Soil water movement and plant available water Flashcards
3 types of water low
- saturated flow
- unsaturated flow
- vapor flow
Saturated flow
- all pores filled with water
- gravity is the driving force (hydraulic gradient)
- can occur both vertically and horizonally
Saturated flow occurs
After a rain fall, in poorly drained soil, when there is a restrictive layer
Darcys law
Governing equation that dictates flow in saturated conditions
J=Q/t=AKsat Change in water potetial/l
J= the flux of saturated flow
A= the cross section area
L= length of column
Ksat= hydraulic conductivity
Saturated Hydraulic conductivity
the ease with which pores of a saturated soil transmit water
More _____, higher Ksat greater rate of saturated flow
Macropores
Which soil texture will typically have the highest Ksat
Sand
Why does no till areas have higher hydraulic conductivity
More micropores and aggregate stability
Preferential flow
Water and its constituents moving by preferred pathways through a ports medium
flows through large micropores to ground water unrestricted
Unsaturated flow
Occurs when soils are not saturated
(large pores filled with water)
____ forces drives the movement of water in unsaturated conditions
Matric
External water vapor movement
movement from the soil surface to air
Internal water vapor movements
Movement between soil pores
Driving force
Vapor moves from areas of high pressure to areas of low vapor pressure
Infiltration
process by which water enters a soils pore space
percolation
process by which water moves through pore space
Infiltability
Rate at which water can enter the soil at the surface
Infiltrability equation
Ask professor
Infiltrability is ____ ______ over time
not constant
Infiltrability when soil is s shrinking clay
Infiltrability will be very high then sharply decline after pore swell
Percolation factors
can be both saturated
Wetting front
steep hydraulic gradient and forms a sharp boundary between wet and dry soil
Perched water tables
accumulation of groundwater that is above the water table in the unsaturated zone (above an impermeable layer)
What creates a perched water table on a slope?
springs or seeps
Why does perched water table occur above sand?
?? ask professor
Maximum retentive capacity (saturation)
All water pores are saturated
Zero water potential
0 kPa
Field capacity
Max amount of water soil can hold that is useful to plants
Little downward movement
micropores filled with air
-10 to -30 kPa
Permanent wilting point
Plants can no longer supply water
Water still in smallest pores
soil appears dusty
-1500 kPa
Hygroscopic water
water remaining in soil after soil has dried beyond wilting point
Available water holding capacity (AWHC)
Water that is held between field capacity and permeant wilting point
Factors affecting the amount of plant available ater
-matric potential
-osmotic potential
-soil depth and layering
Highest water holding capacity
Fine sandy loams, silt loams, silty clay loams
How do plants obtain water from soil
- Capillary movement of water to roots
- root extension into moist soil
Capillary movement of water to roots
- water uptake by roots creates areas of lower potential around root surfaces
- totally movement is usually a max of a few cm
-important during dry periods when root density is high
Limitation of root extension
small proportion of soil comes into contanct with roots
Layers that restrict roots
-bedrock
-compacted or dense zones
- plow pan
- Frangipani or dense glacial till
-cemented zones
- Abrupt change in texture
root depths
- very shallow <25 cm
- shallow 25-50 cm
- moderately deep 50-100 cm
- deep 100-150 cm
- very deep > 150 cm
