soil physics 2010 outline announcements recharge soil structure (pores)
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Soil Physics 2010
Outline
• Announcements
• Recharge
• Soil Structure (pores)
Soil Physics 2010
Homework 5 is posted on the course website. There is a pdf, and an Excel spreadsheet.
• Homework 5 due Wednesday after Spring Break
• I can do a review session over Break if there is interest.
• Quiz!
Announcements
Soil Physics 2010
Quiz
1) Sketch a graph of infiltration rate over time, with infiltration limited by the precipitation rate early, and by the soil later. Label the axes.
time, T
i(t)
, L/T
Rainfall-limited
Soil-limited
Soil Physics 2010
Quiz
2) In the diagram below, water is flowing through the flexible tube.
(a) Describe the force(s) that cause the water to flow.
(b) Describe the force(s) that oppose the water flowing.
hhh
The matric potential in the soil pulls the water into the soil, so the water flows due to capillary forces.
In order for water to leave the Mariotte bottle, air must be pulled down into the watera distance h. Thisis buoyancy or gravitational force.
Soil Physics 2010
Philip model
Recall that horizontal infiltration can be modeled as a diffusion process, with x(t) t½
But for vertical infiltration, the gradient is always at least 1, so x(t) → Ks at large t.
So for vertical infiltration, the short-time infiltration rate is i(t) t½ , but the long-term behavior is more like i(t) t.
Soil Physics 2010
J. R. Philip on infiltration
Infinite series solution, with the first 2 terms dominating:
...22
3
24
3002 tA
tAtKA
t
sti
Kt
sti
2 KttsdttitI
Early time: 1st term dominatesLate time: 2nd term dominates
– conceptually like Green & Ampt
What’s this?
Soil Physics 2010
Illustrating Philip’s concept
Sorptivity is like suction at the wetting front.
s: sorptivity, with units L T-0.5 (like the Boltzmann variable B)
I(t)
t
Sorptivity mostly shows up in Australian research.
Soil Physics 2010
Illustrating Philip’s concept
Infiltration from a circular pond of zero depth:
• Capillary forces are small
• Gravity always dominates
• s is small
Coarse soil (e.g., sand)t = 0
t = 1
t = 2
t = 3
Soil Physics 2010
Illustrating Philip’s concept
Infiltration from a circular pond of zero depth:
• Capillary forces are large
• Gravity is important only as t → ∞
• s is large
Fine soil (e.g., clay)t = 0
t = 1t = 2
t = 3
Soil Physics 2010
Illustrating Philip’s concept
Infiltration from a circular pond of zero depth:
• Capillary forces dominate early
• Gravity more important over time
• s is intermediate
Medium soil (e.g., loam)t = 0
t = 1
t = 2
t = 3
Infiltration
Soil Physics 2010
Infiltration’s friends and relations
Precipitation
Percolation
Groundwater flow
Groundwater Recharge
Runoff
Soil Physics 2010
Infiltration’s friends and relations
Evaporation
Groundwater flow
Groundwater Recharge
Then the rain stops.
Baseflow
Redistribution
Soil Physics 2010
z
After the rain stops: Redistribution
Conservation of mass – blue area must remain constant
Surface soil changes from wettest to driest part of the profile with “new” water – even without evaporation.
For shallow wetting of the profile
Soil Physics 2010
After saturating a deep, fine-textured soil:
z
Lines represent time doubling: ½ day, 1 day, 2 days, 4 days, etc.
ktt i expFor t in days, k can be quite small, e.g. k ≈ 0.0001
Averaged over some depth, we can expect something like
Soil Physics 2010
After saturating a deep, coarse soil:
z
This thought-experiment is like the “long column” method for measuring K().
With several measurements in depth and time, and without evaporation, we could work out K() and h().
Recharge
Evaporation
Groundwater flow
Groundwater Recharge
Baseflow
Redistribution
Soil Physics 2010
courtesy of AJ
Soil Physics 2010
Soil Structure (Pores)
Pore structure is about Transport, so it can be useful to examine other
transportation networks.
Soil Physics 2010
Soil Structure (Pores)
Soil Physics 2010
Soil Structure (Pores)
A soil acquires structure (pores and particles both) over time.
How?
Why one structure versus another?