AGRY 560 Exam II

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Author:
MRK
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289459
Filename:
AGRY 560 Exam II
Updated:
2014-11-18 17:18:53
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soil physics water movement
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The November exam
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  1. Soil water potential
    Ψ total = gravitational + solute (ignored b/c no membrane) + tensiometric pressure
  2. Solute potential
    • change in molar concentration
    • need membrane
  3. Ψg Gravitational/ Elevation potentail
    • ρg(z1-z0)
    • mass
    • elevation
    • gravity
  4. ΨTP Tensiometric pressure
    ie pressure potentail
    • Swelling or non-swelling
    • Non-swelling: matrix, air, and hydrostatic (saturated)
    • Swelling: overburden and wetness
  5. Water retention curves
    • plot of soil water content θ
    • at different values of matric potential
    • for water holding capacity and distribution of pore size
  6. From water retention curves:
    • 1. water content at 0 suction (saturation) = porosity ie BD
    • 2. suction required to start draining pores
    • 3. Steepness of drop in water content as suction increases (gradual if pore sizes are various, straight back if sand)
  7. Hysteresis
    • Drying and wetting are different
    • during wetting the soil is drier than same potentail at drying because ink bottle
    • scanning curves when changes from drying to wetting curve
  8. Estimate the water table depth with piezometer
    water table = water level in the shallowest piezometer with water in it
  9. Methods for water retention
    • Sand table
    • Pressure plate
  10. Sand table
    • very fine sand
    • good to 100 cm of sucction
    • Applies tension
    • keep it humid
  11. pressure plate
    • applies pressure in a pressure cooker
    • cermamic plate with rubber lining
  12. Water content v. water potential
    • depends on objective
    • content = mass/volume fraction of water
    • potential = energy status how tightly held
  13. How to measure water content
    • 1. sampling and drying
    • 2. Eletrical resistance
    • 3. neutron scattering
    • 4.Time domain reflectrometry (TDR)
  14. Sampling and drying method for water content
    • Standard
    • wet and dry weight
    • -direct
    • -can loss water from field to lab
    • -destructive sampling
    • -variability
    • -time-consuming
    • -not good for small plots
    • cheap
  15. electrical resistance method for water content
    • Electrodes in gypsum, fiber-glass, nylon
    • measurement affected by water content and solution concentration
    • calibrate for each soil
    • gypsum dissolves over time ∆ readings
    • limited accuracy - good for lawn irrigation not research
    • cheap
  16. Neutron Scattering method for water content
    • fast nutrons emitted slowed by H
    • more water slower near source
    • measures large yet undefinable volume
    • - pros: non-destructive, same place, rapid, large volume
    • - cons: average soil volume, not near surface, calibration, H in more than water, $10,000
  17. Time domain reflectometry (TDR)
    • two parallel waveguides in soil and measure dialectric constant
    • pros: measure over length of rod, non-destructive, same place
    • cons: only average over the length, longer rods are less Perice, calibration, ~ expensive
  18. Measurements of water potental:
    • Tensiometer
    • good from 0-1 bar
    • only way to measure in the field
    • have to refill/read in person
  19. Saturated flow
    • Poiseuille's Law  and Darcy's Law
    • Q = flux (volume/time)
    • q = flux (cm/time) (Q/A)
    • K = hydraulic conductivity (length/time)
    • velocity = q/volume of water
  20. Ksat
    • Lower for finer textured soils
    • not consistant with time (biological clogging, clay migration, air entrapment, slaking and movement out of pores)
    • Darcy's law is for laminar flow
    • Homogenous conductivity is uniform
    • isotropic- uniform in all directions
  21. Measurement of Ksat
