vadose zone - institut teknologi...
TRANSCRIPT
Medium for plant growth
Regulator of water supplies
Recycler of raw materials
Habitat for soil organisms
Engineering medium
Functions of Soil
Functions of Soil
Medium for plant growth
Physical Support
Gas exchange
Water
Temperature
Nutrient source
Functions of Soil
Regulator of water supplies
Infiltration
Run-off
Storage/Movement
Distribution
Purification
Integral to hydrologic cycle
Ap May June July Aug. Sept Oct
Recharge Recharge Runoff
Evapotranspiration
Precipitation
Soil moisture
utilization
Actual ET
Potential ET
Wate
r am
ou
nt
ET > Precip = Soil moisture utilization
Precip > ET = Recharge, surplus, and runoff
Water Balance Diagram
Answer
• The water moves sideways and downward at the same rate. This is because of adhesion and cohesion.
• Would the movement be different if the soil was saturated? – Yes. The movement would
mainly be downward due to gravity.
WATER
Water Movement
• Water front does not move into sand until loam is saturated
Water
Loam
Sand
t 1
t 2
t3
t4
Water Movement
• Water front moves into clay upon contact with clay, but because it moves slow water builds up above the clay layer.
Water
loam
clay
Summary Points of Water Movement
1. Pore size is one of the most important fundamental properties affecting how water moves through soil. Larger pores as in sand conduct water more rapidly than smaller pores in clay.
2. The two forces that allow water to move through soil are gravitational forces and capillary forces. Capillary forces are greater in small pores than in large pores.
3. Gravitational and capillary forces act simultaneously in soils. Capillary action involves two types of attractions, adhesion and cohesion. Adhesion is attraction of water molecules to solid surfaces; cohesion is the attraction of water molecules to each other. Gravity pulls water downward when the water is not held by capillary action. Thus gravity influences water in saturated soils.
4. Factors that affect water movement through soil include texture, structure, organic matter and bulk density. Any condition that affects soil pore size and shape will affect water movement. Examples include compaction, tillage, decayed root channels and worm holes.
5. The rate and direction of water moving through soil is also affected by soil layers of different material. Abrupt changes in pore size from one layer to the next affect water movement. When fine soil overlies coarse soil, downward water movement will temporally stop at the fine coarse interface until the fine layer above the interface is nearly saturation.
6. When a coarse soil is above a fine soil, the rapid water movement in the coarse soil is greater than through the clay and water will build up above the fine layer as the water front comes in contact with the fine layer. This can result in a build up of a perched water table if water continues to enter the coarse layer.
Capillarity
Spontaneous movement of water
into and through pore spaces in soil
without the aid of gravity.
Soil Pores
Adhesion and Cohesion
Adhesion to the tube or pore wall
Cohesion between water molecules
capillarity
Capillary fringe Capillary pores in the zone of aeration draw up water
from the zone of saturation beneath the water table.
In very fine-grained soils, this capillary fringe can
saturate the soil above the water table
Tensiometer reading is negative
Capillary fringe is a part of vadose zone
Vadose Zone : the upper layer of the earth that contain
a three-phase system of solid, liquid, and gaseous
material.
Also called the zone of aeration or unsaturated zone.
Particle Size Large/coarse Medium Fine/Small
Sand
Loamy Sand
Sandy Loam
Silt
Sandy clay Loam
Silty clay Loam
Silt Loam
Loam
Clay Loam
Sandy Clay
Silty Clay
Clay
Pore Size Large/Macro Meso/Medium Micro/Small
Capillarity Weak Moderate Strong
Texture
High potential Energy
Low potential Energy
Water moves in response to
differences in potential energy,
from high potential energy to
low potential energy.
Gravitational Potential Energy
The greater the difference in height
The greater the difference in
Gravitational potential energy.
Gravitational Potential
Ψg = mgh
The greater the height, the greater the potential energy.
The potential energy of a unit quantity of water.
