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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. Subsurface water. Infiltration Soil moisture Subsurface flow Groundwater flow. Porous Medium Flow. Groundwater

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soil water balance
Soil Water Balance
  • Reading: Applied Hydrology Sections 4.3 and 4.4
  • Topics
    • Soil water properties
    • Soil water flux
    • 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 water
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 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 > 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

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


soil texture triangle
Soil Texture Triangle

Source: USDA Soil

Survey Manual Chapter 3

soil water content
Soil Water Content

Soil Water Content

evaporation mm day
Evaporation (mm/day)


Average = 1.1 mm/day


Average = 3.1 mm/day


Daily Rainfall

Aug 6 – Aug 13

30 min Rainfall

August 6, 2004

soil water content15
Soil Water Content



soil water balance at freeman ranch
Soil Water Balance at Freeman Ranch



0 cm

Net Exchange

10 cm

20 cm

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





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

Element of soil, V


Pore with



Pore with


Element of soil, V


continuity cont
Continuity (Cont.)



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)



Net forceinward


No net force

capillary action






Capillary Action
  • Capillary Pressure
  • Related to saturation
  • Suction Head
  • Total Head
pressure distribution in subsurface
Pressure Distribution in Subsurface



Top of

capillary fringe

Water table

z = 0, P=0

richard s equation
Richard’s Equation
  • Recall
    • Darcy’s Law
    • Total head
  • So Darcy becomes
  • Continuity becomes

Soil water diffusivity