Lecture Panel 8

Question 1

Residence time of soil moisture

Answer 1

15 days to 1 year

Question 2

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

Answer 2

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

Question 3

what is sensible heat?

Answer 3

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

Question 4

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

Answer 4

your prediction will suck

Question 5

important factors for soil formation

Answer 5

• climate
• parent material
• biota
• topography
• time

Question 6

What are the weathering processes

Answer 6

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

Question 7

What is important for weathering of bedrock?

Answer 7

-temperature affects reaction rates - more weathering at equator

Question 8

Where is clay?

Answer 8

warm environments have more clay.

clay is deeper in wet environments

Question 9

soil profile

Answer 9

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

Question 10

soil as a three phase system

Answer 10

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

Question 11

soil texture sizes

Answer 11

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

Question 12

What is the bulk density of soil?

Answer 12

bulk density = Mass of solids/Total volume

Question 13

soil porosity

Answer 13

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

rock density is 2.65g/cm^3

Question 14

What maintains the the architecture of clay particles

Answer 14

intermolecular electrostatic forces

Question 15

Wilting point

Answer 15

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

Question 16

Field capacity

Answer 16

amount of moisture left in soil after excess drains away

Question 17

What will hygroscopic soil do

Answer 17

take water from the air as it is an unstable state

Question 18

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

Answer 18

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

Question 19

volumetric water content

Answer 19

water volume/total volume

Question 20

gravimetric moisture content

Answer 20

mass of water/mass of solids

Question 21

degree of saturation

Answer 21

Vw/(Va + Vw)

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

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

Question 26

What does 100% moisture available soil feel like?

Answer 26

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

Question 27

Traditional gravimetric samling, adv and disadv.

Answer 27

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

Question 28

neutron scattering soil moisture measurement

Answer 28

- 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

Question 29

electrical resistance soil moisture measurement

Answer 29

- porous blocks equilibrate with the matric suction | - problems - effected by hysteresis, salinity, and temp, may not reach equilibrium fast enough, dependent on block type

Question 30

cosmic ray probe for soil moisture measurement

Answer 30

- 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

Question 31

what is a dielectric?

Answer 31

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

Question 32

water as a dielectric compared to soil and air

Answer 32

water has a high dielectric constant (81) - soil is 4-8 - air is 1

Question 33

time domain reflectometry for soil moisture measurement

Answer 33

- 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

Question 34

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

Answer 34

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

Question 35

How can we calibrate the soil probe?

Answer 35

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

Question 36

Sensing soil moisture remotely via thermal sensing

Answer 36

- diurnal temp changes lower for wet soils - dependent on soil type - limited to bare soils

Question 37

Microwaves for remote sensing of soil moisture

Answer 37

- microwaves are sensitive to dielectric of soils (related to volumetric water content) - can be active or passive sensors

Question 38

Active microwave sensing pros and cons for soil moisture

Answer 38

- 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

Question 39

Passive microwave sensing pros and cons for soil moisture

Answer 39

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

Question 40

what is produced with SMOS soil estimates?

Answer 40

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

Question 41

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

Answer 41

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

Question 42

what are semi-variograms?

Answer 42

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