FANR 3060: Final Chapter 9 Study Guide
37 Cards in this Set
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residence time
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the average length of time water spends in each reservoir
residence times for water in soil are short
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cohesion
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the polar water molecule results in high cohesion between molecules because the + H atoms on one molecule are attracted to the - O atom on another molecule, creating a chain of molecules connected weakly by these hydrogen bonds
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adhesion
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water molecules are also attracted to charged surfaces because of the polarity of the molecules and this attraction results in adhesion of water molecules to oxygen-containing and charged surfaces
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capillarity
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the rise of water above free water surface in narrow channels such as fine tubing
a byproduct of molecular adhesion and cohesion
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capillary rise
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the wicking effect caused by small pores
height of rise increases with decreasing particle size
also inversely proportional to diameter of tubing or channels
equation doesn't hold for clay soils
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hydraulic head
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hydrologists convert energy to a unit called the hydraulic head which has units of length
hydraulic head of a fluid, h, is the energy of an equivalent height of water
h = z + p where z is the elevation and p is the pressure head
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pressure head
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a.k.a. pressure potential
can be either + or -
it is energy imparted to water due to the positive pressure of overlying water or to negative pressure due to adhesion with soil surfaces
some refer to
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when does a positive pressure head occur?
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when water molecules are being pushed together or compressed, usually by weight of water column above
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when does a negative pressure head occur?
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water molecules are being pulled apart and are experiencing tension
also called water tension
when soil is unsaturated, water molecules are bound to soil particles by surface attraction at soil-water interface...the magnitude of this force is equal to vacuum required to pull water molec…
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tension (matric potential)
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potential due to adhesion of water to hydrophillic surfaces b/c of hydrogen bonding and dipole interactions
same as negative pressure head, but has a different sign (+/-)
typically in units of pressure, which is units of bars
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___ bars = ____ meters
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1 bar = 10 m
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soil with large vs. small pores
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small pore spaces drains slowly and holds substantial amounts of water even when tension is very high while soil with large pores drains quickly and loses most of its water at small negative pressure potentials
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porosity
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volume of voids (zone not occupied by soil particles) expressed as fraction/percentage of total soil volume
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bulk density and porosity
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inversely proportional
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moisture release curve/soil moisture characteristic curve
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describes relationship between soil moisture and pressure
shows amount of water held at certain tensions or between tensions in that soil
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saturation (SAT)
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water content when pores are completely filled with water, some of this water is not really held by the soil at all but exists in the macropores and would drain our of the soil by gravity if allowed to
tension = 0
corresponds to pressure potentials of 0 and above
at saturation, volumet…
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field capacity (FC)
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water content held after rapid gravitational drainage has occurred; the maximum water-holding capacity of soil
tension = 0.1 bars; equal to 100 and 300 cm
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wilting point (WP)
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amount of water held when plant roots no longer extract water from soil
tension = -15 bars
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air dry (AD)
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amount of water held by soil when it is exposed to the atmosphere
tension = 1000
a function of relative humidity
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oven dry (OD)
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amount of water held once soil has been dried in a 105º (some prefer 60) oven for 48 hours
tension = 10,000
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saturated flow
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when water flow in soils occurs as saturated flow, the water flows circuitously around soil particles thru matrix and flows quickly throughmacropores (larger pore sizes, faster water flows through it)
also dependent on hydraulic gradient in soil...higher graident, faster flow
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sands and gravels are referred to as _____ because they have ______ that can transmit ______ while clays have _________
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highly permeable
large pores spaces
saturated flow
low permeability
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hydraulic gradient
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a measure of how quickly head changes form one point to another
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Darcy's Law
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describes/predicts saturated flow rate in a soil by accounting for head gradient and pore size distribution of the soil
says that water movement thru soil is a linear function of hydraulic gradient and that the constant of proportionality is a soil characteristic called thehydraulic cond…
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Darcy's Law basically says that the amount of water that flows from one region to another thru ground is proportional to 3 factors:
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1. K: hydraulic conductivity or permeability of soil
2. A: area of flow...greater the area of flow, greater the amount of flow
3. G: magnitude of driving force...steeper water energy slope, more flow
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porosity and flow rate
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high porosity = much slower flow
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a soil with a high HC is unsuitable for ____ or _____ because _____.
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for a septic drain field or for land application of wastewater b/c contaminated water migrates too quickly to ground water and will receive insufficient treatment in the soil
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unsaturated soil water movement (a.k.a. capillary flow)
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pores are filled with both air and water and the water flows in films around soil particles
flow at pressures below -0.1 bar (FC)
driven primarily by matric potential differences rather than gravity
can occur in any direction
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saturated vs. unsaturated flow
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major difference is that HC is constant in saturated soils but in unsaturated HC is a function of soil moisture
unsaturated is slower, HC lower than saturated
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hydraulic conductivity decreases as soil becomes ______ or as the pressure potential becomes ______
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drier
more negative
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translocation
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movement of water up thru conducting stem tissues
although elevation head increases as water moves up plant, total head decreases b/c of large negative pressure within the plant tissues
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water availability to plants is limited by two factors:
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1. on the wet end it's limited by the maximum water holding capacity of the soil (field capacity)
2. on dry end, it's limited by root's abilities to overcome soil water attraction to the soil surface (wilting point)
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what is the commonly accepted limit for useful water uptake by plants?
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15 bars
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available water in soils as a continuum
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substantial water can be readily extracted around 0.1 bar tension (field capacity) and progressively less water is available as the tension increases up to about 15 bars, at which point the tension water loss thru the leaves exceeds the uptake rate in the soil and plants wilt
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loamy textures have _____ max. available water than clays because...
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higher because clays hold more water more tightly and therefore have less available
(also sands hold very little water at FC and thus have low max. available water)
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generally, available water is greatest at...
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-0.1 bar and decreases as tension increases
most available water is held b/w -1.0 bar and -15 bars (WP)
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what drives soil water to move?
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osmotic potential
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FANR 3060: ROCKS