Unit quantities: volume
mass
weight
ψg
= mg
Ψg = h (cm) mg
Gravitational Potential
Reference level Ψg = 0
Height (cm)
100
50
a
b
ψga = 100 cm
ψgb = 40 cm 40
soil
Difference in energy determines movement
Independent of soil
properties
Gravitational Potential
Reference level
(Ψg = 0)
Height (cm)
100 a
b
Ψga = 60 cm
Ψgb = 0 cm
Ψga – Ψgb
60 - 0 = 60 cm
40
0
Gravitational Potential
1. Gravitational potential energy is
due only to the height of an object
(water) above some reference point.
2. Gravitational potential energy is
independent of soil properties.
Matric Potential
“suction” potential - capillarity
Narrow capillary tube – high capillary rise h = 0.15
- strong force r
- compared to free water
Small particles, small pores
Applies to unsaturated soils
Primary Factors in Matric Potential
Texture, Density, Aggregation
Pore Size Distribution
Moisture Content
Which Pores are Filled
Capillarity and Soil Texture
Small pores
Strong suction
Strong capillarity
Large pores
Weak suction
Weak capillarity
Porous block
Suction (capillarity)
Capillary Potential
100 cm
Dry soil
Ψm = -100 cm
(suction)
Vertical distance between the surface of the water and the porous cup.
suction
Soil Texture
1000 cm
Dry soil
ψm = -1000cm
(suction)
Vertical distance between the surface of the water and the porous cup.
Sandy Soil Porous block
suction
Soil Texture
10,000 cm
Dry soil
Ψm = -10,000 cm
(suction)
Vertical distance between the surface of the water and the porous cup.
Fine-textured soil
Units of Potential
Centimeters of water
Bars
Pascals
1 bar = 1020 cm water (4oC)
1 KPa = 10 cm water
1 bar = 100 kPa
Total Potential Energy is the sum
of the gravitational, submergence,
and matric potential energies.
Ψg + ψm + ψs = ψT
Gravitational Potential + Matric Potential = Total Potential
Reference level Ψg = 0
Height (cm)
50
20
a
10
Ψm = -65 cm Ψg = 50 cm
ΨT = -15 cm
Gravitational Potential + Matric Potential = Total Potential
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
Ψm = -65 cm
Ψm = -5 cm Ψg = 10 cm
Ψg = 50 cm
ΨT = -15 cm
ΨT = 5 cm
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
ΨTa = -15 cm
ΨTb = 5 cm
ΨTa – ΨTb = (-15cm) - 5cm = -20 cm
Energy Differences
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
ΨTa = -15 cm
ΨTb = 5 cm
ΨTa – ΨTb = (-15cm) - 5cm = -20 cm
Which way will water move?
Determining the Direction of Water Flow
4. Point A – Point B
5. Water moves from high to low energy
Positive Point A to Point B
Negative Point B to Point A
1. Sum the individual potentials at each point
2. Determine if there is a difference in potential
3. Water will move from the higher to the lower energy
Gradient
The difference in potential divided by the
Distance between the two points considered
total potential at point A – total potential at point B
distance between points A and B
The driving force for water flow.
The stronger the gradient,
the greater the driving force
for water movement.
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
ΨTa = -20 cm
ΨTb =-100 cm
Difference in total potential = 80 cm = 2
Distance between the points 40 cm =
Gradient
Difference in potential energy = -20 cm – (-100 cm) = 80 cm
Gradient =
Distance between points A and B = 40 cm
Distance (cm) 0
Height (cm)
50
20
a b
10
Difference in total potential -100 - (-200) = 100 cm = 5
Distance between the points 20 cm 20 cm =
5 25
Ψma = -100 cm
Ψga = 0 cm
Ψmb = -200 cm
Ψgb = 0 cm Ref.
Gravitational Potential + Matric Potential = Total Potential
Reference level Ψg = 0
Height (cm)
50
20
a
10
Ψm = -95 cm Ψg = 50 cm
ΨT = -45 cm
Gravitational Potential + Matric Potential = Total Potential
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
Ψm = -95 cm
Ψm = -25 cm Ψg = 10 cm
Ψg = 50 cm
ΨT = -45 cm
ΨT = -15 cm
ΨTa – ΨTb = (- 45cm) - (-15cm) = -30 cm
Gradient
The difference in potential divided by the
Distance between the two points considered
total potential at point A – total potential at point B
distance between points A and B
The driving force for water flow.
