Soil Moisture and
Groundwater Recharge
Hsin-yu Shan
Department of Civil Engineering
National Chiao Tung University
Soil Physics
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A scientific field that focus on the water
in the vadose zone
Major concern of agricultural
engineering
Relationship between water content and
pressure
Hydraulic conductivity of unsaturated
soil
Soil in the Vadose Zone
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Solids:
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Soil particles/Aggregates
Organic materials
Fluids:
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Water (Aqueous solution)
Air
Other liquid
Porosity and Water Content of
Soil
Weight
Volume
Vv
Va
Air
wa
Air
Vw
Water
ww
Water
ws
Solids
Solids
Vs
Porosity: ratio of volume of the void to the total volume
Vv
Vv
e
n= =
=
V Vv + Vs 1 + e
Void ratio: volume of the voids to the volume of the solids
Vv
e=
Vs
Bulk density:
ws
ρb =
Vs
Gravimetric water content: ratio of weight of water to
the dry weight of solids
ww
w=
x 100%
ws
Volumetric water content: ratio of pore water to the total volume
Vw
θ=
V
Degree of saturation: ratio of the volume of pore water to
the total volume of voids
Vw
Sr =
x 100%
Vv
Capillary and the Capillary
Fringe
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Capillary: water molecules at the water
table subject to an upward attraction
due to surface tension of the air-water
interface and the molecular attraction of
the liquid and the solid phases.
Tension
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If fluid pressures are measured above
the water table, they will be found to be
negative with respect to local
atmospheric pressure
Air Pressure
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If air in the pores is connected, the air
pressure is equal to the local
atmospheric pressure
If air in the pores is not connected and
is in the forms of air bubbles such as in
the capillary fringe, the air pressure is
equal to the pressure of water, which is
negative.
Capillary rise in a tube
σcosλ
i
λ f
s
r
λ
R
σ
λ=0
R=r
hc
water
2σ cos λ
hc =
ρ w gR
Fig. 6.2 IDEALIZED pore diameter in a sediment with
cubic packing. The equivalent capillary tube has a
radius of 0.2 the diameter of the grains
Patm
z (m)
ψh (=0)
0.4
0.3
ψp
ψg
0.2
Patm
0.1
0
water
-4
-2
0
2
4
y (J /kg)
-4
-2
0
2
4
r l y (kJ /cu.m = kP a )
-0.2
0
0.2
0.4 y /g (J /N = m)
-0.4
z (m)
0.4
ψg =gz
ψp
0.3
ψh
0.2
water
0.1
-1
0
1
2
3
4
ψ (J/Kg)
1
2
z (m)
0
-0.2
-0.4
-0.6
-0.8
ψg= gz
-0.8 -0.6
H
h
-1.0
-1.2
-0.4 -0.2
0
0.2 0.4 0.6 0.8 heads (m)
Height of Capillary Rise
Sediments
Fine silt
Coarse silt
Very fine sand
Fine sand
Medium sand
Coarse sand
Very coarse sand
Fine gravel
Grain
Diameter
(cm)
0.0008
0.0025
0.0075
0.0150
0.03
0.05
0.20
0.50
Pore
Radius
(cm)
0.0002
0.0005
0.0015
0.003
0.006
0.010
0.040
0.100
Capillary
Rise (cm)
750
300
100
50
25
15
4
1.5
Capillary Fringe
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Capillary pores in the vadose zone can
draw up water from beneath the water
table below which the pores are
saturated with water.
The zone above the water table, in
which the pores are saturated is termed
capillary fringe
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The liquid pressure in the capillary zone
is negative
Capillary fringe is a part of vadose zone
The zone of aeration is best defined as
the zone where the soil moisture is
under tension
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The capillary fringe is higher in finegrained soils than in a coarse-grained
soils
Smaller pore opening creates greater
tension
Pore-Water Tension in the
Vadose Zone
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Fluid pressures in the vadose zone are
negative
The negative pressure head is
measured in the field with a
tensiometer
Tensiometer
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A tube that is closed at the top and
filled with water
The tube is connected to a pressure
gauge
A ceramic cup at the bottom of the tube
to provide a porous membrane
The ceramic cup should be saturated
with water before use
d
b
∆ z1
Soil Surface
c
∆ z2
a
Tensiometer (1)
C
B
l
D
∆ z1
A
E
∆ z2
Tensiometer (2)
Soil Surface
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If the suction is lower than the entry
pressure of the membrane, only water
can go through it
When the suction in the soil pulls water
out from the tensiometer, the water in
the tube is under tension and will cause
the pressure gauge to indicate the
magnitude of such a tension
z (m)
0.4
0.3
0.2
0.1
0
-0.1
-0.2
Air Entry Pressure
Soil Water
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Water in the vadose zone that is
available to growing plants.
This is not a very exact definition. Avoid
using it.
Field Capacity
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The soil moisture content of a layer at
which the the force of gravity acting on
the water equals the surface tension.
Related to the specific retention
Depends on specific retention,
evaporation depth, and the unsaturated
permeability characteristic curve of the
soil.
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Field capacity is related to specific
retention but has different units
It depends upon specific retention,
evaporation depth, and the unsaturated
permeability characteristic curve of the
soil
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The concept is vague
Gravity drainage may take a long period
to occur
Some definitions:
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Water content of soil after 48 hours of
gravity drainage
Water content of soil under a suction of 0.3
bar
Moisture content of a silt loam as a
function of time since saturation
Time (days)
θ (%)
1
20.2
7
17.5
30
15.9
60
14.7
156
13.6
Wilting Point
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The soil moisture content below which
the plant roots cannot withdraw water
from the soil.
