5. Soil water & groundwater Flashcards
Soil horizon
Layer typically parallel to the soil surface with physical characteristics that are different from those of the layers above and below
What is the soil profile made up of?
Sequences of soil horizons
What are soil horizons delimited by? (5)
- color
- texture
- organic matter
- degree deposition (illuviation)
- removal (eluviation) of material by physical and chemical processes
What is the development of soil profiles dependent on? (5)
- climate
- topography
- disturbances (erosion, deposition)
- parent material
- time elapsed since weathering
Five hydrologic soil horizons in the shallow subsurface
- Root zone
- Intermediate zone
- Capillary fringe (tension-saturated zone)
- Ground water
- Impermeable layer
Which zones are unsaturated?
The intermediate zone, and the capillary fringe
Which zones are saturated?
Ground water
What is the capillary layer?
- a sub zone
- just above the water table
- it is a belt of variable thickness
- water is drawn by capillary action
What happens after infiltration? (2)
- Water is stored in shallow soil layers
- Water is redistributed:
→ vertically: after infiltration, water percolates downward until the saturated zone
→ laterally: governed by principles of unsaturated flow or groundwater flow
Infiltration
movement of water from the soil surface downward
Percolation
downward movement of water in the unsaturated zone
Recharge
addition of water to the saturated zone (groundwater recharge)
Capillary rise
movement of water from the saturate zone upwards into the unsaturated zone
Interflow or subsurface flow
flow occurring in the unsaturated zone in the lateral direction
Groundwater flow
flow occurring in the saturated zone
soil def
- a mixture of mineral grains, air, water, and organic matter
- it is porous
Size classification of sand clay and silt (smallest to largest)
clay < silt < sand
Total porosity (n)
n = V voids / V total
where
- V voids = V water + V gas
- V total = V voids + V solid
Void ratio (e)
e = V voids / V solids
where
- V voids = V water + V gas
Volumetric soil moisture content (θ)
θ = V water / V total
Gravimetric soil moisture (G)
G = ( M wet soil - M dry soil ) / M dry soil
Volumetric soil moisture content (θ)
θ = V water / V total
or
G = ρb / ρw * G
where ρ is bulk density of soil and water density
Degree of saturation (S)
S = V water / ( V water + V gas )
What does soil porosity (n) determine?
Soil porosity determines how much is available in the soil for water to be retained/stored
3 different types of soil water
- gravitational water
- capillary water
- hygroscopic water
Gravitational water
Excess water that drains under the influence of gravity at saturation
Capillary water
Water that is held in soil pores in intermediate to wet conditions
Hygroscopic water
Thin films of water that hold onto soil grains in dry conditions
Field capacity (FC)
Maximum amount of water that a soil can hold after gravitational drainage
When does subsurface flow occur?
Usually when soil moisture content (θ) exceeds field capacity (FC)
Permanent wilting point (WPW)
Minimum soil moisture content produced after gravity drainage and plant evapotranspiration
Characteristics of permanent wilting point (WPW)
- always lower than FC
- corresponds to water content at which plants can no longer extract water from the soil
Plant available water (PAW)
The difference between field capacity (FC) and the permanent wilting point (WPW)
→ PAW = FC - WPW
Soil moisture (θ) in the root zone (as compared to θ PWP, θ FC and n)
θ PWP < θ < n
Soil moisture (θ) in the intermediate zone (as compared to θ PWP, θ FC and n)
θ FC < θ < n
Soil moisture (θ) in the capillary fringe (as compared to θ PWP, θ FC and n)
θ = n
Soil moisture (θ) in ground water (as compared to θ PWP, θ FC and n)
θ = n
How does water move in soils?
From higher pressure to lower pressure
What is the soil water pressure (P) at the water table?
P=0
Is the soil water pressure (P) positive or negative in saturated soils? (below the water table)
Soil water pressure is positive
Is the soil water pressure (P) positive or negative in unsaturated soils? (above the water table)
Soil water pressure is negative
What are other terms to call negative water pressure? (2)
Tension or suction
What is hydraulic conductivity?
Hydraulic conductivity (K) describes the ease with which water can move through pore spaces or fractures in soil
What has larger hydraulic conductivity K values? Sand or clay?
Sand has larger hydraulic conductivity values
What is infiltration?
Infiltration is the flow of water into the ground through the earth surface
What are the two ways in which we can “see” infiltration?
As a rate or a total. So in mm/hr or mm for example
What is the rainfall rate i ?
It is the rainfall intensity measured in units of length over time (e.g. mm/hr)
What is infiltration capacity fc?
It is the maximum rate at which soil can absorb water through its surface
What is infiltration rate f(t) ?
It is the time-variable rate of water entering the soil through the surface (measured in length over time)
What is cumulative infiltration F(t) ?
- The depth of infiltration from the beginning of rainfall to any time t.
- It is also the area below the infiltration rate curve
What is a wetting front?
A wetting front is present when the change of soil moisture content with depth is so great that there is a sharp discontinuity in the soil profile, between the wet soil above and the dry soil below
What is time of ponding H(t) ?
Time at which the surface soil layer becomes saturated after the rainfall rate exceeds the hydraulic conductivity
