Soil Water
• Reading: Applied Hydrology Sections 4.1
and 4.2
• Topics
– Soil water properties
– Soil water measurement
– Soil water balance
Subsurface water
• Infiltration
• Soil moisture
• Subsurface
flow
• Groundwater
flow
Porous Medium Flow
• Groundwater
– All waters found beneath the ground surface
– Occupies pores (void space not occupied by solid matter)
• Porous media
– Numerous pores of small size
– Pores contain fluids (e.g., water and air)
– Pores act as conduits for flow of fluids
• The storage and flow through porous media is affected
by
– Type of rocks in a formation
– Number, size, and arrangement of pores
• Pores are generally irregular in shape because of
– differences in the minerals making up the rocks
– geologic processes experienced by them.
Zones of Saturation
• Unsaturated zone
– Zone between the land surface and
water table
– Pore contains water and air
– Also called as vadose zone or the zone
of aeration
• Saturated zone
– pores are completely filled with water
– Contains water at greater than
atmospheric pressure
– Also called phreatic zone
• Water table
– Surface where the pore water pressure is
atmospheric
– Divide between saturated and
unsaturated zone
• Capillary fringe
– Zone immediately above the water table
that gets saturated by capillary forces
Soil Water
Three categories
1. Hygroscopic water
–
–
–
Microscopic film of water surrounding soil particles
Strong molecular attraction; water cannot be removed by natural
forces
Adhesive forces (>31 bars and up to 10,000 bars!)
2. Capillary water
–
–
–
Water held by cohesive forces between films of hygroscopic water
Can be removed by air drying or plant absorption
Plants extract capillary water until the soil capillary force is equal to
the extractive force
•
Wilting point: soil capillary force > plant extractive force
3. Gravity water
–
•
Water that moves through the soil by the force of gravity
Field capacity
– Amount of water held in the soil after excess water has drained
is called the field capacity of the soil.
Soil Sieves
http://www.rtg.wa.edu.au/loanpool/belmont/sieves.jpg
Soil Particle Sizes
(USDA Soil Classification System
Name of soil
separate
Very coarse sand*
Diameter limits (mm)
2.00 - 1.00
1 mm
0.1 mm
0.01 mm
. 0.001 mm
Coarse sand
1.00 - 0.50
Medium sand
0.50 - 0.25
Fine sand
0.25 - 0.10
Very fine sand
0.10 - 0.05
Silt
Clay
0.05 - 0.002
less than 0.002
http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf
Soil Texture
Triangle
Source: USDA Soil
Survey Manual Chapter 3
Soil Water Content
VolWater
 
TotalVol
Soil Water Content
Soil Water Flux, q
q = Q/A
Soil Water Tension, y
• Measures the suction
head of the soil water
• Like p/g in fluid
mechanics but its
always a suction
(negative head)
• Three key variables in
soil water movement
– Flux, q
– Water content, 
– Tension, y
Total energy head = h
v2
h z
y  z  0
g
2g
p
h1  y 1  z1
h2  y 2  z2
h2  h1
q12   K
z 2  z1
z=0
z1
q12
z2
Soil Water Measurement
• Neutron scattering (attenuation)
– Measures volumetric water content (v)
– Attenuation of high-energy neutrons by hydrogen nucleus
– Advantages:
• samples a relatively large soil sphere
• repeatedly sample same site and several depths
• accurate
– Disadvantages:
• high cost instrument
• radioactive licensing and safety
• not reliable for shallow measurements near the soil surface
• Dielectric constant
–
–
–
–
A soil’s dielectric constant is dependent on soil moisture
Time domain reflectometry (TDR)
Frequency domain reflectometry (FDR)
Primarily used for research purposes at this time
Soil Water Measurement
Neutron Attenuation
Measures
Soil Water
Content, θ
Soil Water Measurement
• Tensiometers
– Measure soil water potential (tension)
– Practical operating range is about 0 to 0.75
bar of tension (this can be a limitation on
medium- and fine-textured soils)
• Electrical resistance blocks
– Measure soil water potential (tension)
– Tend to work better at higher tensions (lower
water contents)
• Thermal dissipation blocks
– Measure soil water potential (tension)
– Require individual calibration
Tensiometer for Measuring Soil Water Potential, ψ
Water Reservoir
Variable Tube Length (12 in- 48 in)
Based on Root Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)
Electrical Resistance Blocks & Meters
Soil Water Tension, y
• Measures the suction
head of the soil water
• Like p/g in fluid
mechanics but its
always a suction
(negative head)
• Three key variables in
soil water movement
– Flux, q
– Water content, 
– Tension, y
Total energy head = h
v2
h z
y  z  0
g
2g
p
h1  y 1  z1
h2  y 2  z2
h2  h1
q12   K
z 2  z1
z=0
z1
q12
z2
Darcy’s Law
• K = hydraulic conductivity
• q = specific discharge
• V = q/n = average velocity through the
area
Q   KA
h
L
hdown  hup
Q
q   K
A
L
q z  K
h
z
Definitions
V  gross volume of element
Vv  volume of pores
Element of soil, V
(Saturated)
Pore with
water
solid
Vs  volume of solids
Vw  volume of water
Vv
 porosity
V
V
S  w  saturation; 0  S  1
Vv
n

Vw
 nS  moisture content; 0    n
V
Pore with
air
Element of soil, V
(Unsaturated)
Continuity Equation
V  dxdydz  volume of element
qz 
z
Vw  dxdydz  volume of water
dx
q z
dz
z
dy
dz
d
0    w d    wV  dA
dt CV
CS
y
x
d
d
d



d




dxdydz


dxdydz
 w
w
w
dt CV
dt
dt

  wV  dA   w  q 
CS

qz 
Q
h
 K
A
z
q 
q
dz dxdy   w qdxdy   w dzdxdy
z 
z
Continuity (Cont.)
0
d
  w d    wV  dA
dt CV
CS
d
d
 wdxdydz

d


 w
dt CV
dt


V

dA


 w
wq 
CS
d
  w dxdydz
dt
d
q
0   w dxdydz
  w dzdxdy
dt
z

q 
dz dxdy   w qdxdy
z 
  w dzdxdy
q
z
 q

0
t z
Continuity
Equation
Surface Tension
• Below surface, forces act
equally in all directions
• At surface, some forces are
missing, pulls molecules
down and exert tension on
the surface
• If interface is curved, higher
pressure will exist on
concave side
• Pressure increase is
balanced by surface tension,
s
• s = 0.073 N/m (@ 20oC)
Interface
water
air
Net force
inward
No net force
Richard’s Equation
• Recall
– Darcy’s Law
– Total head
• So Darcy becomes
D K


Soil water diffusivity
• Richard’s eqn is:
h
z
h  z
q z  K
   z 
z
  

  K
K
  z

 

  D  K 
 z

q z  K
q z  K

q   

   D  K
t
z z  z


K
z