UNSATURATED ZONE HYDROLOGY (GEO4-4417)
Lecture 1: Course introduction
Unsaturated zone: part of the soil between
top of groundwater level and the surface =
critical zone: because a lot of the activities
that we do depend on this small layer
- Terrestrial ecosystems
- Crop growth
- Partioning of energy: dependent on
soil moisture
- Runoff generation: also dependent
on soil moisture
- Carbon sequestration
- Contamination
Lecture part 1: Soil water
The unsaturated zone is part of the soil situated between the
soil surface and the water table (phreatic level) where some of
the spaces between the soil particles are filled with air. The soil
consists of solid substances, the matrix, and interstices or the
pore space. As a complicating factor there is usually air in the
pores. Therefore, this zone is called the unsaturated zone,
unsaturated meaning not-saturated with water.
An alternative name for the unsaturated zone is the vadose
zone, or sometimes the zone of aeration, and the water stored
there is called soil water. Part of the unsaturated zone are:
- Root zone: part of the soil surrounding the roots of
plants
- Percolation zone: zone where water moves in the
direction of the water table
- Capillary fringe: the fringe above the water table, where
water is sucked into pores by capillary forces and/or
other forces
The vertical water flow at different depths in the unsaturated zone can be downward: infiltration
from precipitation, percolation and recharge of the groundwater, or upward: capillary rise, and
evaporation that includes transpiration. The unsaturated zone can be temporarily saturated, but
usually it’s not.
Soil water can be considered important in many ways:
- Land degradation processes at the surface: overland flow, erosion
- Land degradation processes below the surface: mass movement, availability of moisture and
growth of plants (natural vegetation, agriculture)
- Groundwater recharge or replenishment
- Protecting groundwater from pathogenic bacteria and viruses
,Soil components:
- Minerals: substances that are formed naturally in the earth, they are usually inorganic and
have a crystal structure.
o Coarse sand (200-2000 μm)
o Fine sand (50-200 μm)
o Silt (2-50 μm)
o Clay (<2 μm)
- Organic matter: composed of organic compounds that have come from remains of organisms,
such as plants and animals, and their waste products in the environment.
- Water: in pores
- Air: in pores
Porosity (n) is the measure of the pore spaces in a soil: it is the fraction of the volume of pore spaces
over the total soil volume, usually in cm3/cm3, thus dimensionless between 0 and 1, or as a
percentage between 0 and 100%. The porosity is the maximum volume fraction of water in a soil: it is
the maximum volumetric moisture content of a soil, and therefore also presented as 𝜃𝑠 ; the s stands
for saturation. There is a large range of soil porosity.
𝑛 = 𝜃𝑠 = 𝑣𝑜𝑙𝑢𝑚𝑒 𝑝𝑜𝑟𝑒𝑠/𝑡𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒
- Sandy soils: 0.37 (0.30-0.56)
- Silty soils: 0.45 (0.39-0.56)
- Loamy soils: 0.50 (0.30-0.55)
- Clayey soils: 0.53 (0.35-0.70): difficult because grow/shrink with water content
- Peaty soils: 0.80-0.85: difficult because of high porosity
The soil's ability to retain water is strongly related to particle size and pore size; water molecules hold
more tightly to the fine particles and pores in clay soil than to the coarser particles of a well-sorted
sand; so, clays generally retain more water. The average porosity for sand is lower than for clay. Sand
has large grains that are closely packed: and in a well-sorted sand, one predominant pore size is due
to the predominant grain size. Sand has a granular structure with much water held in pores between
the grains by capillary forces, whereas clay minerals have a laminated structure. Due to their mineral
structure, small sheets of silicon-oxide tetrahedrons and aluminium hydroxyl octahedrons, clay
minerals are negatively charged at their surface. The mineral’s negative charge is neutralized by
cations, positively charged ions in the soil water. Thin films of water and neutralizing cations adhere
to the clay minerals by electrostatic forces, and the same amount of water is held more strongly in
clay than in sand, which is also why a sand layer in the subsurface can be quite permeable, and clay
and mudstones are not; why it is easier to remove water from a sand than clay layer; and why sand
layers in the subsurface constitute the aquifers, and clay layers the aquitards.
Volumetric moisture content: θ is the fraction of the volume of moisture (water) over the total soil
volume, usually in cm3/cm3, thus dimensionless. It is usually determined from a stainless steel core
sample ring (Kopecky ring). Sampling volume of 100 cubic centimetres.
- Measure the soil sample mass 𝑚𝑤
- Saturate the sample and measure the mass 𝑚𝑠
- Dry 24 hours in the oven at 105 Celcius
- Measure the sample mass again: 𝑚𝑑
- V is the volume of the core sample ring: 𝜃 = (𝑚𝑤 − 𝑚𝑑 )/( 𝜌𝑤 𝑉). 𝜃𝑠 = (𝑚𝑠 − 𝑚𝑑 )/( 𝜌𝑤 𝑉).
,Moisture content characteristics:
- Volumetric moisture content: θ = volume water/total soil volume (cm3/cm3)
- Gravimetric moisture content or wetness w = mass water/mass solid matter (g/g)
- Relative moisture content or wetting phase saturation θ𝐸 = θ/n. (0-1)
- Dry bulk density: 𝜌𝑑 = mass of dry soil/total soil volume (g/ cm3)
Hydrostatics = study of forces in the soil-water system, when there is hydrostatic equilibrium. All
forces are in equilibrium: there is no water flow; the fluxes (rates) in the soi are zero; the moisture
content (state) does not change. There is no vertical water movement because of infiltration,
percolation or evaporation. The moisture content differs at different depths. Forces in the soil:
- Gravity
- Capillary: especially when soil is dry
- Adsorption: where soil water accumulates on solid soil surfaces forming a thin molecular film
of water, when the soil is dry
- Electrostatic: in clay
- Osmotic pressure: in saline soils
- etc
In soil water zone, water does not per definition have to flow from a wetter to a drier part of the soil.
This is because you also need to take the elevation differences into account.
Unifying concept: the concept of negative pore water pressures in the unsaturated zone when we
take the existing air pressure at the water table as a reference or zero level.
- Groundwater (saturated zone): Under
hydrostatic conditions (no vertical water
flow) and take the water table as a
reference level. Hydraulic head = elevation
+ pressure head. Horizontal axis: energy
level and the axis is positioned at the level
of the water table. Y-axis is elevation. When
descending into groundwater, elevation
head decreases by the same amount and
pressure head increases. The hydraulic head
saturated zone at all elevations equals zero,
and thus is independent of elevation. No groundwater flow in vertical direction
, - Unsaturated zone: hydrostatic equilibrium.
Zone above water table. Soil pores contain
water and air. The total energy, total potential,
equals the sum of the elevation head and
pressure head. We move upward from the
water table, than matric potential (= suction (-
matric) = negative pressure) is negative and
gravitation potential (elevation) is positive. NO
water flow: the total potential should remain
zero. Because the pores contain air, the total
potential cannot be as easily determined as
under the water table. Therefore not the
hydraulic head is used but the total (water) potential or hydraulic potential.
In specific instances, other potentials may also be important, such as the pneumatic
potential, when air is entrapped in the soil, or the envelope potential, caused by overburden
pressure from overlying soil. Also the osmotic potential may sometimes be important to
consider e.g. irrigation. Osmosis is a natural process that causes water to move from locations
with a low solute (ion) concentration to a location with a higher solute concentration. It is
negative (dry conditions draws water)
Choose reference level that is most handy. Sometimes land surface in stead of water level.
