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Water entering a dry or partly wet soil does not always push every air parcel neatly ahead of it. Menisci advance through irregular throats, reconnect around grains and can isolate gas in pores. Sudden capillary rearrangements appear as Haines jumps; trapped gas may compress, dissolve or persist. The result is a near-saturated soil that still contains gas and conducts water differently from an air-free sample.
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Chapter contents
Choose the route closest to the decision, or read from Chapter 1. The chapters remain open; these grouped routes are a navigable index.
Route 1 · Define the decision
Route 2 · Work the ledgers
Route 3 · Transfer the method
Route 4 · Check limits and evidence
Route 5 · Ask and continue
CHAPTER 1 OF 12 · DEFINE THE DECISION
1. Separate infiltration from complete saturation
Infiltration is water entry across a boundary. Saturation is a pore-volume state. A strong wetting front can pass while disconnected gas remains behind it, so reaching a high water content does not prove that all air has escaped.
Boundary conditions control the escape route. Open sidewalls, a vented top, upward wetting and slow application give air different pathways from a sealed, rapidly ponded column. Record them before interpreting a final water content.
CHAPTER 2 OF 12 · DEFINE THE DECISION
2. Follow menisci through pore bodies and throats
Capillary pressure depends on curvature, interfacial tension, contact angle and pore geometry. A meniscus can remain pinned at a throat while pressure changes, then move rapidly when the local entry condition is crossed.
Pore networks also allow snap-off and cooperative filling. One pathway can surround gas before that gas reaches a vent. Connectivity, not merely total porosity, decides whether the gas remains mobile.
CHAPTER 3 OF 12 · WORK THE LEDGERS
3. Work a pore-volume closure ledger
Consider a specimen with 500 mL of connected pore volume. It initially contains 100 mL water and 400 mL air. After infiltration, measured water storage is 420 mL and 300 mL of gas has left through the vent.
| Quantity | Initial | Measured change | Final or implied |
|---|---|---|---|
| Water in pores | 100 mL | +320 mL | 420 mL |
| Geometric gas space | 400 mL | −320 mL | 80 mL |
| Vented gas at reference conditions | 400 mL available | −300 mL | 100 mL implied |
| Closure difference | — | 20 mL | Check compression, dissolution and measurement |
CHAPTER 4 OF 12 · WORK THE LEDGERS
4. Recognise a Haines jump without overextending it
A Haines jump is a rapid interfacial rearrangement between capillary configurations, often recorded as a pressure drop or burst of pore filling. Classic observations emphasise drainage by a non-wetting phase, but analogous burst dynamics can occur during complex imbibition and gas redistribution.
Not every pressure fluctuation is a Haines event. Pump compliance, trapped tubing bubbles, sensor resonance and boundary breakthrough can create similar traces. Pair pressure with time-resolved imaging or local saturation when possible.
CHAPTER 5 OF 12 · WORK THE LEDGERS
5. Change the application rate and gas boundary
Faster water supply can compress air faster than it vents, raising gas pressure and diverting the wetting front. Slower supply may allow escape or dissolution, yet small throats can still isolate gas topologically.
| Change | Likely response | Alternative explanation | Measurement |
|---|---|---|---|
| Seal gas outlet | More compression | Apparatus leakage | Gas pressure |
| Increase flux | Earlier pressure rise | Dynamic capillarity | Flux and pressure |
| Degas inflow | Faster dissolution capacity | Temperature shift | Dissolved gas |
| Reverse wetting direction | Different escape paths | Gravity effect | Replicated imaging |
CHAPTER 6 OF 12 · TRANSFER THE METHOD
6. See how trapped gas alters hydraulic behaviour
Residual bubbles reduce the cross-section and connectivity available to water, so measured saturated or near-saturated hydraulic conductivity can remain below the air-free value. Compressible gas also adds storage and delayed pressure response.
Repeated wetting may dissolve some bubbles or mobilise them through coalescence, while drying can create a different gas topology. This history dependence contributes to hydraulic hysteresis and makes one endpoint insufficient.
CHAPTER 7 OF 12 · TRANSFER THE METHOD
7. Connect the established soil-water routes
The pressure-overshoot guide treats dynamic wetting fronts and fingers. The hysteresis guide tracks path-dependent retention.
Use the dynamic-capillarity guide for rate-dependent pressure and the preferential-flow guide when channels bypass the bulk matrix.
CHAPTER 8 OF 12 · TRANSFER THE METHOD
8. Design a defensible infiltration experiment
Measure bulk volume, porosity, initial water, inflow, outflow, specimen mass, vented gas, gas pressure and temperature. Add spatial water-content or imaging data so a volume balance can distinguish uniform filling from isolated clusters.
Run vented and sealed controls, more than one flux and a repeated cycle. Leak-test the apparatus and calibrate dead volumes. Report gas volumes at stated reference conditions and estimate dissolved gas rather than forcing every discrepancy into “entrapped air.”
CHAPTER 9 OF 12 · CHECK LIMITS AND EVIDENCE
9. Know the limits and common misconceptions
Residual gas saturation is not a universal soil constant. It depends on pore geometry, wettability, initial state, flow direction, rate, gas solubility, pressure history and boundary access.
A sample can look saturated by mass while still containing compressed microbubbles, and a conductivity plateau does not prove that gas topology stopped changing. Conversely, an isolated pore-scale jump does not determine field-scale infiltration by itself.
CHAPTER 10 OF 12 · CHECK LIMITS AND EVIDENCE
10. Use an evidence and source routine
Record soil structure, pore volume, wettability, initial saturation, fluid and gas composition, pressure and temperature, wetting direction, flux history, vent condition, sensor response, jump definition, imaging resolution, gas-volume correction and full water–gas closure.
Berg and colleagues use real-time three-dimensional imaging to resolve Haines jumps and fluid rearrangement. Tekseth and colleagues connect millisecond pore-filling events to multiscale drainage dynamics. A transient-flow tracer study examines how entrapped residual air bubbles affect transport in variably saturated porous media.
CHAPTER 11 OF 12 · ASK AND CONTINUE
11. Frequently asked questions
Why does air remain if water has crossed the sample?
Water can find a connected path while enclosing gas in side pores or disconnected clusters.
Does slower infiltration eliminate trapped air?
It can improve venting and dissolution, but geometry and wettability can still isolate gas.
Can pressure data alone identify Haines jumps?
Not reliably; apparatus dynamics and boundary events should be excluded with imaging or independent sensors.
CHAPTER 12 OF 12 · ASK AND CONTINUE
12. Continue with a closure transfer check
Repeat the ledger if only 260 mL of gas is measured at the vent while final water storage remains 420 mL. Calculate the new gas-balance mismatch. Then name three measurements needed before deciding whether the extra difference reflects compression, dissolution, leakage or a pore-volume estimate.
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