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
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
Three categories
• 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
Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system.
Name of soil separate
Diameter limits (mm)
Very coarse sand* 2.00 - 1.00
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 0.05 - 0.002
Clay less than 0.002
0.001 mm
0.1 mm
.
0.01 mm
1 mm
http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf
Soil Texture Triangle
Source: USDA SoilSurvey Manual Chapter 3
Soil Water Content
TotalVol
VolWater
Soil Water Content
Soil Water Flux, q
q = Q/A
Soil Water Tension,
• Measures the suction head of the soil water
• Like p/ in fluid mechanics but its always a suction (negative head)
• Three key variables in soil water movement– Flux, q– Water content, – Tension,
02
2
zg
vz
ph
Total energy head = h
111 zh
222 zh
z=0
z1
z2
12
1212 zz
hhKq
q12
• Neutron scattering (attenuation)Neutron scattering (attenuation)– Measures volumetric water content (Measures volumetric water content (vv))– Attenuation of high-energy neutrons by hydrogen nucleusAttenuation of high-energy neutrons by hydrogen nucleus– Advantages: Advantages:
• samples a relatively large soil sphere samples a relatively large soil sphere • repeatedly sample same site and several depths repeatedly sample same site and several depths • accurateaccurate
– Disadvantages: Disadvantages: • high cost instrument high cost instrument • radioactive licensing and safety radioactive licensing and safety • not reliable for shallow measurements near the soil surfacenot reliable for shallow measurements near the soil surface
• Dielectric constantDielectric constant– A soil’s dielectric constant is dependent on soil moistureA soil’s dielectric constant is dependent on soil moisture– Time domain reflectometry (TDR)Time domain reflectometry (TDR)– Frequency domain reflectometry (FDR)Frequency domain reflectometry (FDR)– Primarily used for research purposes at this timePrimarily used for research purposes at this time
Soil Water MeasurementSoil Water Measurement
Soil Water MeasurementSoil Water MeasurementNeutron AttenuationNeutron Attenuation
Measures Soil Water Content, θ
• 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
Soil Water MeasurementSoil Water Measurement
Tensiometer for Measuring Soil Water Potential, Tensiometer for Measuring Soil Water Potential, ψψ
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)Vacuum Gauge (0-100 centibar)
Water ReservoirWater Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
Electrical Resistance Blocks & MetersElectrical Resistance Blocks & Meters
Soil Water Tension,
• Measures the suction head of the soil water
• Like p/ in fluid mechanics but its always a suction (negative head)
• Three key variables in soil water movement– Flux, q– Water content, – Tension,
02
2
zg
vz
ph
Total energy head = h
111 zh
222 zh
z=0
z1
z2
12
1212 zz
hhKq
q12
Darcy’s Law
• K = hydraulic conductivity
• q = specific discharge
• V = q/n = average velocity through the area
L
hKAQ
z
hKqz
L
hhK
A
updown
Definitions
solid
Pore withair
Pore withwater
Element of soil, V(Saturated)
Element of soil, V(Unsaturated)
n0content;moisturenSV
V
S0;saturationV
VS
porosityV
Vn
waterofvolumeV
solidsofvolumeV
poresofvolumeV
elementofvolumegrossV
w
v
w
v
w
s
v
1
Continuity Equation
CS
wCV
wddt
ddAV0
waterofvolume
elementofvolume
dxdydzV
dxdydzV
w
y
z
x
dy
dz
dx
dzz
qq z
z
z
hK
A
Qqz
dt
ddxdydzdxdydz
dt
dd
dt
dww
CVw
z
qdzdxdyqdxdydxdydz
z
qq www
CSw
dAV
Continuity (Cont.)
CS
wCV
wddt
ddAV0
dt
ddxdydz
dxdydzdt
dd
dt
d
w
wCV
w
z
qdzdxdy
qdxdydxdydzz
w
wwCS
w
dAV
z
qdzdxdy
dt
ddxdydz ww
0 0
z
q
t
ContinuityEquation
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,
• = 0.073 N/m (@ 20oC)
water
air
No net force
Net forceinward
Interface
Richard’s Equation
• Recall – Darcy’s Law– Total head
• So Darcy becomes
• Richard’s eqn is:
z
hKqz
Kz
D
Kz
K
z
zKqz
KD
Soil water diffusivity
Kz
Dzz
q
t
Kz
Kqz
