Soil%20Water%20Balance

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Soil Water Balance

• Reading Applied Hydrology Sections 4.3 and 4.4
• Topics
• Soil water properties
• Soil water flux
• Soil water balance

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Subsurface water

• Infiltration
• Soil moisture
• Subsurface flow
• Groundwater flow

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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.

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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

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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 upto 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
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Soil Particle Sizes(USDA Soil Classification
System
Table 1. Size limits (diameter in millimeters) of
soil separates in the USDA soil textural
classification system.
Name of soil separate Diameter limits (mm)
Very coarse sand 2.00 - 1.00
Coarse sand 1.00 - 0.50
Medium sand 0.50 - 0.25
Fine sand 0.25 - 0.10
Very fine sand 0.10 - 0.05
Silt 0.05 - 0.002
Clay less than 0.002

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
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Soil Water Flux, q
q Q/A
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Energy fluxes at Freeman Ranch
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Evaporation (mm/day)
November
Average 1.1 mm/day
August
Average 3.1 mm/day
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Daily Rainfall Aug 6 Aug 13
30 min Rainfall August 6, 2004
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Soil Water Content
Rainfall mm/30min
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Soil Water Balance at Freeman Ranch
E
P
0 cm
Net Exchange
10 cm
20 cm
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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
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Continuity (Cont.)
Continuity Equation
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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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Capillary Action

• Capillary Pressure
• Related to saturation
• Suction Head
• Total Head

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Pressure Distribution in Subsurface
Clay
Sand
Top of capillary fringe
Water table z 0, P0
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Richards Equation

• Recall
• Darcys Law
• Total head
• So Darcy becomes
• Continuity becomes

Soil water diffusivity