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Outline. Announcements Where were we? Water retention curve Hysteresis. Soil Physics 2010. Announcements. Homework 3 is due now Slides or Blackboard? (blackboard was preferred) A brief run through problems 2 & 4. Soil Physics 2010. Homework problem 2. A. 5 cm water. B. 10 cm soil.
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Outline • Announcements • Where were we? • Water retention curve • Hysteresis Soil Physics 2010
Announcements • Homework 3 is due now • Slides or Blackboard? (blackboard was preferred) • A brief run through problems 2 & 4 Soil Physics 2010
Homework problem 2 A 5 cm water B 10 cm soil C 15 cm soil D Soil Physics 2010
Homework problem 2 A 5 cm water B 10 cm soil C 15 cm soil D Soil Physics 2010
Homework problem 4a Mass in air: 1.1 kg Density of iron: 5.5 kg / L Volume of iron = 0.2 L This is not the volume of the chunk! Soil Physics 2010
Homework problem 4 ? Soil Physics 2010
The hard way (part 1) Find Volchunk Soil Physics 2010
The hard way (part 2) f = 1/3 or 33.3% Soil Physics 2010
The easy way Buoyancy for iron only (f = 0): 1.1 kg – 0.9 kg = 0.2 kg Actual buoyancy: 1.1 kg – 0.8 kg = 0.3 kg Volchunk = 1.5 x Voliron so f = 1/3 Soil Physics 2010
Where were we? Water retention curve Basic idea: If the soil were a bunch of capillary tubes, we could figure out everything about how water and air move in it… …if we also knew the size distribution of those capillary tubes. The water retention curve is our best estimate of the soil’s pore size distribution. Soil Physics 2010
With that warning, let’s look at water retention Start with a soil core that’s saturated: Atmospheric pressure Known height Known dry mass Known porosity q = f So we know the water’s potential everywhere Soil Physics 2010
So we know the water’s potential everywhere L (0) 0 Atmospheric pressure 5 Known height L If it can drain out the bottom, then q0 < f, and mean h0= L/2 At saturation: q = f h = 0 Soil Physics 2010
Then I talked about sponges Soil Physics 2010
We pull lightly on the water L/2 • new points: • h1 = Dh1 + L/2 • q1 = f – • (Swater drained/ Vol) Dh1 Soil Physics 2010
Repeat with a bigger Dh Dh2 > h1 • new points: • h2 = Dh2 (+ L/2) • q2= f – • (Swater drained/ Vol) L/2 Soil Physics 2010
Plot the points Suction Potential, h, tension, etc Water content Wetness, q, etc Soil Physics 2010
Plot the points Suction Potential, h, tension, etc Water content Wetness, q, etc Soil Physics 2010
Why use this one? Suction Potential, h, tension, etc Height Water content Wetness, q, etc Soil Physics 2010
Different regions Dry Suction Potential, h, tension, etc Middle Wet Water content Wetness, q, etc Soil Physics 2010
Wet region Pore only drains if: Big enough Not isolated Air can get to it h Air entry Air access Structural pores Wet q Soil Physics 2010
A model porous medium being drained Drainage allowed: Pore radius: Big Small Soil Physics 2010
A model porous medium being drained Drainage allowed: Pore radius: Big Small Soil Physics 2010
A model porous medium being drained Drainage allowed: Pore radius: Big Small Soil Physics 2010
A model porous medium being drained Drainage allowed: Pore radius: Big Small Soil Physics 2010
A model porous medium being drained Drainage allowed: Pore radius: Big Small Soil Physics 2010
Wet region Pore only drains if: Big enough Not isolated Air can get to it h Air entry Air access Structural pores Wet When wetting, air entrapment limits the final q < f q Soil Physics 2010
Middle region Air and water are both continuous Reasonable reflection of pore size distribution Mixed textural & structural pores at wetter part Textural pores at drier part Hysteresis Middle h q Soil Physics 2010
Dry region Most water is in films sorbed to solid surface Water retention mostly determined by surface area Little or no hysteresis (if at equilibrium) Water flow in films is very slow q → 0 as h → ∞ (for example, drying at 105° for 24 hrs) Dry h q Soil Physics 2010
Hysteresis • Thermostats • Speedboats • “Ink bottle” pores History-dependent or direction-depedent Soil Physics 2010
