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Hydrology > Week 7 > Flashcards

Week 7 Flashcards

1
Q

What is the drainable porosity ~equivalent to in an aquifer?

A

Specific yield

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2
Q

Moisture content, theta =

A

vol water in voids/total rock volume

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3
Q

Water saturation, Sw =

A

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
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4
Q

Surface tension, concept

A

Particle on water/gas interface

Stronger forces from liquid

NET ATTRACTION = SURFACE CONTRACTS

each pore is a water/gas interface

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5
Q

(H) =

A

contact angle

Smooth surface = low

Rough surface = high

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6
Q

Capillary pressure Pc =

A

Po-Pw >=Pe

Po = air pressure

Pw = water pressure

Pe = air entry pressure

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7
Q

Pe =

A

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

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8
Q

What value must Po have in order for air to enter tube/to empty tube?

A

Po >=Pw+Pe

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9
Q

Air entry pressure head, w(e) =

A

Pe/(water density) x g

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10
Q

For a tube of diameter D the air entry pressure, Pe =

A

(4 x (surface tension) x cos((H)) ) /D

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11
Q

“Cumulative probability distribution of pore sizes within a given pore volume” the plot

A

y (left) = Sw

y (right) = probability of non-exceedance

x (top) = D (pore size, decreases left to right)

x (bottom) = Pc

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12
Q

“Cumulative probability distribution of pore sizes within a given pore volume” results

A

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

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13
Q

Ignoring Po, Pc =

and w =

A

-Pw

w = -Pc/(water density) x g

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14
Q

Why does w (pressure head) decrease from the saturated zone to the unsaturated zone?

A

Unsaturated = higher Pc = lower w

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15
Q

Why does z (elevation head) increase from the saturated zone to the unsaturated zone?

A

h increases with elevation

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16
Q

What does h (hydraulic head) do from the saturated zone to the unsaturated zone?

A

Balanced from w and z

17
Q

If w(e) < -w

A

Pores = empty

18
Q

If w(e) > -w

A

Pores = filled with water

19
Q

w > 0

A

Saturated zone

20
Q

w < 0

A

Unsaturated zone

21
Q

Plot of:
y = moisture content
x = pressure head, w (i.e. decreasing elevation/increasing saturation, opposite to left READ THIS WAY)

PATTERNS:

A
  1. 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

  1. 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

22
Q

When does the hydrostatic profile occur?

A

After a sustained absence of rainfall, evaporation and water table movement

UNREALISTIC

23
Q

What happens to the hydrostatic profile when it rains?

A

Increase saturation

Decrease Pc

Increase w

Increase h

24
Q

When does a divergent zero flux plane (DZFP) develop?

A

After rain has stopped

Water at topped pulled upwards (evapotranspiration)
= decreases h

Water at bottom still infiltrating downwards
= increases h

25
Q

When does a convergent zero flux plane (CZFP) develop?

A

Post DZFP

Rainfall again exceeds evapotranspiration

(May move down and cancel out DZFP)

= @ top increasing h

= @ bottom decreasing h

26
Q

Zero flux plane cycles over the year

A

November - mid April = winter drainage through whole profile

Mid April - November = DZFP

Mid September - November = CZFP

27
Q

Measurement of moisture content, theta

A

Gravimetric method

Neutron probe

Dialectric methods

Profile probe

28
Q

Measuring moisture content; gravimetric method

A

Weigh/dry samples of known volume

29
Q

Measuring moisture content; neutron probe

A

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

30
Q

Measuring moisture content; dialectric methods

A

Property related to amount of electrical energy it can store
Water = 80 (high)

  1. Time domain reflectometry
  2. Impedance technique
  3. Capacitance technique

:) can be continually logged
:( require in situ calibration

31
Q

Measuring moisture content; profile probe

A

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

32
Q

Measurement of pressure head, w

A
  1. TENSIOMETERS

2. POROUS MATERIAL SENSORS

33
Q

Measuring pressure head; tensiometers

A

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

34
Q

Issue with tensiometers

A

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

35
Q

Measuring pressure head; porous material sensors

A

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:

  1. 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
  2. Dialectric methods applied to porous blocks rather than soil
36
Q

Which are the most established measurement techniques?

A

Neutron probe

Tensiometer

37
Q

Frequent dry soil readings …

A

Porous block sensor

38
Q

Wet condition readings …

A

Pressure transducer

Hydrology (9 decks)

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