ENVS 2060 Lab 3
docx
keyboard_arrow_up
School
McMaster University *
*We aren’t endorsed by this school
Course
2L03
Subject
Geography
Date
Dec 6, 2023
Type
docx
Pages
8
Uploaded by CorporalWillpowerDeer12
Lab # 3 Water in Soil
Recommended Reading: Weil and Brady, 2017: The nature and properties of soils. 15
th
ed., Chapters 5 and 6 Digging into Canadian Soils 114-116, 118-132 Part 1. Water Content a) Use the data provided in table 1 to calculate the: Mass of water in each soil, volume of water, mass based water content (sometimes called gravimetric water content) (θm), Volumetric water content (θv), depth equivalent water content in the 50 cm depth of each soil (Dw), soil bulk density (BD), soil pore space ratio (PSR), and the percentage of the pore space occupied by water. For each calculation provide your rough work for the calculations on one of the soils and provide your answers in table 2 below. Remember that the particle density will remain constant at 2.65 g cm
-3
. Table 1 Data on soil cores for use in calculation in part 1 a. Core # Wet weigh of soil (g) Dry weight of soil (g) Volume of soil (cm
3
) 1 172 128 126 2 198 166 126 3 221 173 126 Table 2 Core # Mass of
water (g) Volume of
water (cm
3
) Θm (%) Θv (%) Dw cm/cm BD (g cm
-3
) PSR (%) % of pore space
filled with water 1 2 3
Part 2: Soil water potential at equilibrium a)
Assuming equilibrium
, determine the value of ψ
h
and its components (ψ
m
, ψ
p
and ψ
g
) at
points A, B and C in the above diagram. Express the potentials in units of cm of water in
table 3. Figure 1 soil water potential diagram Table 3 soil water potential at equilibrium using position “B” as the gravitational reference POINT Potential A (at surface) B C ψ
p ψ
m ψ
g ψ
h b)
Currently in figure 1 the gravitational reference point is given at position “B” (the water
table). The gravitational reference point could however be placed anywhere in the soil
profile as it is just a measure of the relative gravitational energy. Move the gravitational
reference point to position “A” and determine the value of ψ
h
and its components (ψ
m
, ψ
p
and ψ
g
) at points A, B and C in the above diagram. Express the potentials in units of cm of
water in table 4. Table 4 soil water potential at equilibrium using position “A” as the gravitational reference POINT Potential A B C ψ
p
ψ
m ψ
g ψ
h c)
Make note that even though the absolute values of ψ
h
changed in table 3 and table 4 that
the relative values between positions stays the same. Part 3 Soil water potential at non-equilibrium 1. In non-equilibrium condition soil matric potential cannot be determined based on the position relative to the water table. As such the matric potential must be measured at each of the desired positions in a soil. Tensiometers, which measure metric potential, were installed at four different depths (0, 10, 30 and 60 cm below the soil surface) in a
soil profile. On three different days (day 1, 3 and 10) readings of ψ
m
(in cm H
2
0) are obtained. The data are presented below in Table 5. a) Using position A in table 5 as the gravitational reference point, complete table 5 by filling in values for ψ
g
and ψ
h
and indicate in the fourth column for each day whether the water is
flowing upward (↑) or downward (↓) or is at a standstill (0) between adjacent positions (ie between position “A” and “B”, “B” and “C” etc.). e.g A water flow is from ↑ point B to point A B Table 5 Non-equilibrium soil hydraulic potentials. Day 1 Day 3 Day 10 Depth (cm) ψ
m
ψ
g
ψ
h
↑ ↓ 0 ψ
m
ψ
g
ψ
h
↑ ↓
0 ψ
m
ψ
g
ψ
h
↑ ↓
0 0 A -125 -4 -45 10 B -60 -25 -30 30 C -15 -25 -12 60 D -290 -320 -258 c)
What can you say about the most likely weather pattern during this ten-day period?
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
d)
In this case (table 5), explain why was the pressure potential (ψ
p)
not determined? Part 4. Available Water a) Compare the moisture characteristic data for the following 3 soils (figure 2) and complete
table 6 assuming the soil is uniform in texture and structure to 50 cm soil depth. To
determine the volume of water per hectare in table 6 multiply the depth of available water
in meters by the surface area of 1 hectare (10000 m
2
). Also remember that there are 1000L
in 1 m
3
of water. Figure 2 water release curves of 3 soils for use in Part 4 a. Matric Potential Ψm (bars)
Table 6 determination of plant available water from figure 2 Water content
(θv) at Field Cap. Water content
(θv) Wilting Point Plant available
water content (θv) depth of available water in top 50 cm of soil Volume (L/ha) of water plant available in top Soil Water Content θv (% m
3
m
-3
)
50 cm of soil Soil 1 Soil 2 Soil 3 Part 5. Water Movement in Saturated Soils In class we discussed Darcy’s experiment. Figure 3 gives a representation of an experiment to determine the flux rate of water coming through a soil. Figure 3 Darcy’s experiment example to be used to complete table 7 and 8 a)
Determine the hydraulic potential (ψh) at both point A and B represented in figure 2. Present the data in table 7 Table 7 hydraulic potential for the experiment indicated in figure 3 Position in figure 2 Potential A B ψ
p ψ
m Soil
Water
15 cm
12 cm
Ref point
A
B
10 cm
12 cm
11 cm
13 cm
ψ
g ψ
h b)
Use the information you determined in Table 7 and the information in figure 3 to determine
for each of the soils in table 8: the flux rate of water (Jw) (equation below), the change in hydraulic potential (
ψh) (from table 7), and the hydraulic conductivity (k) (equation △
below) and record the information in table 8. Be sure to show your work. Vw
Jw =
AT
Jw = -k
Ψ
h
Z
Table 8 presentation of results for Part 5 b. Volume of water collected (Vw) Surface Area of
soil (A) Time (T) Flux rate of
water (Jw) ψh △
Z △
k Soil 1 115 cm
3
45 cm
2
60 seconds Soil 2 30 cm
3
45 cm
2
60 seconds Part 2 Water movement in unsaturated soils Watch the following video that describes water movement in soils. https://www.youtube.com/watch?v=DmTNFIEc2VA
and/ have a look at the displays in the lab (note they show the same thing as the video) a) At approximately three minutes 10 seconds in the video (3:10) there is a demonstration of capillary rise use two glass plates. They are separated on one site and clamped directly together on the other to represent a range of pore size from large on the open side to very small on the other side. Examine the wedge capillary on the demonstration table and the height of rise above the free water surface at the closed edge of the wedge. If the water on the edge of the glass that was closed together was drawn up to a total hight of 20 cm, use the following equation calculate the theoretical radius of the pore at the edge of the wedge. 0.15 h = r
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
b)
Once again from the video, describe what happened when water was moving through a non-saturated loam textured soil and encountered a sand layer (3:48). Explain why this effect happens using your knowledge of soil hydraulic potentials and hydraulic conductivity. c)
Similarly describe what happened when water was moving through a silt loam textured soil and encountered a clay layer (6:36). Explain why this effect happens using your knowledge of soil hydraulic potentials and hydraulic conductivity. d)
Similarly explain what happened when the water encountered a diagonal band of sandy soil that extended to the soil surface (10:50). Explain why this effect happens using your knowledge of soil hydraulic potentials and hydraulic conductivity. Be sure to explain the difference in effect caused by this sand band compared to the one in part “b”