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)
Movement of water in the unsaturated zone
- 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:
- q = -K * dh/dz = -K * d(z+Ψ)/dz
= -K * d(z + p/(ρg))/dz
- K: hydraulic conductivity, function of saturation (Fig 8.5)
- 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
Resources
- Basic
Hydrogeology
- Dingman, S.L. (1994) Physical Hydrology. Prentice Hall,
Englewood Cliffs, 575pp.