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Soil Water

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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 ... – PowerPoint PPT presentation

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Title: 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.

6
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

8
http//www.uga.edu/srel/kidsdoscience/soils-planet
s/soil-particle-size.pdf
9
Soil Texture Triangle
Source USDA Soil Survey Manual Chapter 3
10
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
13
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

14
Soil Water MeasurementNeutron Attenuation
Measures Soil Water Content, ?
15
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

16
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)
17
Electrical Resistance Blocks Meters
18
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
19
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20
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21
Darcys Law
  • K hydraulic conductivity
  • q specific discharge
  • V q/n average velocity through the area

22
Definitions
Element of soil, V (Saturated)
Pore with water
solid
Pore with air
Element of soil, V (Unsaturated)
23
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
26
Richards Equation
  • Recall
  • Darcys Law
  • Total head
  • So Darcy becomes
  • Richards eqn is

Soil water diffusivity
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