Soil water Flashcards
What is the rate of water loss determined by?
Leaching, runoff, and evapotranspiration
How is the hydrogen of one water molecule bonded to the oxygen of another?
Hydrogen bonds
How is the hydrogen and oxygen bonded on the same molecule?
Covalent bonds
What is the difference between cohesion and adhesion?
Cohesion is the attraction of water molecules to each other. Adhesion is the attraction of water molecules to solid surface.
Is water potential always going to be a positive or negative value?
Negative
Where do water molecules form strongest bongs?
At the surface.
What is water potential?
The difference in potential energy between a given water sample and pure water.
What is matric potential?
Attraction of water to solid surface.
What is gravitational potential?
Force of gravity on soil water.
What is osmotic potential?
Presence of solutes dissolved in water.
What is pressure potential?
Weight of overlying water in saturated soils, only operational in saturated zones below the water table.
Where does water move from?
Water moves from high water potential (wet areas) to lower water potential (dry areas)
What kind of soil has highest water potential?
Clay
Describe the gravimetric method.
This method measures the soil water content in a standard method whereas the other methods are calibrated. It is does by placing the soil in an oven at 105 degrees for 24hrs and measuring the mass of the soil before being placed in the oven and after.
What are some disadvantages of the gravimetric method?
Destructive method, slow, hard to monitor temporal change.
What is a neutron scattering probe?
It is a device used to measure the number of neutrons scattered after contact with hydrogen molecules in the water.
What are some disadvantages of using the neutron scattering probe?
Radiation permit needed, expensive equipment, not good in high organic matter soils.
What is the dielectric method of measuring soil water and potential?
It measures soil moisture by sending an electromagnetic pulse along a probe instead into the soil. Speed of pulse is converted to the volume of water in soil. It is good for most typed of soils.
What is a disadvantage of the dielectric method?
Expensive equipment.
What equipment is used in the dielectric method?
Time-domain reflectrometry
How do capacitance sensors work?
They measure the rate of change of voltage along a thin metal to determine the dielectric constant of soil.
How does a tensiometer work to measure the soil water and water potential?
It measures the water potential.
What does the pressure membrane apparatus do?
It measures water potential. Accurate measurements of water content over a wide range of potentials in a short time. Used with gravimetric method to construct soil water characteristic curve.
What are the two types of water movement in soil?
- Infiltration
- Percolation
What is infiltration?
The process by which water on the ground surface enters the soil. Infiltration slows as soil becomes saturated. Sandy soils have a higher rate of infiltration.
What is Percolation?
Percolation is the downward movement of water in the soil profile mainly by gravity.
What is saturation?
The amount of water in the soil when both the macro and micropores are filled with water. The water potential is close to 0.
What is field capacity?
The amount of water in the soil after it has been saturated with water and the excess water has drained away. Air has moved into the macropores but the micropores are still full of water. Ideal for plants and aerobic microbial activity. Water potential is generally between -10 and -30 kPa
What is permanent wilting point?
Water content at which plants can no longer extract sufficient water from the soil for its growth. Water potential is about -1500 kPa.
What is the Hygroscopic coefficient?
Water is strongly held by soil particles by means of adhesion forces. Water potential is about -3100 kPa
What is available water holding capacity?
The capacity of soils to hold water available for use by most plants. It is commonly defined as the difference between the amount of soil water at field capacity and the amount at the wilting point.
What kind of soil has the greatest available water holding capacity?
Clay soils
