Subsurface Water

1
Subsurface Water

• unit volume of subsurface consists of soil/rock,
  and pores which may be filled with water and/or
  air
• total porosity volume voids/total volume
• water contentvolume water/total volume
• saturationvolume water/volume voids
• degree of saturation delineates various zones of
  subsurface water

2
Definitions

• soil water - Ground surface to bottom of root
  zone depth depends on soil type and vegetation.
  May become saturated during periods of rainfall
  otherwise unsaturated (soil pores partially
  filled with air). Plants extract water from this
  zone. Evaporation occurs from this zone.
• intermediate vadose zone - Between soil water
  zone and capillary fringe. Unsaturated except
  during extreme precipitation events. Depth of
  zone may range from centimeters to 100s of
  meters.

3
Definitions Continued

• capillary zone - Above saturated zone. Water
  rises into this zone as a result of capillary
  force. Depth of this zone is a function of the
  soil type. Fractions of a meter for sands (mm)
  to meters for fine clays. All pores filled with
  H2O, p lt 0. Effect seen if place bottom of dry
  porous media (soil or sponge) into water. Water
  will be drawn up into media to a height above
  water where soil suction and gravity forces are
  equal.
• saturated zone - All pores filled with water, p gt
  0. Formations in this zone with ability to
  transmit water are called aquifers.

4
Unsaturated Zone

• Water can exist in all its phases in the
  unsaturated zone.
• Liquid water occurs as
• hygroscopic water - adsorbed from air by
  molecular interaction (H-bonds)
• capillary water - held by surface tension due to
  viscosity of liquid
• gravitational water-water in unsaturated zone in
  excess of field capacity which percolates
  downward due to gravity ultimately reaching
  saturated zone as recharge.

5
Unsaturated Zone

• Hygroscopic and capillary waters are held by
  molecular electrostatic forces (between polar
  bonds and particles -- surface tension) in thin
  films around soil particles ? drier soil, smaller
  pores ? hygroscopic and capillary forces
• Hygroscopic water - held at -31 to -10,000 bars.
  Water is unavailable to plants or for recharge to
  groundwater.
• Capillary water - Held at -0.33 to -31 bars.
  More water filling pores but discontinuous except
  in capillary fringe. This water can be used by
  plants.

6
Definitions

• Permanent wilting point tension (suction,
  negative pressure) below which plant root system
  cannot extract water. Depends on soil and type
  of vegetation. Typically -15 bars (-15x105 Pa,
  -15000cm
• Field capacity tension (suction, negative
  pressure) below which water cannot be drained by
  gravity (due to capillary and hygroscopic forces)
  Depends on soil type. Typically about -0.33
  bars

7
Typical Moisture Profiles

• rain after a long dry period

moisture content
root zone
direction of moisture movement
depth
wilting point
field capacity
hygroscopic
saturation
8
Typical Moisture Profiles

• Drying process

moisture
1 - Drying in upper layers by ET. 2 - Bottom part
of wetting front continues ?. Upper part
continues to dry. 3 - At some point ? and ?
movement results in no moisture gradient 4 -
Dry front established. Lower zones are
being depleted to satisfy PET at surface.
Drying continues until capillary forces are
unable to move water to surface.
depth
field capacity
saturation
9
Dacry-Buckingham law

• Flow in unsaturated porous media governed by a
  modified Darcys law called Darcy-Buckingham law
  
• ? - suction head (capillary head) or negative
  pressure head. Energy possessed by the fluid due
  to soil suction forces. Suction head varies with
  moisture content, ? ? n, ?? 0, ? lt n , ? is
  negative.
• K(?) - hydraulic conductivity is a function of
  water content ? ?,? K(?) because more
  continuously connected pores, more space
  available for water to travel through, until at
  ? n, K(n) Ksat

10
Measuring Soil Suction

• Soil Suction (?) head measured with tensiometers,
  an airtight ceramic cup and tube containing
  water.
• Soil tension measured as vacuum in tubes created
  when water drawn out of tube into soil. Comes to
  equilibrium at soil tension value.
• Tensiometers often used to schedule irrigation.

11
Estimating water flux from tensiometer
measurements
h z ? h1 100 cm - 65 cm 35 cm h2 50 cm
- 50 cm 0 cm
h1 z1 ?1
?1 (negative) (-65 cm)
z1 100 cm
h2 z2 ?2
?2 (negative) (-50 cm)
z2 50 cm
z 0
12

• Look at components of flux
• Capillary gradient cause upward flow,
  gravitational gradient causes downward flow. Net
  flux is down.
• What would it take to get net flux upward?