1. Soil Water Balance Reading: Applied Hydrology Sections 4.3 and 4.4
Topics
Soil water properties
Soil water flux
Soil water balance

2. Subsurface water Infiltration Soil moisture Subsurface flow
Groundwater flow

3. Porous Medium Flow Groundwater Porous media
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 Saturated zone Water table
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 Field capacity 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.

6. Soil Sieves

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.
Soil Particle Sizes (USDA Soil Classification System
Name of soil separate
Diameter limits (mm)
Very coarse sand*
Coarse sand
Medium sand
Fine sand
Very fine sand
Silt
Clay
less than 0.002
1 mm
0.1 mm
0.01 mm .
0.001 mm

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. Energy fluxes at Freeman Ranch

13. Evaporation (mm/day) November Average = 1.1 mm/day August

14. Daily Rainfall
Aug 6 – Aug 13
30 min Rainfall
August 6, 2004

15. Soil Water Content
Rainfall
mm/30min

16. Soil Water Balance at Freeman RanchP E
0 cm
Net Exchange
10 cm
20 cm

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
z=0 z1
q12 z2

21. Darcy’s 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
air
Element of soil, V
(Unsaturated)

23. Continuity Equation z dx dy 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 = N/m 20oC)
Interface
air
Net force inward
water
No net force

26. Capillary Action Capillary Pressure Related to saturation Suction Head
Total Head
Solid
Water
Air R

27. Pressure Distribution in Subsurface
Clay
Sand
Top of
capillary fringe
Water table
z = 0, P=0

28. Richard’s Equation Recall So Darcy becomes Continuity becomes
Darcy’s Law
Total head
So Darcy becomes
Continuity becomes
Soil water diffusivity