Lec 11: Soil water & groundwater 1 Flashcards
Name the 8 components of hydrological cycle diagrams
- precipitation
- evaporation
- storage
- runoff
- river discharge
- interception
- transpiration
- evapotranspiration
Define precipitation
water condensed in clouds – which is too heavy to remain suspended in the air – that falls on land as rain, sleet, hail or snow
Define evaporation
Evaporation: liquid water in lakes, oceans, rivers and soils heated by the sun
that is converted to a gas (i.e., water vapor) and returned to the atmosphere
Define storage
- Storage: water held in contained areas (e.g., in soil as soil moisture, in snow and
ice caps, in depressions as depression storage) for a given amount of time
Define runoff
Runoff: water that moves away from a terrestrial location, via surface and/or
subsurface flow processes
Define river discharge
River discharge: flow of water within a river channel
Define interception
Interception: water that is prevented from reaching the ground by plant leaves
and branches
* Transpiration: liquid water from plant leaves and stems that is converted to
water vapor and returned to the atmosphere
* Evapotranspiration: combination of evaporation and transpiration
Define transpiration
- Transpiration: liquid water from plant leaves and stems that is converted to
water vapor and returned to the atmosphere
Define evapotranspiration
Evapotranspiration: combination of evaporation and transpiration
Define water table
Water table = subsurface boundary below which the ground is completely
saturated with water
Define percolation
water that moves vertically through the soil toward the water
table
Define capillary fringe
Capillary fringe: saturated zone above the water table, in which groundwater
seeps upward due to capillary forces
Define groundwater flow
Groundwater flow: water that is flowing below the water table
What happens after infiltration?
Water is stored in shallow soil layers as soil moisture
Water is redistributed vertically and laterally after infiltration
* Vertically: water percolates downward until the saturated zone (groundwater recharge)
* Laterally: governed by principles of unsaturated flow (or soil water flow) or
groundwater flow
What are horizons?
What are the 3 major horizons of most soils?
Distinct layers of a soil profile
differentiated by:
- color
- texture
- organic matter
- the degree of deposition (illuviation) or removal (eluviation) of material by physical and chemical processes
Surface horizon, subsoil, and substratum
What are the hydrologic horizons in the shallow subsurface?
root zone
intermediate zone
tension-saturated zone (capillary fringe)
ground water
impermeable layer
Why is soil referred to as porous?
because it is permeated by an interconnected network of
pores (i.e., voids) filled with fluids (either
liquid or gas).
what does soil texture describe?
Soil texture is a term used to describe the
grain size distribution (by weight) for particles
of less than 2 mm (upper boundary for sand)
What determines the size of soil pores?
grain size
soil is a mixture of grain sizes
rank the soil particle types (silt, sand, and clay) from smallest to largest particle size
clay (<0.002 mm)
silt (0.002-0.5 mm)
sand (0.05-2 mm)
Porosity is a function of…
Total porosity (n) is a function of…
soil texture
the volume of the voids and the total volume of soil
n = v(voids)/v(total)
v(voids) = v(water) + v(gas)
v(total) = v(voids) + v(solids)
What are the formulas for:
- soil porosity
- void ratio
- soil moisture content by volume and by mass
- degree of saturation
soil porosity (n) = v(voids)/v(total)
void ratio (e) = v(voids)/v(solids)
volumetric soil moisture (teta) = v(water)/v(total)
gravimetric soil moisture (G) = ((mass wet soil - mass dry soil)/mass dry soil)
degree of saturation (s) = v(water)/((vwater)+(vgas))
What determines
the maximum amount
of space available in
the soil for water to be
retained/stored?
porosity
What is soil profile according to hydrologists?
from highest to lowest
*vadose zone (unsaturated zone)
*capillary fringe
zone just above water table where water is drawn up by capillary action
*water table
upper limit of saturated zone
*phreatic zone (saturated zone)
Define capillary action
Which two forces make capillary action possible?
When does capillary action occur?
Capillary action is the process of a liquid
moving in a narrow space, without the
assistance of (and sometimes even
against) gravity
cohesion (water molecules like to stay close together) and adhesion (water molecules are attracted to and stick to different materials)
occurs when the
adhesion forces of water to a solid
material are greater than the cohesive
forces between the liquid water
molecules
What are the scientific terms for:
excess water
water held in pores
thin films of water
When do each of these occur?
excess water = gravitational water (occurs at saturation
water held in pores = capillary water (occurs in intermediate to wet conditions)
thin films of water = hygroscopic water (occurs in dry conditions)
Define field capacity
The field capacity (FC) of a soil: maximum amount of water that a soil can hold after
gravitational drainage losses
in relation to field capacity, when does subsurface flow occur?
Subsurface flow rarely occurs unless soil moisture content exceeds field capacity
Define the permanent wilting point (WPW) of a soil
soil moisture content produced
after gravitational drainage + plant evapotranspiration losses
‒ Always lower than field capacity
‒ Corresponds to water content at which plants can no longer extract water from
the soil
Define plant available water
the difference between field capacity and the permanent
wilting point
For which grain size is plant available water highest?
Highest plant available
water occurs in intermediate grain sizes.
Define soil water pressure and tension
*Water pressure is a force per unit area
applied by water in all directions
‒ It is conventionally measured relative to
atmospheric pressure
- Soil water tension is a measure of how
tightly water is held to soil particles
‒ It is expressed as a negative pressure
Soil water tension is the minimum
pressure that must be exerted to extract
water from soil
Define soil water pressure at different hydrological horizons
vadose zone: soil water pressure < atmospheric pressure
water table: soil water pressure = atmospheric pressure
saturated zone: soil water pressure > atmospheric pressure
What is the relationship between soil water pressure and tension?
Describe soil water pressure in unsaturated vs saturated soils
Tension is negative pressure
unsaturated: P<1
water table: P = 0
saturated P>1
What does a tensiometer measure?
How does it work?
for measuring soil suction in moist settings
Tube is partly filled
with water, low-permeability porous cup at bottom allows pressure
equilibration with soil, pressure gauge measures suction (i.e., pore pressure
relative to atmosphere)
How does water move in the subsurface?
A water pressure gradient promotes the migration of water:
‒ from higher pressure (e.g., where water is less tightly bound to the soil grains, so
low tension)
‒ to lower pressure (e.g., where water is held more tightly, so high tension)
Define hydraulic conductivity
Rank silt, sand, clay, and gravel from highest to lowest K
Hydraulic conductivity K describes the ease with which water can move through
pore spaces or fractures in soils
gravel, sand, silt, clay
