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.
What is the permanent wilting point?
Is the water content when evaporation and transpiration can’t remove any more water after the field capacity is reached, the evapotranspiration can’t overcome the forces holding the water to the soil grains. (symbol is 0pwp)
What is the available water content?
Is water available to plants and is the difference between field capacity and the permanent wilting point. eqn = 0a = 0fc - 0pwp
In the intermediate zone, what would the water content be at? Why?
At field capacity, this is because the excess water has drained out and after that the water that remains will be when the negative pressure head and the elevation head are at equilibrium. It won’t go lower than field capacity because there’s no roots to suck up water.
In the root zone what would the water content be?
It would be between field capacity and the permanent wilting point if no rain events.
What is the Leiden frost effect?
Is when water drops float on hot surfaces do to a water vapor forming beneath the drop, stops water from boiling away rapidly
What is infiltration?
Is the process by which water arrives at the surface as rain or snowmelt and enters the soil
What is the water input rate (w)?
rate at which water arrives at the soil surface (e.g. rain rate, snowmelt rate), ex rainfall intensity can be (mm/hr)
What is in infiltration rate (f)?
rate at which water enters the soil
What is the infiltration capacity? (f*)
its the maximum infiltration rate
What are the three general conditions in infiltration?
No ponding
Ponding (Saturation from above)
Ponding (saturation from below)
What is no ponding?
This is when the soil soaks up all the water that falls on it so the infiltration rate is the same as the water input rate, the maximum infiltration rate is not exceeded
What is ponding (saturation from above)?
The water input rate exceeds the infiltration maximum rate (infiltration capacity) so we get ponding above.
What is ponding (saturation from below)?
The water table has risen to the surface and there’s no infiltration at all
Infiltration rate _______ over time?
decreases
Why does infiltration rate decrease over time?
Because when it initially gets wet that adds pressure decreasing the tension (negative) pressure head, but the layers below the initial wet one still have negative tension head and differences in elevation head so it’ll move fast to them, as infiltration continues the soil will become saturated so there will be no differences in pressure head as everything saturated and only force of elevation head (gravity) keeps infiltration going which slows it down.
What are the four other controls (besides tension and soil pores clogging and reducing) that effects the infiltration rate? Explain them?
Organic surface layers (have high infiltration capacity, because it contains leaf litter and other junk that tends to have large openings)
Frost (doesn’t permit infiltration, but in permafrost where water content is low can increase)
Rain compaction (permits infiltration because the rain has caused no pores)
Chemical characteristics (if plants have coating which make them hydrophobic and are in soil can reduce infiltration)
What type of flow can ponding become?
Overland flow
How do we measure infiltration? What do we use?
A ring infiltrometer, have two rings and measure the decrease in water over time after filling the inside one
Why do we measure the inner ring when using a ring infiltrometer?
Make the measurements in the inner ring because the water flowing down the inside ring would flow out as dry soil will have strong tension and suck water out, so we get measurements that are too high. By making the ring you make sure the soil surrounding the measurement area is wet as well.
What’s the difference between the saturate front and wetting front? How does it interact with the water table?
The saturate front is when no more water can be put in, the wetting front is the downwards movement of water, if the wetting front hits the water table it’ll be recharged if the saturation front hits the water table then the whole column is saturated and infiltration stops.
What are the three ways groundwater can recharge?
Infiltration/percolation (dominant)
Surface water
Artificial
Why would we do artificial recharge?
Conserve and dispose of runoff/flood waters
Supplement natural recharge of groundwater
Reduce or balance salt water intrusion
Suppress ground subsidence
Store water in off-seasons for use during the growing seasons; can also reduce pumping
Remove suspended solids by filtration through the ground
