Transcript Subsurface Water - University of Florida

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 content=volume water/total volume
• saturation=volume water/volume voids
• degree of saturation delineates various
zones of subsurface water
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.
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
< 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 > 0.
Formations in this zone with ability to transmit water are
called aquifers.
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.
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.
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
Typical Moisture Profiles
• rain after a long dry period
moisture content
root zone
direction of
moisture
movement
depth
wilting
point
hygroscopic
field
capacity
saturation
Typical Moisture Profiles
• Drying process
moisture
1 - Drying in upper layers by ET.
depth
field
capacity
saturation
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.
Dacry-Buckingham law
• Flow in unsaturated porous media governed by a modified
Darcy’s law called Darcy-Buckingham law :
h


qz  K 
z
hz
•  - 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,  < 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
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.
Estimating water flux from tensiometer
measurements
h1 = z1 + 1
1 (negative)
(-65 cm)
h2 = z2+ 2
z1 = 100 cm
z=0
z2 = 50 cm
2 (negative)
(-50 cm)
h=z+
h1 = 100 cm - 65 cm = 35 cm
h2 = 50 cm - 50 cm = 0 cm
 35cm  0cm
 h1  h2 
h
   K  
q z   K     K  
 100cm  50cm
z
 z1  z 2 

 35cm  0cm
 0.18cm / day
 100cm  50cm


  0.13cm / day






• Look at components of flux:
h h 
   z   2  z 2  
   2 
   K   1
  K  
q z   K   1 2    K   1 1
z1  z 2
 z1  z 2 


 z1  z 2 
  65  (50)
  K  
 100  50
  15
  K  
 50

  K  



  K  


• Capillary gradient cause upward flow, gravitational
gradient causes downward flow. Net flux is down.
•
What would it take to get net flux upward?