Expert answer:Soil Physics Problem Set

Answer & Explanation:I will need this by tomorrow at the end of the day, Sunday 9/26.  Good explanations for each one to make sure I understand each piece.  Thanks.2015_Problem Set 2 copy.pdf
2015_problem_set_2_copy.pdf

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Problem Set #2
SCOR470 Fall 2015
Topics: Soil water potential, unit conversions, soil water content
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1. Unit conversions
a. You are curious about the matric potential of an air-dried soil. This depends on the
relative humidity of the air of course but you find a value of -2.20 x 105 J/Kg in a journal
article. Express this in MPa and m. (Note that the permanent wilting point for plants is
typically assumed to be -1.5 MPa.)
b. You need a value for the osmotic pressure of maple tree sap and find a table in a
reliable older reference stating 2.23 x 106 erg/cm3. Convert this to m, ft., and Pa.
c. Your grandfather’s WWII submarine could withstand about 1.5 MPa of water
pressure. Assuming a saltwater density of 1.03 g/cm3, calculate the crush-depth of the
submarine in m and ft.
2.
a. Calculate the energy/volume (in J/m3) of water a plant root must overcome to withdraw
water laterally from a soil with a matric potential of -15000 cm with an EC of 8 dS/m.
b. Assuming 15 oC, calculate the relative humidity in the soil air for the soil in part (a).
Note that this soil is near the wilting point for many plants.
Helpful information: The EC, electrical conductivity, is a measure of the total dissolved salts in a
system. For a wide range of soils, the following empirical relationship relates the EC of the soil
solution to the osmotic pressure (Π) of the solution; Π(bars)= 0.36 x EC (dS/m). The EC unit is
decisiemen per meter.
3. Consider a soil profile above a shallow water table:
Soil surface
water table
silt loam
0.20 m
loam
0.15 m
loamy sand
0.30 m
clay loam
0.40 m
Suppose the θ(h) in this profile is described by the Brooks-Corey formulation:
θ(h) = θS
for
|h| ≤ |he|
θ(h) = (θS -θr)[(he/h)λ] + θr
for
|h| > |he|
Using the parameters in the following table, calculate and plot the equilibrium water content
profile, θ(z), from the soil surface to the water table.
θS
θr
|he|
(cm)
λ
Silt Loam
0.49
0.10
12
0.21
Loamy sand
0.40
0.02
3
0.47
Loam
0.44
0.05
7
0.23
Clay loam
0.51
0.14
19
0.18
Soil
4. The following system is at equilibrium. Determine each component of the total soil water
potential in energy/volume and energy/weight at the points indicated (A, B, …). If a component
is zero, state why.
Loamy
sand
5cm
No evaporation
Pair=1.0 atm
T=25 oC
Clay loam
A∙
B∙
Soils slightly saline
EC=0.75 dS/m
Liquid saturated porous
plate
60 cm
Semi-permeable
membrane
40 cm
Pair(absolute)=
0.90 atm
15 cm
0.05 M KCl
M is moles/liter
5. Refer to Problem# 4.
a. Consider a microorganism at point A in the loamy sand. Assuming the cell wall of the
microorganism is semipermeable, what osmotic (solute) potential is necessary within the cell
to avoid loss of cellular fluid to the soil water? You may assume that osmotic adjustment of
the cellular fluid is the only defense against lower water potentials outside of the cell.
b. Using the θ(h) parameters for the soils listed in Problem 3, calculate the water content at
Point A and Point B in the figure.
c. Suppose the microorganism at point A has had enough of fighting the water robbing forces
in her loamy sand home and dreams of moving to a wetter environment. Her life coach
recommends she follow her dreams so she packs up and moves to the wetter soil(?) at
position B (the clay loam). What cellular solute potential is necessary to prevent
dehydration in the new home?
6. A common representation of the moisture retention function first used by van Genuchten
(1980) is as follows:
(θ −θ )
S
r
θ(h) = [1+(α|h|)
n ]m + θr
where m = (1-1/n)
where h(cm) is the soil water pressure head (matric potential in head units), θs is the saturated
water content, θr is the so-called residual water content, and α (cm-1) & n are shape parameters.
The purpose of this problem is to give you practice working with and interpreting this function.
Consider the following soils with their typical parameter values (based on texture);
θs
Soil Texture
sandy loam
silt loam
clay
0.39
0.44
0.55
α(cm-1)
θr
0.03
0.07
0.12
0.070
0.025
0.010
n
1.60
1.35
1.20
a. For 0.1<|h|<50000 cm, plot θ(h) for each soil in the table. I recommend that you use a log scale for h. b. If water is available for plant uptake over the matric potential range of -0.0100 to -1.500 MPa, calculate the cm of plant available water (PAW) in a 60 cm deep root-zone of each texture. c. If you allow a crop to deplete 60% of PAW prior to irrigating, estimate the time between irrigations in each of the soils if the evapotranspiration loss is 0.7 cm/day. You may ignore soil water drainage below 60 cm depth and assume no rainfall. d. Suppose the sandy loam is at θ=0.100, the silt loam is at θ=0.150, and the clay is at θ=0.350. Samples of each soil are then placed in horizontal contact; Sandy loam Silt loam Clay Upon contact, what is the direction of flow between the layers? Explain. 7. Tensiometers are used to measure matric potential (soil water pressure) and thus can be helpful in determining the direction of water flow and estimating soil water content if θ(h) is known for the soil. The three tensiometers shown here are in the silt loam and clay soils of problem # 6. Sealed headspace #1 Water filled tube #2 #3 10 cm 20 cm 40 cm Water saturated porous cup 60 cm Silt Loam Clay Readings (the gauge pressure of the air in the headspace) are reported for three dates during the early summer: June 1 June 15 June 30 Tensiometer 1 -80 cm -240 cm -450 cm Tensiometer 2 -110 cm -200 cm -400 cm Tensiometer 3 -150 cm -300 cm -350 cm a. For the three dates, calculate the water content at each depth and sketch the water content profile between 20 and 60 cm depth. b. For each date, determine the total soil water potential at each depth and then indicate the direction of water flow between 20 and 40 cm depth and between 40 and 60 cm depth. Note that you can assume that the tensiometer is at equilibrium with the soil water. ... Purchase answer to see full attachment

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