Some defined it to be the soil moisture
content under a suction of 15 bars.
The available water capacity of a soil is
the difference between the field
capacity and the wilting point.
Fig. 6.5 Hypothetical
fluctuation of soil
moisture for a sandy
loam soil through an
annual cycle in a region
with a moderate
amount of rainfall (500
to 750 mm per year)
and heavy rains in the
spring
Fig. 6.6 Water-holding properties of soils based on
texture. The available water supply for a soil is the
difference between field capacity and wilting point
Water Potential
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The potential energy, or force potential
of ground water consists of two parts:
elevation and pressure (velocity related
kinetic energy is neglected)
Suction of Water in Soils
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Fluid pressures in the vadose zone are
negative, owing to tension of the
soil-surface-water contact
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The negative pressure head is
measured in the field with a
tensiometer
d
b
∆z1
∆z2
Soil Surface
c
a
Suction – Calibrate Before Use
Suction
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Matric suction
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Elevation head
Pressure head
Osmotic suction
Head (Water Potential)
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Gravity potential, Z
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Elevation head
Moisture potential, ψ
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Suction head
Can be several orders of magnitude
greater than the gravity potential
1 bar ≈ 10 m of water column
Soil Water Characteristics
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Defines the relationship between water
content and water potential (suction)
0 0
0.1
0.2
0.3
0.4
-0.2
-0.4
-0.6
-0.8
-1.0
hm (m)
SWCC of a Coarse Sand
θ
SWCC of various soils
Soil Water Characteristic
Curve
Absorption and desorption characteristics with primary
scanning curves
-1 0 7
-1 0 6
-1 0
P o o r ly so r te d
5
-1 0 4
P ressu re h ead
( c m ) -1 0 3
W e ll so r te d
-1 0 2
Entry pressure hb
hb
-1 0
1
-1 0 0
0
0 .1
0 .2
0 .3
W a te r c o n te n t,θ
0 .4
Drying scanning curve
ψ
Main drying curve
Main wetting curve
Wetting scanning curve
Volumetric water content,θ
Absorption and desorption characteristics with primary
scanning curves
Hysteresis
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Ink bottle effect
Trapping of air
Advancing and receding contact angle
Determination of SWCC
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Laboratory
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Pressure plate apparatus
Filter paper
Thermocouple Psychrometer
Centrifuge
Field
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Tensiometer
Water content measurement
z (m)
soil
Ceramic
plate
0
-0.2
water
-0.4
Effluent port
that can vary
elevation
ψg= gz
-0.6
-0.8
H
-1.0
h
-1.2
-1.6 -1.2
-0.8 -0.4
0
0.4 heads (m)
Theory of Unsaturated Flow
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Gravity potential, Z
Moisture potential, ψ(θv)
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Negative value – suction resulted from soil-water
attraction
At moisture contents close to specific
retention, the gravity potential is greater
When the soil is very dry, the moisture
potential may be several orders of magnitude
greater than gravity potential
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Darcy’s law is valid for flow in the
unsaturated zone
Flow in water saturated pores
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Flow in pores with air in them
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Larger cross-sectional area to conduct flow
Smaller cross-sectional area to conduct
flow
Total potential φ
φ = ψ (θ v ) + Z
Fig. 6.7 The relationship between hydraulic
conductivity and volumetric water content
-300
-250
-200
Drying
ψ -150
cm water
-100
W etting
-50
0
1
2
3
Hydraulic conductivity (10 -4
4
Relationship between hydraulic conductivity and soilmoisture head
Fig. 6.9 Idealized curves showing relationships of
volumetric water content, hydraulic conductivity, and soilmoisture head
Fig. 6.10 Moisture
profiles showing the
downward passage
of a wave of
infiltrated water. The
soil is saturated at a
water content of 0.29
and has a field
capacity water
content of 0.06
Coarse vs. Fine-Grained
Materials
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At lower volumetric water content:
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Coarse material may have very few
saturated pores
Fine-grained soils may have most of the
pores still saturated
Thus, the unsaturated hydraulic
conductivity of a clay may be greater than
that of a sand or gravel
Fig. 6.11 Typical
soil-moisturepotential-hydraulic
conductivity curves
for a sandy soil
showing the
crossover effect for
increasing moisture
potential
Water-Table Recharge
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When the front of infiltrating water
reaches the capillary fringe, it displaces
air in the pore spaces and cause the
water table to rise.
Fig. 6.12 Hydrograph of a shallow well in a watertable aquifer in Long Island.
Rate of Recharge
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The rate of water-table recharge
depends on:
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Thickness of the unsaturated zone
The thinner the zone, the faster the rise of
water table
May generate a localized ground-water
mound
Fluctuation of Ground-Water
Table
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Water table shows a seasonal
fluctuation
Rising during periods of recharge
Falling when there is no precipitation or
when evapotranspiration exceeds
precipitation
Fig. 6.13 Monthly hydrograph of water levels in a watertable monitoring well on eastern Long Island, New York
Depth to water (feet)
Fig. 6.14 Hydrograph of a water-table monitoring well
showing effect of discharge by evaporation on the water
table elevation
Depth to water (feet)