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26 Cards in this Set
- Front
- Back
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soil water affects (3) |
plant growth
quality of groundwater
quality of freshwater lakes |
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soil-water interactions influence (3) |
ecological functions (plants, microbes)
practices of soil mgmt (tillage, seeding, irrigation)
harvesting (crop, timber) |
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soil water measurement techniques
indirect and direct |
indirect: involving measurement of physical or chemical proeprty of a soil that depends on its water content (dielectric, electrical conductivity, heat capacity, hydrogen content)
direct: involve some form of removal or separation of water from the soil matrix with a direct measurement of the amount of water removed |
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Gravimetric method
principle, advantages, and disadvantages |
principle: wet sample weighed, oven dried at 105*C, ratio of mass loss, to total mass of fully dried soil, correction for stone content (SC)
= (1-SC) x water content
advantages: simple, least equipment required
disadvantages: destructive, not suitable for dynamic measurements, time consuming, not in situ, not applicable to organic soils |
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neutron moisture meter
priniciple, advantages, disadvantages |
principle: a neutron is the same mass as an H atom, after collision with H fast neutrons slow down. By releasing fast neutrons in soil and counting slow neutrons, figure out the number of H in the soil
advantages: least destructive, total water content, large measurement volume
disadvantages: radioactive, calibration, cannot measure surface water content, insentivie to abrupt change |
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gamma attenuation
principle, advantages, disadvantages |
principle: gamma ray is electromagnetic radiation of high frequency and therefore high energy and is classically produced by the decay from high energy states of atomic nuclei
gamma attenuation due to scattering and adsorption depends on consitiuents, and bulk density
advantages: least destructive, high resolution, total water content, buik density
disadvantages: radioactive, slow |
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time domain reflectometry |
capacitance - ability to store electric charge
capacitor - device able to store electric charge
dielectric constant - ratio of capacitance of a capacitor with the given substance as dielectric to the capcitance of the same capacitor with air as the dielectric |
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time domain reflectometry
principle, advantages, disadvantages |
principle: measure the velocity of electromagnetic signals travelling to the end of probe and reflecting back
advantages: nondestructive, nonradioactive, fast and accurate, no need for onsite calibration, clear boundary, potential to be multipurpose and automation
disadvantages: gap between rod and soil (installation and shrinking), salinity problems (high EC in soil). |
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cohesion
adhesion
surface tension
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cohesion: attraction of water molecules for each other
adhesion: attraction of water molecules from solid surface
surface tension: the tension is the force per unit length of any straight line on the liquid surface. Depends on liquid, temp, solvent |
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consequences of a soil with hydrophobic material (oil contaminated site) |
land degradation by increasing runoff and erosion
delayed germination
patchy growth
slow down bio-remediation of oil contaminated soils |
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capillarity |
measures how strong the soil suction of water (higher capillary rise, the stronger the soil suction)
suction is inversely related to pore diameter (the smaller the pore size, the larger the suction) |
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water content definition
water potential definition |
water content: describes the amount of water in the soil
water potential: describes the energy state of water in soil |
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potential energy |
the energy due to position in a force field and is a form of mechanical energy
the difference in potential energy per unit quantity of water between soil water and reference water
only differences in potential energy can be measured
the work done per unit quantity of water by the soil water to move reversible and isothermally from the soil state to the reference state |
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Factors affecting the potential energy of water (4) |
-adsorption to solid surface
-position in gravitational field
-salt/salute content
-applied pressure (hydrostatic, pneumatic, overburden) |
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Reference point/state |
water potential always zero for reference state.
if PE of soil water < the reference then = -
if PE of soil water > the reference then = + |
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gravitational potential |
Definition: potential energy of soil water at a higher elevation than standard reference state
-reference usually soil surface or water table
-/+ depends whether soil location is above or below reference elevation |
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pressure potential |
Definition: the portion of water potential due to pressure applied to soil water from standing water, air, or overburden (reference = atmospheric)
-hydrostatic pressure in soils calculated from depth of standing water
For calcs: is the soil saturated? yes = hydrostatic = 0 no = hydrostatic = depth of standing water
is the point of interest exposed to atm? yes = hydrostatic = 0 |
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matric potential |
definition: the potential energy due to the attraction of water by soil particle surfaces
*always negative = - soil suction
higher the water content, higher the matric potential
matric potential sand> silt> clay...
no soil means no matric potential exists when soil is unsaturated |
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tensiometer |
measuring the negative pressure, or tension, of water in soil in situ.
composed of : a porous, permeable ceramic cup, a tube to a manometer or vacuum gauge. |
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tensiometer reading + the height of the tensiometer = |
matric potential at the tip of the tensiometer |
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why is salinity bad? (4) |
-salts decrease osmotic potential(selectively allows ions) of soil, reverse the direction of water flow from soil to roots
-some salts are toxic to plants
-salts destroy soil structure, slowing down water movement
-nutritional inhibition |
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Soil water retention curve
definition and what is used to determine it? |
definition: relationship between soil matric potential and the soil water content
capillarity: pore size determines the soil water potential. the volume of a specific pore size determines water content
therefore, pore size distribution can be used to determine water retention curve, or vice versa |
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water retention curve
soil physical properties that affect curve (4) |
porosity soil texture soil structure soil organic matter |
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field capacity |
amount of water content held in soil after excess water has drained away and the rate of downward movement has materially decreased
matric potential -330cm |
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permanent wilting point |
the minimum soil water content at which a plant wilts and can no longer recover its turgidity when placed in a saturated atm for 12 hours
matric potential = -15000 cm |
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available water |
the range of available water that can be stored in soil and be available for growing crops
can be calculated: water content at field capacity - water content at permanent wilting point = available water |
