Week 7 Flashcards
What is the drainable porosity ~equivalent to in an aquifer?
Specific yield
Moisture content, theta =
vol water in voids/total rock volume
Water saturation, Sw =
vol water in voids/vol voids accepting water
i.e.
moisture content/effective porosity
- how much of the effective porosity you’ve actually used
- better indication of how wet/dry it is
Surface tension, concept
Particle on water/gas interface
Stronger forces from liquid
NET ATTRACTION = SURFACE CONTRACTS
each pore is a water/gas interface
(H) =
contact angle
Smooth surface = low
Rough surface = high
Capillary pressure Pc =
Po-Pw >=Pe
Po = air pressure
Pw = water pressure
Pe = air entry pressure
Pe =
air entry pressure
= pressure needed to increase the air pressure (Po) by to push water back out of the tube i.e. for air to enter
What value must Po have in order for air to enter tube/to empty tube?
Po >=Pw+Pe
Air entry pressure head, w(e) =
Pe/(water density) x g
For a tube of diameter D the air entry pressure, Pe =
(4 x (surface tension) x cos((H)) ) /D
“Cumulative probability distribution of pore sizes within a given pore volume” the plot
y (left) = Sw
y (right) = probability of non-exceedance
x (top) = D (pore size, decreases left to right)
x (bottom) = Pc
“Cumulative probability distribution of pore sizes within a given pore volume” results
Increase Pc = decrease Sw (b/c increasing Pe)
Decrease D = decrease Sw
60% pores <288um
288um Pe = 1000Pa
Pc>=Pe, so 60% of pore volume has Pe >1000Pa
= 60% will remain saturated with water
Even though Po > Pw (indicated by +ve Pc), pores resist air because their Pe’s are so high
Ignoring Po, Pc =
and w =
-Pw
w = -Pc/(water density) x g
Why does w (pressure head) decrease from the saturated zone to the unsaturated zone?
Unsaturated = higher Pc = lower w
Why does z (elevation head) increase from the saturated zone to the unsaturated zone?
h increases with elevation
What does h (hydraulic head) do from the saturated zone to the unsaturated zone?
Balanced from w and z
If w(e) < -w
Pores = empty
If w(e) > -w
Pores = filled with water
w > 0
Saturated zone
w < 0
Unsaturated zone
Plot of:
y = moisture content
x = pressure head, w (i.e. decreasing elevation/increasing saturation, opposite to left READ THIS WAY)
PATTERNS:
- Stable, drops suddenly, stable
= sandstone
= pores pretty much all same size, lose all saturated once Pe exceeded
N.B. Also lower than 2) = suggests low Pe = suggests large D
- Decreases (not linearly)
= clay
= lots of different pore sizes
N.B. Hydraulic conductivity patterns not the same because it is a function of porosity and PORE SIZE
When does the hydrostatic profile occur?
After a sustained absence of rainfall, evaporation and water table movement
UNREALISTIC
What happens to the hydrostatic profile when it rains?
Increase saturation
Decrease Pc
Increase w
Increase h
When does a divergent zero flux plane (DZFP) develop?
After rain has stopped
Water at topped pulled upwards (evapotranspiration)
= decreases h
Water at bottom still infiltrating downwards
= increases h
When does a convergent zero flux plane (CZFP) develop?
Post DZFP
Rainfall again exceeds evapotranspiration
(May move down and cancel out DZFP)
= @ top increasing h
= @ bottom decreasing h
Zero flux plane cycles over the year
November - mid April = winter drainage through whole profile
Mid April - November = DZFP
Mid September - November = CZFP
Measurement of moisture content, theta
Gravimetric method
Neutron probe
Dialectric methods
Profile probe
Measuring moisture content; gravimetric method
Weigh/dry samples of known volume
Measuring moisture content; neutron probe
In situ
Neutrons emitted from radioactive source
Collide with H atoms = slow down
= “slow neutron counter”
:( can’t be continually logged
:( health/safety
:( sphere of observation decreases with drier conditions
Measuring moisture content; dialectric methods
Property related to amount of electrical energy it can store
Water = 80 (high)
- Time domain reflectometry
- Impedance technique
- Capacitance technique
:) can be continually logged
:( require in situ calibration
Measuring moisture content; profile probe
Measures soil dialectric constant using capacitance method
~1m long, 100MHz signal
Stainless steel rings transmit electromagnetic field extending ~100mm into soil
- field passes easily through access tube walls but less easily through air gaps
Dialectric constant affects electromagnetic field
Measurement of pressure head, w
- TENSIOMETERS
2. POROUS MATERIAL SENSORS
Measuring pressure head; tensiometers
Porous cup connected through tube to P-transducer
All parts filled with water
Initially at atmosphere pressure
Unsaturated soil < atmospheric pressure = sucks water out of porous cup
= reduced pressure in tensiometer
= recorded by pressure transducer
Issue with tensiometers
Limited by water boiling point
20’C, boiling occurs at -927hPa; below 20’C = constant value measured of -927hPa
If soil becomes subsequently wetter, tensiometer sucks in soil water - contains dissolved gases = problems
If soil dry for too long; tensiometer water sucked out of soil = pressure reading of 0
Measuring pressure head; porous material sensors
Porous block –> soil, absorbs until pressure equilibrium reached
Moisture content OF BLOCK measured and translated to P head
:) dry conditions (no boiling/drying problems)
:( moisture content of block insensitive in wet conditions
Measure e.g. with:
- Electrical resistance sensors
- gypsum and electrodes
- resistance = w
- gypsum block slowly dissolves to maintain saturated conc of CaSO4 = electric conductivity insensitive to changes in solute cons - Dialectric methods applied to porous blocks rather than soil
Which are the most established measurement techniques?
Neutron probe
Tensiometer
Frequent dry soil readings …
Porous block sensor
Wet condition readings …
Pressure transducer
