Soil Water

1
Soil Water

• Reading Applied Hydrology Sections 4.1 and 4.2
• Topics
• Soil water properties
• Soil water measurement
• Soil water balance

2
Subsurface water

• Infiltration
• Soil moisture
• Subsurface flow
• Groundwater flow

3
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.

4
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

5
Soil Water
Three categories

• Hygroscopic water
• Microscopic film of water surrounding soil
  particles
• Strong molecular attraction water cannot be
  removed by natural forces
• Adhesive forces (gt31 bars and up to 10,000 bars!)
• 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 gt plant
  extractive force
• 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.

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Soil Sieves
http//www.rtg.wa.edu.au/loanpool/belmont/sieves.j
pg
7
Soil Particle Sizes(USDA Soil Classification
System
Table 1. Size limits (diameter in millimeters) of
soil separates in the USDA soil textural
classification system.
1 mm
0.1 mm
0.01 mm
.
0.001 mm

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http//www.uga.edu/srel/kidsdoscience/soils-planet
s/soil-particle-size.pdf
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Soil Texture Triangle
Source USDA Soil Survey Manual Chapter 3
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Soil Water Content
Soil Water Content
11
Soil Water Flux, q
q Q/A
12
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, q
• Tension, y

Total energy head h
z0
z1
q12
z2
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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 soils dielectric constant is dependent on soil
  moisture
• Time domain reflectometry (TDR)
• Frequency domain reflectometry (FDR)
• Primarily used for research purposes at this time

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Soil Water MeasurementNeutron Attenuation
Measures Soil Water Content, ?
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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

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Tensiometer for Measuring Soil Water Potential, ?
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root
Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)
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Electrical Resistance Blocks Meters
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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, q
• Tension, y

Total energy head h
z0
z1
q12
z2
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Darcys Law

• K hydraulic conductivity
• q specific discharge
• V q/n average velocity through the area

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Definitions
Element of soil, V (Saturated)
Pore with water
solid
Pore with air
Element of soil, V (Unsaturated)
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Continuity Equation
z
dy
dx
dz
y
x
24
Continuity (Cont.)
Continuity Equation
25
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 (_at_ 20oC)

air
Interface
Net forceinward
water
No net force
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Richards Equation

• Recall
• Darcys Law
• Total head
• So Darcy becomes
• Richards eqn is

Soil water diffusivity