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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

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soil water
Soil Water
  • Reading: Applied Hydrology Sections 4.1 and 4.2
  • Topics
    • Soil water properties
    • Soil water measurement
    • Soil water balance
subsurface water
Subsurface water
  • Infiltration
  • Soil moisture
  • Subsurface flow
  • Groundwater flow
porous medium 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
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 water5
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 (>31 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 > 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.
soil sieves
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.

Soil Particle Sizes(USDA Soil Classification System

1 mm

0.1 mm

0.01 mm

.

0.001 mm

slide8

http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdfhttp://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf

soil texture triangle
Soil Texture Triangle

Source: USDA Soil

Survey Manual Chapter 3

soil water content
Soil Water Content

Soil Water Content

soil water tension y
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

z=0

z1

q12

z2

slide13

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
Soil Water MeasurementNeutron Attenuation

Measures Soil Water Content, θ

slide15

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
slide16

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)

soil water tension y18
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

z=0

z1

q12

z2

darcy s law
Darcy’s Law
  • K = hydraulic conductivity
  • q = specific discharge
  • V = q/n = average velocity through the area
definitions
Definitions

Element of soil, V

(Saturated)

Pore with

water

solid

Pore with

air

Element of soil, V

(Unsaturated)

continuity cont
Continuity (Cont.)

Continuity

Equation

surface tension
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)

air

Interface

Net forceinward

water

No net force

richard s equation
Richard’s Equation
  • Recall
    • Darcy’s Law
    • Total head
  • So Darcy becomes
  • Richard’s eqn is:

Soil water diffusivity