
~LONDON~
- population live in towns around rather than the city itself
- city underexploits water resources
- towns around over exploit
~ESSEX~
- expanding population without much water supply or recharge
What does the size (height) of the capillary fringe depend on?
Sediment porosity - think smaller pores = bigger suction for a given amount of water
The capillary fringe moves up and down with the water table
Confined aquifer
Bound by aquitards
Water comes up under pressure
ARTISAN BASIN
Unconfined aquifer
Not bound or has exposure at the surface
Has direct recharge from/”through” the unsaturated zone when precipitation etc occurs
Perched aquifer
Bound beneath by an aquitard
Situated above the regional water table
= SPRING LINES
- provides year round water supply to villages
- saturated rock above the water table
Potentiometric surface =
The water table for an aquifer which isn’t at the surface (confined)
- not within the aquifer
- the point the water table would rise to if it could
Keep drilling down and eventually water will come above the ground = ARTESIAN WELL
- flows under own pressure
- “artesian basin”
Artesian basin =
Where the potentiometric surface comes above the ground surface
Classic desert oasis
Elastic storage =
When confined aquifers under pressure behave SLIGHTLY elastically and depressurise, giving more water than thought
E.g. Dakota sandstone produces more water than was thought possible due to a fall in the potentiometric surface
What is the flow in aquifers governed by?
1) the area it flows through
2) the head gradient
3) hydraulic conductivity
Head =
Elevation + pressure (m)
WATER WILL ONLY FLOW WHERE THERE IS A HEAD DIFFERENCE
Hydraulic conductivity =
Intrinsic material property, the measure of:
1) ease with which a fluid flows
- due to pore size
2) ease the porous rock allows passage
- due to tortuosity
Suction =
Negative pressures ABOVE the water table which prevents unsaturated flow into wells
- water table; water pressure = atmospheric pressure
- negative pressures above
What causes suction?
1) CAPILLARY FORCES
- small grain size = small pores = large capillary forces
2) ADSORPTION
- surface charge dependent and therefore grain size dependent
- also slightly mineral dependent
- :):)organic matter :)clay :(quartz
3) OSMOSIS
- requires a semi permeable membrane (roots)
- due to differences in ionic concentrations
Total head =
Elevation head + suction head + osmotic head
Moisture characteristic =
Matric suction (cm) vs volumetric water content (%)
- suction increases as water content decreases because there is a greater force for a given amount of water
- it’s shape depends on pore sizes; smaller pore sizes = greater suction i.e. sand < silt < clay (can use the moisture characteristic to measure this)
Air entry value =
The matric suction value that must be exceeded before air enters the soil pores
- air won’t enter the system until the suction of the largest pore space is overcome
- largest pore space = smallest suction
- also last place water will go because of this
Wilt point =
Suction at which roots find it difficult to extract the water needed (permanent = death)
Field capacity =
Amount of soil moisture/content held in soil after excess water drained away and rate of downward movement has decreased
Relate to air entry value; suction at which pores won’t drain under gravity
E.g. of pores are too big it won’t be able to hold water against gravity
Hysterisis =
Amount of water contained in a pore at a given suction due to the ink bottle or contact angle effects
PORES ALWAYS EMPTY AT LARGER SUCTIONS THAN THEY FILL AT
Ink bottle effect: drying
(Pores = wide bodies and narrow necks)
Won’t drain last neck because there’s too much suction in neck
- water held by this
= hold onto more water than expected
Ink bottle effect: wetting
Can’t get water in because suction is too low in the body
= takes in less water than expected
Contact angle effect: drying
Contact angle decreases = bit of water there held higher
= hold more than expected
Contact angle effect: wetting
Meniscus turns corner in neck = contact angle increases
(“Artificially increases the contact angle”)
= takes in less water
Flow and hydraulic conductivity
Water moves from low to high suction
K(unsat) < K(sat)
- less effective cross section
- increased tortuosity
Hydraulic conductivity of clay
Clay is always tortuous BUT it’s low pore size means that it is good at holding water
- low water contents = high effective porosity
I.e. K(unsat clay) > K(unsat sand)
Zero flux plane =
Plane which divides water moving up to water moving down
Water moves up when evaporation and therefore suction takes place
Water moves down in a soil at equilibrium
- gravity does also pull down here = zero flux plane
Convergent = drying
Divergent = wetting
Measuring unsaturated properties
1) depth to water table
- piezometer
2) moisture content
- gravimetric method
- capacitance probe
- gypsum
- neutron probe
3) suction
- tensiometer
Gravimetric method
Weigh/dry samples of known volume
:) easy once you have the sample
:( disruptive
Capacitance probe
Two probes, measure the dielectric constant (which is related to moisture content)
:) favoured
:( measures average
:( disruptive
Gypsum
Sparingly soluble
= measure resistance/electrical conductivity with two electrodes
:( have to bury = disruptive
Neutron probs
Uses a fast neutron source and measures the “back scatter” off the water molecules
:) only requires a dipwell
:) most info for disturbance
Tensiometer
Semi porous cup which loses/gains water
Measure the pressure change with a barometer
:( quite disruptive and interferes with the system
Infiltration capacity =
The capacity of soil to take in water (m/s)
Bypass flow =
Flows rapidly to the water table - macropore/lateral/overland flow
Macropores =
Between 30 microns to 2mm big
Drain under gravity
Active at suctions lower than field capacity
Types of macropore
FAUNAL
- burrows/wormhole
ROOT HOLES
CRACKS
NATURAL SOIL
- e.g. cutans
Measuring macroporosity
SUCTION PLATE
:( assumes macropores operate at suction which they don’t
TRACERS
- measures pores doing transporting under gravity
- anything at the bottom = g = macropores
Nature of macropore flow =
Balance between input rate (1) and infiltration capacity (2)
(1)(2)
- either the rate is too fast or infiltration capacity has decreased
= unsaturated overland flow and unsaturated macropore flow
If (2) is exceeded because the soil is saturated
= saturated overland flow and saturated macropore flow
Two flow domains
1) MATRIX
- darcian flow
- slow
2) MACROPORES
- non darcian (turbulent pipe)
- fast
Lateral interflow =
Subsurface, horizontal/subhorizontal macropore flow
- layers within the soil have an infiltration capacity and can act like a soil surface
Normal macropore flow is VERTICAL
Issues with bypass follow
POLLUTION
- rapist movement of pollutants off fields and through soil
- limits opportunity for degradation/adsorption to retard or diminish pollution
Predict times of bypass flow = protect water intakes
UK WATER QUALITY SINCE 1990
36% rivers have increased biological quality
44% chemical quality
Who defines water quality?
