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(solution) Hello, I'd like help with going over this test review.


Hello, I'd like help with going over this test review. 


Exam 1 Test questions- At least 90% of the points on your exam will be a selection of these

 

questions. Problems involving calculations will have altered numbers. 1) Define or describe the following;

 

a) Reference state for soil water potential

 

b) Diffuse double layer

 

c) Soil bulk density

 

d) Soil water potential equilibrium

 

e) Cohesion

 

f) Soil water hysteresis

 

g) Porosity

 

h) Matric potential

 

i) Total soil water potential

 

j) Specific surface

 

k) Contact angle

 

l) Air entry matric potential

 

m) Volumetric water content

 

n) Time Domain Reflectometry

 

o) Gravimetric water content

 

p) Shrink-swell

 

q) Adhesion

 

r) Solute potential

 

s) Head units

 

t) Isomorphous substitution

 

u) Water characteristic function 2) Short answera. If water exist in soil as films only (that is, water is only coating particle surfaces),

 

explain why you would expect more water in a clay soil than in a sand.

 

b. As a soil drains, is it true that small diameter pores drain before larger diameter pores?

 

Explain.

 

c. For a given soil, is the bulk density a constant? Explain.

 

d. Explain why soil water would move from a drier to a wetter soil. Or could it?

 

e. Does water drip from unsaturated soil? Explain based on interfacial curvature.

 

f. It is often said that water always flow downhill. Is this true? Explain.

 

g. Why do we need a reference state for the soil water potential? List the properties of the

 

reference state.

 

h. Can an unsaturated sand pull water from an unsaturated clay? Briefly explain.

 

i. Why does free water move against gravity into narrow diameter pores? 3) Concise essay

 

a. Briefly describe the forces in soil which reduce the potential energy of water relative to

 

the reference state.

 

b. Explain the purpose and basic operation of the pressure plate.

 

c. . Briefly describe the principle of operation of Neutron Attenuation (neutron probe) and

 

Time Domain Reflectometry. What do these measure and how?

 

d. Concisely discuss the key physical and chemical characteristics of the soil solid phase

 

involved with the adsorption and retention of water.

 

e. Briefly explain why some soils change dimension (swell) upon wetting while others do

 

not.

 

f. A friend suggests that you use a tensiometer for the indirect measurement of soil water

 

content. Briefly discuss the operation of the tensiometer, what it measures, and list pros

 

and cons of using it for measuring soil water content.

 

g. A certain soil is known to change dimensions with changing water content if the

 

dominant cation in the soil is calcium but not if the dominant cation is potassium. Why?

 

4) Using a sketch, compare and contrast ?(h) for a fine and a coarse textured soil. Explain the

 

similarities and differences (if any) between the curves.

 

5) A volume of soil (Vtotal) is the sum of the volume of pores (Vpores) and the volume of solids

 

(Vsolids). Derive in a few steps the porosity ? expression =1? where ?b is the soil bulk density and ?s is the particle density. Quantitative

 

6) A bucket (20 cm diameter by 10 cm depth) contains a loam soil with a particle density of 2.7

 

g/cm3 and a porosity of 40%. The soil is at a volumetric water content of 0.10. If the bucket

 

receives 2.0 cm of rainfall,

 

a. Determine the soil water content after the rainfall (you may assume that the rainfall

 

mixes uniformly throughout the soil volume).

 

b. Determine the weight of the bucket of soil after the rainfall (you may disregard the

 

weight of the empty bucket). 7) A cylinder (4 cm diameter by 10 cm long) contains 210.0 g of oven-dry mineral soil. Estimate

 

the grams of water required to fully saturate the soil in the cylinder.

 

8) Suppose a layer of soil (20 cm thick, overlying impermeable bedrock) has a known ?(h)

 

relationship

 

?5 0.25

 

(?) = 0.46 ? ?

 

? If the soil is initially at h= -1000 cm, how much rainfall would be required to increase h to -100 cm

 

throughout the soil layer? (You may assume that the added water is mixed uniformly throughout

 

the 20 cm layer and that there is no evaporation or drainage losses.)

