Soil Water

Question 1

Definition of cohesion

Answer 1

Attraction of water molecules for each other

Question 2

Definition of adhesion

Answer 2

Attraction of water molecules for solid surfaces

Question 3

What is surface tension

Answer 3

Occurs at liquid air interfaces and results from greater attraction of water molecules for each other than for air above

Causes water to behave as if it’s surface were covered with a stretched elastic membrane

Question 4

Formula for gravimetric

Answer 4

Moist weight - oven dry weight/ oven dry weight

Question 5

Formula for volumetric water content

Answer 5

Volume of water / total oven dry volume

Question 6

Definition of free energy

Answer 6

Energy available to do work

Question 7

Soil energy water concept: how does water move

Answer 7

Water moves from high free energy ( wet soil) zone to a low free energy zone ( dry soil)

Question 8

What are the forces affecting free energy

Answer 8

Gravity: free energy of soil water at a given elevation is higher than that of pure water at a lower elevation

Matric force: due to adhesion ( attraction of soil solids for water) -> reduces free energy of water molecules

Osmotic force: due to attraction of ions + solutes for water -> reduces free energy

Question 9

Soil water potential

Answer 9

Gravitational potential - due to force of gravity ( increases free energy of water)

Matric potential - due to adhesion + capillary ( reduced free energy of water )

Osmotic potential - solutes reduce free energy of water

Question 10

Soil water potential and soil water content

Answer 10

Water in wet soils has more energy than in dry soils

At set water potential, clay holds more water

At set water content, water held tightly by clay

Question 11

What are the different types of soil water movement

Answer 11

Saturated flow: micro and macropores are completely filled with water but soil is freely drained ( E.G: during heavy rainfall)

Unsaturated flow: when larger pores are filled with water

Vapor movement: when vapor differenced develop in relatively dry soils

Question 12

What is tortuosity

Answer 12

The paths that soil pores provide for fluid flow are not straight, but tortuous and meandering

Question 13

Describe the unsaturated flow in soils

Answer 13

Macropores filled with air

The unsaturated flow is governed by:
- matric potential gradient = difference in matric potential of moist and dry soil areas
- not gravitational potential
- hydraulic conductivity: a soil property
- decreases as water content decreases

Question 14

How do you measure soil water potential

Answer 14

Tensiometer: water drawn from fine porous cup into soil until equilibrium is reached

Gypsum blocks
Pressure plates