The stronger the gradient,
the greater the driving force
for water movement.
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
ΨTa = -20 cm
ΨTb =-100 cm
Difference in total potential = 80 cm = 2
Distance between the points 40 cm =
Gradient
Difference in potential energy = -20 cm – (-100 cm) = 80 cm
Gradient =
Distance between points A and B = 40 cm
Distance (cm) 0
Height (cm)
50
20
a b
10
Difference in total potential -100 - (-200) = 100 cm = 5
Distance between the points 20 cm 20 cm =
5 25
Ψma = -100 cm
Ψga = 0 cm
Ψmb = -200 cm
Ψgb = 0 cm Ref.
Soil Water Content
Water content by weight
Moist weight – Dry weight
Dry soil weight =
Water weight
Dry soil weight
Multiply by 100 to yield % water by weight
V = Πr2h
Water content by Volume
Volume Water
Volume Soil
Multiply by 100 to yield % water by volume
Example:
You collect a 200 cm3 soil sample. Its moist weight is
150 g. After drying, the dry weight is 100 g.
Gravimetric water content:
Moist weight – Dry weight
Dry weight =
Water weight
Dry weight
150 g - 100g
100g =
50 g = 0.5 or 50%
100g
Example:
You collect a 200 cm3 soil sample. Its moist weight is
150 g. After drying the dry weight is 100 g.
Volumetric water content:
150 g - 100g
200 cm3 = = 50 cm3 water = 0.25 or 25%
200 cm3 soil
Volume Water
Volume Soil Density of water
1 g/cm3
50 g
200 cm3
Energy-Based
Characterizing Soil Moisture Status
Relating water content and matric potential (suction)
Characterizing Soil Water
Suction applied in
discrete increments.
Water
Remaining
In soil
Suction applied (cm) 0 10,000
One soil
saturated
*
Soil Core
Moisture Release Curve
Texture, Density
Water
Remaining
In soil
Suction applied (cm) 0 10,000
saturated
*
A
B
Two Soils
coarser
finer
Soil Moisture Status
Field Capacity: Water content of soil after drainage from saturation by gravity
Suction equivalent: -0.33 bars (or –0.10 bars)
- 33 KPa
- 330 cm water
Permanent: Water can no longer be accessed by plants
Wilting point Suction equivalent: -15 bars
-1500 KPa
- 15,000 cm water
Saturation: Water content of soil when all pores are filled
Suction equivalent: 0 bars
0 KPa
0 cm water
Plant Available water: Field Capacity - PWP
Texture Field
Capacity
Perm. Wilting
Point
Sandy Loam 17 9
Loam 24 11
Clay 36 20
Heavy Clay 57 28
Energy and Texture
Smaller particles and pores
Water Content (%) at
Hydraulic Conductivity
Strongly responsible for water distribution within the soil volume. Determines the rate of water movement in soil.
Texture Density Structure Water content
The ease with which water moves through soils
h
L
A
Volume time
h * A L W
A T E R
Determining Saturated Hydraulic Conductivity
Volume time
= h * A L
K
K = V * L
h * A * t
Soil
Approximate Ksat and Uses
Ksat (cm/h) Comments
36 Beach sand/Golf Course Greens
18 Very sandy soils, cannot filter
pollutants
1.8 Suitable for most agricultural,
recreational, and urban uses
0.18 Too slow for most uses
<3.6 x 10-5 Extremely slow; good if compacted
material is needed
Saturated hydraulic conductivity
Reference level Ψg = 0
Height (cm)
50
20
a
b 10
ΨTa = -20 cm
ΨTb =-100 cm
Difference in total potential = 80 cm = 2
Distance between the points 40 cm =
Gradient
Difference in potential energy = -20 cm – (-100 cm) = 80 cm
Gradient =
Distance between points A and B = 40 cm
Darcy’s Equation
Volume flow
Area * time = Q = Ksat * gradient
(Q) = 5 cm/hr * 2
= 10 cm/hr
Difference in total potential = 80 cm = 2
Distance between the points 40 cm = Gradient =