1) drinking water inspectorate
- water from companies that ENTERS property
2) environment agency (sepa)
- “controlled waters” e.g. rivers/lakes/groundwater/estuaries/bathing waters
Physical characteristics of water quality
Temperature
Transparency
Colour
Turbidity
Suspended solids
Total Dissolved Solids (TDS)
Conductivity
pH
Measuring temperature
Seasonal/diurnal variations
Hot water = pollutant
- salmon :( 22 degrees
Some water bodies show stratification
Measuring transparency
Secchi disk
Indicators of biological activity
Typical values 0.5-15m
Measuring colour
By light absorbable
Brown/yellow = organic matter
- react with Cl to disinfect = chloroform
- can’t use too much if it’s really bad because it’s carcinogenic
Measuring turbidity
Particulate materials prevents light penetration
Measure with a nephelometer
Measuring suspended solids
Dry weight of particles retained by a 0.4um filter
TDS
Once through the suspended solids filter, evaporate down
Weigh
Similar to salinity
Measuring conductivity
More TDS = more conductive
Use electrodes
If in v saline environment easier to use TDS as an indicator
Chemical water quality
Dissolved in organic substances
Organic substances
Dissolved oxygen
Dissolved CO2
Dissolved inorganic substances
Rainwater picks up atmosphere gases
54% evaporates = concentrated in surface waters
Cations: Na, K (agricultural), Ca, Mg
Anions: HCO3- (major buffer of natural waters), Cl-, SO42-, nitrate, phosphate
Organic substances
Particulate organic carbon (POC)
- bacteria/dead organisms etc
- organic carbon content of residue from filtering sample
Dissolved organic carbon (DOC)
- WATER COLOUR
- organic carbon content of filtrate of sample
Total organic carbon (TOC) = POC + DOC
Dissolved oxygen
Saturation is lower at higher temperatures
Seawater holds less oxygen
Algal mats can boost it to 150-200% saturation
Vital for life
BOD vs COD
For water quality control
Biochemical Oxygen Demand = amount of oxygen required for bacteria to degrade organic components (5 days)
Chemical Oxygen Demand = total chemicals (inorganic and organic) in water (on the spot)
BOD/COD indicates toxicity
High ratio = low toxicity
Low ratio = requires treatment
Dissolved CO2
Hardness and alkalinity
- harder water has a higher buffering capacity
Water hardness =
The capacity to precipitate Ca and Mg compounds with soap or heating
Advantages of biological surveys
Integrates time and conditions
- physical properties e.g. conductivity only measure at THAT time or condition
Environmental Quality Standard contains biological standards to meet
Need them for discharge consents
Valuable in management studies
Biological water quality characteristics
Indicator species
Community measures
Aquatic flora
Aquatic fauna
Disease
Indicator species
Benthic macroinvertebrates
Their presence/absence
Community measures
Kick samples (RIVER SPECIFIC)
Plankton studies
Fish counts
Aquatic flora
Bacteria
Pathogens e.g. salmonella
Cyanobacteria
- dominant phytoplankton
- deplete O and release toxins
Algae
- can see long term core trends using chlorophyll concentrations and cell counts
Aquatic fauna
Microbenthic invertebrates
Kick samples
Flatworms, worms, leeches, snails, limpets, shrimp, waterlice etc etc
Impacts of pollutants on water quality
Organic wastes
Nutrients
Inert solids
Metals
Pesticides
Waste heat
Pathogens
Acid deposition
Critical loads
Organic wastes
From domestic sewage/farms
Causes oxygen depletion so you measure BOD/COD
Also causes eutrophication
Nutrients
Mainly nitrates and phosphates
Cause eutrophication by oxygen depletion
- algal blooms
Inert solids
Sediment e.g. sand/clay stops light penetration and causes excessive sedimentation
Metals
Most are toxic except for Na/Ca/K/M
Some companies add phosphates to make insoluble metal phosphates
Accumulate in sediments and organisms
Pesticides
Toxic
UK = herbicides
Climate change may lead to using them further north
Waste heat
From power stations
Reduces oxygen stability and increases oxygen consumption rates
= decreases oxygen concentration
Pathogens
Cholera Typhoid Salmonella Polio Hepatitis Weil’s disease Cryptospiridia
Acid deposition
Industrial gases NOx and SOx
Causes acidification and eutrophication
Critical loads
Assessing the extent a system can accept a pollutant
Largely developed for acidification
Rivers have more…
Si Fe Al P N DOC
Oceans have more…
Na Ca Mg K Cl SO4
