4. Soil Water Flashcards
clay soil porosity is about __%
25
5 Main Properties of a soil
• Particle size distribution • Pore size distribution • Moisture content – gravimetric – volumetric – saturated or unsaturated • Porosity – m3 of pore space/m3 of soil or % •Dry bulk density – kg m-3 for dry soi
Forces on water in soil
1.• Gravitational force – Direction • Downward – Effect • Drains the soil
2.• Water retention forces
– Capillary forces
– Adsorption forces
.water retention forces= exerts tension/suction on the water which moves water from wet to dry soil
.soil water has potential energy, but it varies
-water will flow from high to low potential energy
-called “soil water potential”
Effect • Keeps water in the soil • Exerts tension or suction on water – matric suction • Water flows from wet to dry soil – up, down, sideways
Angle between hydrogen atoms on a water moleculeis ___ degrees
105
total soil water potential =
Above ground water table pressure potential is called=
Below ground water table pressure potential is called=
total soil water potential = gravitational potential + pressure potential
- Above ground water table – Pressure potential is called matric potential
- Below ground water table – Pressure potential is called piezometric potential
- Potential energy per unit mass=
- Potential energy per unit volume=
- Potential energy per unit weight=
• Potential energy per unit mass
– J kg-1
-Jis Joule which is a unitof energy
• Potential energy per unit volume
– J m-3 = N m m-3 = N m-2
-Nm is a nutonmeter
• Potential energy per unit weight
– J N-1 = N m N-1 = m
-expressed in length
hydraulic head=
_____+______
gravitational head+pressure head
–Joule is the unit of energy
- Above ground water table – Pressure head is called matric head
- Below ground water table: – Pressure head is called piezometric head
.Soil water potential expressed as a _______ is =Force per unit area N m-2
Nm=Nutonmeter
.Soil water potential per unit weight is expressed as a ______
pressure
length
“head”means
- “head” means energy potential per unit length(m)
- so we are working in Soil water potential expressed per unit weight
Gravitational Head
• Always the same • Reference level – Soil surface – hg = 0 m • hg decreases downward – 1 m per m
- 1D=1 dimensional/directional
- anytime we draw a soil profile, put 0 at the top with depth increasing downwards
Pressure Head at equilibrium conditions
- pressure head varies with wetness/dryness of the soil
- so we have to discuss it within “equilibrium conditions”
- in equilibrium conditions we know exactly what will happen
Equilibrium Conditons mean
• Equilibrium conditions
– No evaporation
– No precipitation
– Capillary rise completed
AND
Hydraulic head stays constant with depth
-no waterflow
Pressure Head at equilibrium conditions
– At groundwater table: hp = 0 m – Above groundwater table • hm < 0 m • Decreases upward • Rate: 1 m per m – Below groundwater table • hp > 0 m • Increases downward • Rate: 1 m per
.pressure head will be 0 at the groundwater table no matter the conditions(never changes)
.above the groundwater table, the pressure head is always negative
-decreases upward at equilibrium conditions
-rate 1 m per m
.pressure head below the groundwater table will always be positive
m
Steps in building soil water diagram
Step 1: find water table depth and surface
Step 2: find gravitational head, start at 0 and decreases at 1m per m
Step 3: find pressure head ,start at 0
Step 4: compare hm at equilibrium with actual hm
To figure out which way the water is flowing we should be looking at the _________ head
To figure out which way the water is flowing we should be looking at the HYDRAULIC HEAD
- look at green curve and notice that it goes down
- closer to 0 is bigger on the negative side
- if it moves negatively away from 0 then the water is going up
- if it moves positively towards 0 then the water is going down
capillary fringe
.capillary fringe: zone where all pore space is saturated and is above water table
- matric head is negative, under tension which must be overcome to access the water
- decreases at a rate of 1m per m as we move towards the surface
unsaturated zone
.Unsaturated zone: in this zone, moisture content decreases as we go towards the surface
Matric Suction:
is the absolute value of the Matric head whch is always negative, i.e matric suction is positive
pF
.pF is the 10 base log of matric suction so 104 m has a pF of 4
any soil has a moisture content of between __-__% VOLUMETRIC WATER CONTENT at saturation(0)
40-55%
On a log scale there is no __ on the y-axis
0
__cm matric suction is found 1 cm above the water table
1
If the y axis is pf instead, the bottom of the y axis would be __ because the 10 log base of 1 is __
0
If talking pf, matric head and matric suction must be in __
cm
.Permanent wilting point: plants can no longer suck water out of soil(water is held to tight)
-happens at a pF of ___for any soil and any plants
-at a pF of ___ the soil is at field capacity
.Permanent wilting point: plants can no longer suck water out of soil(water is held to tight)
- happens at a pF of 4.2 for any soil and any plants
- knowing this, we can check to see at what volumetric moisture content this occurs at by looking at the curve
- at a pF of 2.0 the soil is at field capacity
Volumetric moisture content=
_______-______
field capacoty-permanent wilting point
-water that is actually available to the plant
Field capacity-wilting point= % amount of water available
there is always still water available in the soil for plants after there wilting point but they cannot access it because of..
tension is too strong for plants to access
Hystersis
.graphs before have not taken into account hysteresis
-hysterisis takes into account real world inputs rather then 1 to 1
-have two main curves, one for when the soil is drying, and one for when the soil is wetting
.by drawing horizontal line across from the Y axis, we can determine that soil stays wetter for longer then soil stays drier.
What is a 1-1 relationship and why does hystersis show different results?
Why isn’t there a 1-1 relationship?
.1-1 relationship between matric suction and moisture content
-but in real world the curved surface(meniscus) effects
-as the water table goes down and the pore begins to dry out, the meniscus is dropping but has very high curvature from adhesions
-as the water table goes up the the curvature is very low
.real pores are not tubes, so pores shapes hold water when drying because of wet soil
-when wetting, moisture content in soil is much dryer so it wont be able to pull the water in as far/well
Darcy’s Law
FOUND IN TEXT EQUATION 4.13
-used to calculate hydraulic head
q is the flux (m s-1 or cm day-1) K( y ) is the hydraulic conductivity (m s-1 or cm day-1) H is the difference in hydraulic head between two points (m) l is the distance between the twopoints (m)
- saturated sand has very high conductivity
- saturated clay has very low conductivity
- ALWAYS REPORT WATER FLUX AS A POSITIVE NUMBER
- (-) is just there to make water move from low to high
Continuity Equation
Moisture content/time= -(flux in-flux out)/change in distance
Continuity Equation:
.picture a block of soil, 1 square meter block of soil a few cm thick
-a flow of water flowing from left to right through the soil (qin->qout)
-if the flow/flux in is the same as the flow out then the soil moisture is constant
-if the flow/flux in is more then the flow/flux out then the soil moisture increases
-if the flow/flux is less then the flow/flux out then the soil moisture decreases
ONLY __________ PORES CONTRIBUTE TO HYDRAULIC CONDUCTIVITY
water filled
Hydraulic Conductivity
•Measure of how much water will flow if the hydraulic head gradient is 1 m m-1 •Function of the moisture content – K( ) •Function of the matric head – K( y ) • Only water-filled pores contribute to the hydraulic conductivity
