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Hydrology > Lecture Panel 8 > Flashcards

Lecture Panel 8 Flashcards

1
Q

Residence time of soil moisture

A

15 days to 1 year

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2
Q

what parts of the hydrologic cycle is soil moisture important for?

A
  • evapotranspiration
  • controls runoff
  • groundwater recharge
  • biogeochemical cycles
  • controls the distribution of vegetation
  • affects sensible and latent heat exchange with atmosphere
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3
Q

what is sensible heat?

A

heat exchanged with the atmosphere without changing volume or pressure - heat you can feel

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4
Q

What happens if you use the wrong soil moisture data for your weather prediction?

A

your prediction will suck

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5
Q

important factors for soil formation

A
  • climate
  • parent material
  • biota
  • topography
  • time
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6
Q

What are the weathering processes

A

physical (eg. freeze-thaw, root growth), chemical (oxidation, reduction, solution)

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7
Q

What is important for weathering of bedrock?

A

-temperature affects reaction rates - more weathering at equator

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8
Q

Where is clay?

A

warm environments have more clay.

clay is deeper in wet environments

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9
Q

soil profile

A 
O - organic horizon
A - strong mix OM and minerals
E - zone of max leaching - elevation
B - zone of illuviation - layer of max accumulation
C - unconsolidated materials
R - consolidated bedrock
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10
Q

soil as a three phase system

A

Mineral 45%
OM 5%
air (20-30%)
Water (20-30%)

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11
Q

soil texture sizes

A

sand - 2- 0.5mm
silt - 0.05- 0.002mm
clay - <0.002mm
-although there are different classifying sizes in different countries

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12
Q

What is the bulk density of soil?

A

bulk density = Mass of solids/Total volume

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13
Q

soil porosity

A

porosity = (Va + Vw)/Vs = 1 - bulk density/rock density

rock density is 2.65g/cm^3

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14
Q

What maintains the the architecture of clay particles

A

intermolecular electrostatic forces

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15
Q

Wilting point

A

where a plant cannot exert enough osmotic pressure to draw up water

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16
Q

Field capacity

A

amount of moisture left in soil after excess drains away

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17
Q

What will hygroscopic soil do

A

take water from the air as it is an unstable state

18
Q

What is the general ranking of bulk density/porosity of soil textures?

A

Sand (course) has high bulk density and low porosity
Loam is mid
Clay (fine) has low bulk density and high porosity

19
Q

volumetric water content

A

water volume/total volume

20
Q

gravimetric moisture content

A

mass of water/mass of solids

21
Q

degree of saturation

A

Vw/(Va + Vw)

22
Q

What does 0-25% moisture available soil feel like?

A

dry, loose, will hold together if not disturbed, loose grains on fingers with applied pressure

23
Q

What does 25-50% moisture available soil feel like?

A

slightly moist, forms a very weak ball with well defined finger marks. Light coating of loose and aggregated sand grains on fingers.

24
Q

What does 50-75% moisture available soil feel like?

A

Moist, forms a weak ball with loose and aggregated sand grains on fingers, darkened colour, moderate water staining on fingers, will not ribbon

25
Q

What does 75-100% moisture available soil feel like?

A

Wet, forms a weak ball, loose and aggregated sand grains remain on fingers, darkened colour, heavy water staining on fingers, will not ribbon

26
Q

What does 100% moisture available soil feel like?

A

Wet, forms a weak ball, moderate to heavy water/soil coating on fingers, wet outline of soft ball remains on hand

27
Q

Traditional gravimetric samling, adv and disadv.

A

estimate soil moisture – wet mass/dry mass

  • oven dry sample at 105C
  • problems - time consuming and destructive, often can loose parts of sample or have other soils fall in
  • fairly straightforward, simple, and reliable
28
Q

neutron scattering soil moisture measurement

A
  • probe with source of fast neutrons and a detector of slow neutrons
  • rate meter monitors flux of slow neutrons scattered and attenuated due to collision with hydrogen
  • scattering is in proportion with volumetric water content
  • sphere of influence depends on soil moisture (0.1m wet, 0.25m dry)
  • probe lowered into soil with access tube
  • high cost, low spatial resolution, physical danger with prolonged use
29
Q

electrical resistance soil moisture measurement

A
  • porous blocks equilibrate with the matric suction

- problems - effected by hysteresis, salinity, and temp, may not reach equilibrium fast enough, dependent on block type

30
Q

cosmic ray probe for soil moisture measurement

A
  • calculates low energy cosmic ray neutrons above soil level
  • number of neutrons in the air above soil is inversely related to soil moisture
  • measures over large area
31
Q

what is a dielectric?

A

non-conductor that polarizes in an electric field based on the dielectric constant of the material (relative permittivity)

32
Q

water as a dielectric compared to soil and air

A

water has a high dielectric constant (81)

  • soil is 4-8
  • air is 1
33
Q

time domain reflectometry for soil moisture measurement

A
  • indirect measure of soil water content based on the travel time of an electromagnetic pulse through the soil
  • measure time it takes for pulse to travel down the probe and be reflected back
  • travel time depends on the dielectric constant of the material in contact with the probe
  • mostly independent of soil type relationship and salinity (at low concentration)
  • accuracy 2%, precision 1%
  • expensive and can break
34
Q

What can be used to a long term record of soil moisture?

A

dielectric probes using time domain reflectometry

  • probes installed in ground in permanent monitoring network
  • data loggers record measurements at set intervals
  • good for long term record for climatology, irrigation scheduling, crop assessment prediction
35
Q

How can we calibrate the soil probe?

A

by soil type, eg clay, sand - slight variations in volumetric water content vs dielectric constant by soil type

36
Q

Sensing soil moisture remotely via thermal sensing

A
  • diurnal temp changes lower for wet soils
  • dependent on soil type
  • limited to bare soils
37
Q

Microwaves for remote sensing of soil moisture

A
  • microwaves are sensitive to dielectric of soils (related to volumetric water content)
  • can be active or passive sensors
38
Q

Active microwave sensing pros and cons for soil moisture

A
  • enables higher pixel resolution (5x5m) but covers smaller area
  • signal very sensitive to other features that can confound estimation
  • less frequent revisit time
  • can see top 5 cm or so
39
Q

Passive microwave sensing pros and cons for soil moisture

A

-measures energy emitted from Earth
-enables global coverage and frequent revisit time
-pixel resolution is course (10km)
eg ESMOS

40
Q

what is produced with SMOS soil estimates?

A

bi-weekly map of percent saturation surface soil moisture is produced by AAFC

41
Q

How representative is you soil estimate? How many samples do you need to estimate variability?

A

number of samples necessary at 95% confidence

  • for porosity, bulk density, saturated water content n=9
  • for moisture of silt, clay, sand fraction 9 to 96
  • for saturated hydraulic conductivity, infiltration rate n>96
42
Q

what are semi-variograms?

A

tell you range at which spatial pattern becomes random for soil samples compared to your sample (autocorrelation)
-important for permanent monitoring networks that typically only have single measurement points across distances

Hydrology (10 decks)

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