Lecture Panel 8 Flashcards
Residence time of soil moisture
15 days to 1 year
what parts of the hydrologic cycle is soil moisture important for?
- evapotranspiration
- controls runoff
- groundwater recharge
- biogeochemical cycles
- controls the distribution of vegetation
- affects sensible and latent heat exchange with atmosphere
what is sensible heat?
heat exchanged with the atmosphere without changing volume or pressure - heat you can feel
What happens if you use the wrong soil moisture data for your weather prediction?
your prediction will suck
important factors for soil formation
- climate
- parent material
- biota
- topography
- time
What are the weathering processes
physical (eg. freeze-thaw, root growth), chemical (oxidation, reduction, solution)
What is important for weathering of bedrock?
-temperature affects reaction rates - more weathering at equator
Where is clay?
warm environments have more clay.
clay is deeper in wet environments
soil profile
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
soil as a three phase system
Mineral 45%
OM 5%
air (20-30%)
Water (20-30%)
soil texture sizes
sand - 2- 0.5mm
silt - 0.05- 0.002mm
clay - <0.002mm
-although there are different classifying sizes in different countries
What is the bulk density of soil?
bulk density = Mass of solids/Total volume
soil porosity
porosity = (Va + Vw)/Vs = 1 - bulk density/rock density
rock density is 2.65g/cm^3
What maintains the the architecture of clay particles
intermolecular electrostatic forces
Wilting point
where a plant cannot exert enough osmotic pressure to draw up water
Field capacity
amount of moisture left in soil after excess drains away
What will hygroscopic soil do
take water from the air as it is an unstable state
What is the general ranking of bulk density/porosity of soil textures?
Sand (course) has high bulk density and low porosity
Loam is mid
Clay (fine) has low bulk density and high porosity
volumetric water content
water volume/total volume
gravimetric moisture content
mass of water/mass of solids
degree of saturation
Vw/(Va + Vw)
What does 0-25% moisture available soil feel like?
dry, loose, will hold together if not disturbed, loose grains on fingers with applied pressure
What does 25-50% moisture available soil feel like?
slightly moist, forms a very weak ball with well defined finger marks. Light coating of loose and aggregated sand grains on fingers.
What does 50-75% moisture available soil feel like?
Moist, forms a weak ball with loose and aggregated sand grains on fingers, darkened colour, moderate water staining on fingers, will not ribbon
What does 75-100% moisture available soil feel like?
Wet, forms a weak ball, loose and aggregated sand grains remain on fingers, darkened colour, heavy water staining on fingers, will not ribbon
What does 100% moisture available soil feel like?
Wet, forms a weak ball, moderate to heavy water/soil coating on fingers, wet outline of soft ball remains on hand
Traditional gravimetric samling, adv and disadv.
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
neutron scattering soil moisture measurement
- 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
electrical resistance soil moisture measurement
- porous blocks equilibrate with the matric suction
- problems - effected by hysteresis, salinity, and temp, may not reach equilibrium fast enough, dependent on block type
cosmic ray probe for soil moisture measurement
- 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
what is a dielectric?
non-conductor that polarizes in an electric field based on the dielectric constant of the material (relative permittivity)
water as a dielectric compared to soil and air
water has a high dielectric constant (81)
- soil is 4-8
- air is 1
time domain reflectometry for soil moisture measurement
- 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
What can be used to a long term record of soil moisture?
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
How can we calibrate the soil probe?
by soil type, eg clay, sand - slight variations in volumetric water content vs dielectric constant by soil type
Sensing soil moisture remotely via thermal sensing
- diurnal temp changes lower for wet soils
- dependent on soil type
- limited to bare soils
Microwaves for remote sensing of soil moisture
- microwaves are sensitive to dielectric of soils (related to volumetric water content)
- can be active or passive sensors
Active microwave sensing pros and cons for soil moisture
- 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
Passive microwave sensing pros and cons for soil moisture
-measures energy emitted from Earth
-enables global coverage and frequent revisit time
-pixel resolution is course (10km)
eg ESMOS
what is produced with SMOS soil estimates?
bi-weekly map of percent saturation surface soil moisture is produced by AAFC
How representative is you soil estimate? How many samples do you need to estimate variability?
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
what are semi-variograms?
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