 

9) Two soil samples, A and B, are placed next to each other with good contact. Soil A is at

 

?=0.28, while soil B is at ?= 0.15. The soil water characteristic curve for each soil is

 

Soil A Soil B 300 Soil water tension (cm) 250

 

200

 

150

 

100

 

50

 

0

 

0 0.1 0.2 0.3 0.4 0.5 Water Content (vol/vol) a. Will soil water move from one soil to the other? If so, which sample will lose

 

water? Briefly explain.

 

b. Which soil would you argue has the narrower distribution of pore sizes and why?

 

10) Suppose you have a homogeneous soil sample with a volume of 100 cm3 and a known

 

moisture retention function (for h<-8 cm),

 

?8 0.15

 

(?) = 0.48 ? ?

 

? If the average matric potential of the sample is -2170 cm,

 

a. What is the water content of the sample?

 

b. How much water would you need to add to the soil sample (assuming the water is

 

mixed uniformly with the soil) in order to increase the matric potential to -100 cm?

 

c. What is the physical interpretation of the -8 cm in the moisture retention function?

 

11) a. What do you need to know in order to determine the direction of water flow between

 

two points?

 

b. The matric potential (or water pressure) is measured at three soil depths. Use the data

 

in the following table to determine the direction of water flow between depths 1 and 2

 

and depths 2 and Soil depth (cm)

 

100

 

300

 

400 soil texture

 

sandy loam

 

clay loam

 

loamy sand h or p (cm)

 

-400

 

-80

 

+5 Pair (atm)

 

1

 

1

 

1 12) A field soil is instrumented with tensiometers at three depths as shown below. You wish to

 

know the direction of water flow (up or down) in the soil between the measurement depths. Pair(gauge)=-260 cm Pair(gauge)=-510 cm Pair(gauge)=-310 cm

 

capped air pocket soil surface 0

 

15 40 water filled tube Scale (cm) 100 porous ceramic cup a. Describe (without a calculation) how you would use the information in the diagram to

 

assess the direction of water flow.

 

b. Now use the data provided in the figure in a calculation to determine the direction of

 

water flow in the soil between 15 to 40 cm depth and in the soil between 40 to 100 cm depth. 13) A long capillary tube (radius=0.0015 cm) with a semi-permeable membrane on the lower

 

end is oriented vertically and placed in a dilute sodium chloride solution at T=20OC. If the height

 

of rise of water in the tube is 20 cm,

 

a. What is the solute potential (in head units) of the solution? State assumptions.

 

b. Estimate the concentration of the sodium chloride solution.

 

14) Consider the following cylindrical pores. Determine the height of rise in each configuration.

 

You may assume a contact angle of zero and 20 oC. 10 cm 10 cm 15) r=0.10 cm r=0.01 cm 10 cm 10 cm r=0.01 cm r=0.10 cm Suppose you have a homogeneous soil sample with a volume of 400 cm3 and a known

 

moisture retention function,

 

?5 0.40

 

(?) = 0.43 ? ?

 

? If the average matric potential of the sample is -1000 cm, how much water would you need

 

to add to the soil sample (assuming the water is distributed uniformly in the soil) in order to

 

increase the matric potential to -100 cm? 16) Suppose you have a homogeneous soil sample with a known moisture retention function;

 

(?) = 0.48 10 0.20 (?) = 0.48 ?|?|? |?| ? 10 |?| > 10 If you have a 100 cm3 sample that is water saturated, how many grams of water can you

 

remove by applying a suction of 0.40 atm? 17) Two soil samples, A and B, are placed next to each other with good contact. Both soils are

 

at ?=0.25. The soil water characteristic function for each soil is given below. Will soil

 

water move from one soil to the other? If so, which sample will lose water? Briefly

 

explain.

 

Soil A: Soil B: 18) (?) = 0.55 ?

 

(?) = 0.48 ? 1 ?18 3

 

? ? ?

 

1 ?12 2

 

? ? ? The horse-shoe shaped soil system below contains a homogeneous loam and is at

 

equilibrium. Prior to setting the horse-shoe into water as shown below, the soil on the left

 

was saturated with water while the right side was oven dry. divider

 

Initially water saturated Initially air dry 10 m water a. Provide a qualitative sketch of the water content vs. elevation above the water table

 

for the two sides of the horse-shoe. Provide a brief explanation.

