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
