Soil water infiltration Flashcards
How do we define water content in the saturated zone? The unsaturated zone?
The water content in the saturated zone would equal the porosity because all the pores are filled, in the unsaturated zone the water content would be the volume of water present over the toal soil volume.
What is a saturation term?
Is saturation = water content/porosity to see the porportion of pores that contain
In the unsaturated zone is water content less than the porosity? What is the saturation term in the unsaturated zone?
Yes, the saturation term is less than one.
In the unsaturated zone what happens to pressure head? In the saturated zone? At the water table?
In the unsaturated zone pressure head is less than atmopsheric pressure, in the saturated zone pressure head is higher as water is adding pressure, in the water table pressure head is equal to the atmospheric pressure
Why do we have pressure head become less than atmospheric pressure in the unsaturated zone?
capillary rise, the water goes up due to the charges of the water molecules and clings to pores in the soil pulling the other water molecules up, the smaller the pores the higher it goes up, the pressure is now negative as the water is under tension
What is a tensiometer?
Measures pressure head in the unsaturated zone
How does a tensiometer work?
You have a tube of water, at the bottom you have a permeable bulb that water can move through, you stick it in the soil and then since the pressure is negative (pressure head is less than atmospheric pressure) it will suck the water out of the tube and then the pressure of the tube will change and the bar gage on the tensiometer will tell how much negative pressure (tension) there is.
Pressure head will generally be more what in drier soils?
Negative
How does a negative pressure head in dry soils effect waters movement?
Since we want to go from high pressure head to low, water will move from wet soil to dry soil, as the the soil becomes more wet the movement of water slows
The drier the soil the more _______ the hydraulic head?
negative, because dry soil means lower pressure head, and lower pressure head means lower hydraulic head
Can we use Darcy’s law in the unsaturated zone?
Yes!
How does hydraulic conductivity (K) behave in the unsaturated zone?
The hydraulic conductivity increases with increasing water content
Given that the drier the soil the more negative the pressure head in the unsaturated zone, causing water to move upwards, but that the hydraulic conductivity decreases with decreasing water content, how does the water move up in the unsaturated zone?
It will intially move up but as the soil becomes drier and drier and the water content reduces, the hydraulic conductivity will decreases and the movement of water will become slower and slower.
What is field capacity? How does it come about?
Field capacity is when rain falls on the soil and then the soil gets saturated at the top, then since it’s saturated the pressure head becomes not negative as the pressure of water uptop combined with atmospheric pressure removes the tension, this then causes water to move just due to elevation head (water just moves from high elevation to low). The field capacity is the water content at which the drainage stops after a rain event. (symbol 0fc)
Why is the field capacity important? What does it signify about the relationship between the (tension) pressure head and the elevation head?
Drainage will only stop when the elevation head and (tension) pressure head are at equilibrium, so field capacity tells us the amount of water content that’s present for this to occur.