 

b. If the divider at the top of the horse-shoe is removed, will water move to the right?

 

Explain.

 

c. If a hole is drilled in the sidewall of the arc holding the soil, will water drip out?

 

Explain. 19) Consider the following soils separated by a no flow barrier. The soils are resting in water

 

at equilibrium.

 

20 m sand clay 0m

 

a. Provide a qualitative sketch of the soil water content distribution (that is, ? vs.

 

distance) above the water-table in the two soils. Explain the similarities or

 

differences in the sketches for the two soils.

 

b. Suppose that a portion of the no flow barrier at 3 m above the water is carefully

 

removed allowing contact between the two soils. Will water move from the clay to the

 

sand? From the sand to the clay? Explain.

 

20) A freely draining field soil (soil profile shown below on left) has a uniform matric potential

 

of -150cm several days after a rain-storm that had saturated the soil profile. Provide a qualitative

 

diagram (no calculations needed) of the relative water content versus depth using the axes on the

 

right. Briefly explain your rationale.

 

?v Soil surface Clay loam (30% sand, 35% clay) Sandy loam (60% sand, 10% clay) Loam (45% sand, 15% clay) Sand (90% sand, 5% clay) D

 

e

 

p

 

t

 

h Equilibrium diagram problems- These examples cover the type problems that

 

will appear on the exam but the actual exam problem may be altered.

 

21) A sandy loam soil sample is placed on a saturated porous plate in contact with water as

 

diagramed below. Determine the equilibrium matric potential or hydrostatic pressure

 

(in your choice of unit systems) in the soil at point A. Would you expect this soil to be

 

saturated? Why or why not. A? 1.00 Pair= 850 mbar porous plate

 

m water Pair (gauge)= -200 mbar

 

Air chamber 0.10

 

0 22) The following system is at equilibrium.

 

Pair=0.90 atm air chamber Pair=1.50 atm

 

silt sand 80 cm

 

70 cm water saturated porous plate water

 

10 cm

 

0 a. Find the matric potential (at the midpoint) in the sand and in the silt.

 

b. At equilibrium, how do you expect the water content to compare in the two soils (circle one): ?sand = ?silt

 

?sand < ?silt

 

?sand > ?silt Explain your reasoning. 23) The following system is at equilibrium. Find the water pressure term (h or P) of the total soil

 

water potential at point A and point B. You may express your response in either energy/volume or

 

energy/weight of water. Pair=813 mbar

 

3.0 m Pair=1000 mbar s=-100 cm Saline solution

 

(?=1.0 g/cm3) Solute free 2.0 m A

 

sandy loam Semi-permeable

 

membrane B

 

Not

 

semipermeable to

 

solutes 1.0 m clay loam 0.5 m 24) The following soil-water systems are at equilibrium (two separate problems). Find the

 

requested information (indicated by the question mark) in each figure. You may express your

 

response in either energy/weight or energy/volume of water. a. capped air pocket Pair = ? b. capped air pocket Pair=-0.20 m

 

Pair=1 atm Pair=1 atm water filled

 

tube

 

rigid,

 

unsaturated

 

soil water filled tube 50cm

 

60 cm rigid,

 

unsaturated

 

soil water table

 

saturated soil porous cup 10 cm h=? 25) A soil sample is placed on a saturated porous plate and then subjected to a series of

 

manipulations. Determine the equilibrium matric potential at the midpoint of the soil sample (in

 

your choice of unit systems) at each step in the sequence.

 

Step 1: soil Notes- ?

 

?

 

? 10 cm

 

water saturated porous plate No evaporation

 

Surrounding air pressure=1 atm

 

?= 4.9 x 104 Pascal water Step 2: 60 cm 10 cm Step 3: Step 4: 40 cm Semipermeable

 

membrane Semipermeable membrane Pair (absolute) = 0.60 atm 20 cm 10 cm

 

saline solution, s = -? saline solution, s = -? 26) The following soil-water system is at equilibrium. Find all the components of the soil water

 

potential head at locations A, B, and C. Pair= 1 atm

 

No evaporation C ? Pair=1.15 atm s=-200 cm 50 cm

 

B ? 50 cm saline solution

 

A Porous ceramic

 

water ?

 

semipermeable membrane

 


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