    • Lab permeameter (constant or falling head)
    • Constant head well permeameter (keep the well full)
    • auger hole method (dig under water table, measure rate of refill)
    • block method (cut out huge block take to lab)
    • perc test (not very accurate)
  22. Buckingham-Darcy Flux Law
    • unsturated
    • steady state
    • 3-D flow add x and y no gravity
  23. Unsaturated flow
    • function of matirc and tensiometric pressure head
    • cross over point when sand and cl curve cross -> sand has lower conductivity at low pressure head than cl
  24. Richards Equation
    • extends darcy's law for Transient flow (unsaturated, non-steady state)
    • (∂theta/∂t) = (∂/∂z)[K(h)(∂hp/∂z + 1)]
    • use water content form or matric potential form
  25. Steady-state
    constant water content and flux with time
  26. Steady state methods to measure unsaturated conductivity
    • 1. maintain constant hp at both end of column and measure the flux
    • 2. induce constant fluc by evaporation or infiltration, then measure hp (with tensimeter)
  27. Transient Methods lab methods to measure unsaturated conductivity
    • step-outflow ( increase pressure by increment and measure decrease in water outflow rate with time)
    • do at several pressures
    • takes alot of time
  28. Transient Methods field methods to measure unsaturated conductivity
    • 1. Sprinkling infiltrometer (apply constant flux, measure content or matric potential)
    • 2. Instantaneous profile (internal drainage, saturate area, neutron probe for content and tensiometers for matric potential)
    • 3. Crust test (same way but takes weeks)
  29. Why does infiltration decline with time?
    • decreases in suction gradient (the drier soil is further away)
    • swelling of clays
    • air entrapment
    • crusting or sealing
    • migration of particles
  30. Soil Infiltrability depends on
    • time
    • inital water content
    • ksat
    • surface ocnditions
    • impeding layers
    • chemical
    • texture
    • organic matter
  31. Green and AMPT
    • infiltration equation
    • simple approximate
    • initaly dry soil
    • sharp wetting front
    • coarse textures
  32. Horton Equ.
    it = iF + (io - iF)exp(-ßt)
  33. Assumptions of infiltrability equations
    • distant and definable wetting front
    • suction constant at front, independent of x, z, and T
    • Uniform content and conductivity in transmission zone
  34. Non-uniform wetting
    • from cracks, channels, and instablities
    • verticle mulch works if they stay open
    • if covered hole in the soil will not fill till surroundings soil is saturated
  35. Infiltration measuring methods
    • 1. double-ring infiltrometer (standard)
    • pro: simple and quick, inexpensive
    • con: highly variable w/ 30 cm ring, No rain drop impact
    • 2. Sprinkling infiltrometer (need slope, known rainfall rate, measure run off and find infiltration)
    • pro: fain fall impact, larger area so less variable
    • con: equipment, generoator, water, labor, time
  36. Lab approximations of field capacity
    • -0.3 bar
    • -0.1 bar for sands and those with perched water tables
  37. what factors affect F.C.
    • texture
    • OM
    • type of clay
    • impeding layer
    • depth of wtting
    • evapotranspiration
  38. OM effect on field capacity
    increases both FC and WT but FC more
  39. Max water content at any pressure
    clay, SIL, SL, S
  40. highest plant availbe water
    sil, c, sl, s
  41. highest Ksat to lowest
    sand, sl, sil, c
  42. MWD
    • Mean weight Diameter = Sum:Xi * Fi
    • Xi = (opening in sieve passed + opening in sieve retained) / 2
    • Fi = fraction in that sieve
  43. Crust test Method
    • pond water on top and measure what comes through
    • pencil tenisometer right under the crust
    • steady-state
  44. Calculation with Hg Tensiometer
    • PhitotA = PhitotB
    • PhiPA + PhiZA = PhiPb + PhiZB
    • rho(water)*g*Hpa + rho(water)*g*Hza = rho(Hg)*g*Hpb + rho(water)*g*Hzb
    • rho(Hg) = 13.65 g/cm3
  45. Converting tensiometer metrics
    1 cbar =? cm
    10 and switch sign

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