Hydrology BC ENV 3025

Water in the unsaturated zone

Definitions

experiment: wetting soil in a beaker, what makes the grains stick to each other?
the zone between the ground surface and the water table where pore spaces of the rock or soil may be partly filled with air and partly with water is referred to as the unsaturated zone or vadose zone and water in this zone is referred to as soil moisture (Fig)
the capillary fringe is a saturated zone above the water table where water is affected by capillary forces
distribution of moisture in the vadose zone (Fig 8.1)
grain sizes in soils (and thus pore sizes) vary over several orders of magnitude (Fig)
look at sand gauges
definitions of porosity, field capacity, specific retention, specific yield (Fig)
porosity, field capacity, and wilting points for typical soils (Fig)

the potential in the unsaturated and saturated zone are similar: h = z + p/(ρg) = z + Ψ
this pressure is negative in unsaturated soils and is often called pressure, suction or tension head (Ψ), Ψ is given as "cm water column"
Water pressure within capillary tubes is less than atmospheric pressure
Ψ can be measured with a tensiometer (Fig)
Ψ as a function of moisture content for a fine sand (Fig 8.4)(Fig)
soil water status as a function of pressure (tension) (Fig)
driving forces for flow of water in the unsaturated zone are: gravity and adhesion/cohesion
Darcy's law describes the flow in porous media:

pressure head and elevation as a function of depth in saturated and unsaturated zone under no flow conditions (=> Ψ = -z) (Fig 8.8)
wetting fronts
A Capillary Barrier

Basic Hydrogeology
Dingman, S.L. (1994) Physical Hydrology. Prentice Hall, Englewood Cliffs, 575pp.