Week 7 Flashcards
(38 cards)
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
