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A single-valued water-retention curve assumes that pressure head determines water content uniquely. Real soils can follow different paths while drying and wetting. Entrapped air, pore geometry and contact-angle effects create hysteresis, while reversals generate scanning curves inside the main boundary loop. The current state therefore depends partly on how the soil arrived there.
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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. Define the state and its hydraulic history
Let θ denote volumetric water content and h pressure head. In a non-hysteretic model, one θ(h) relation is used. In a hysteretic model, main drying and main wetting curves bound a family of scanning paths followed after reversals.
The variables, sign convention and reference state must be stated. A pressure head of −100 cm on a drying path need not carry the same water content or conductivity as −100 cm on a wetting path.
CHAPTER 2 OF 12 · DEFINE THE DECISION
2. Link the loop to pore-scale mechanisms
Ink-bottle geometry allows a pore body to empty through a narrow throat at one suction and refill at another. Advancing and receding contact angles differ, and wetting can trap air that drying had displaced.
These mechanisms need not contribute equally in every soil. Structure, texture, organic matter, compaction, air access and cycle amplitude affect the observed loop; hysteresis parameters are model summaries rather than unique pore diagnoses.
CHAPTER 3 OF 12 · WORK THE LEDGERS
3. Work an original same-head ledger
Use an illustrative soil whose main drying water content at h=−100 cm is 0.30, main wetting water content is 0.24 and an intermediate scanning path gives 0.27. The head is identical, but hydraulic history changes storage.
| Path at h=−100 cm | Water content θ | Recent history | Safe reading |
|---|---|---|---|
| Main drying | 0.30 | Drained from wetter state | Upper boundary value |
| Scanning drying | 0.27 | Rewetted, then reversed | Interior path |
| Main wetting | 0.24 | Wetted from drier state | Lower boundary value |
| Single-curve model | One chosen value | History erased | Possible storage error |
CHAPTER 4 OF 12 · WORK THE LEDGERS
4. Construct scanning curves after reversals
When drying reverses to wetting before reaching the main drying endpoint, the state follows a wetting scanning curve toward the main wetting boundary. A later reversal generates a drying scanning curve from that new reversal point.
A model needs rules for reversal points, curve shape and closure. Numerical implementations may scale scanning curves from boundary curves; the exact rule must be reported because it controls memory and can produce different storage changes.
CHAPTER 5 OF 12 · WORK THE LEDGERS
5. Change the path and keep the endpoints
Compare two experiments that both end at h=−100 cm. Experiment A dries monotonically from saturation. Experiment B dries to −300 cm, rewets to −50 cm and dries again to −100 cm. The final head matches, but θ and entrapped-air history can differ.
| Experiment | Head sequence | Final head | Expected path |
|---|---|---|---|
| A | 0 → −100 cm | −100 cm | Main or primary drying |
| B | 0 → −300 → −50 → −100 cm | −100 cm | Nested scanning path |
| Endpoint-only record | Only −100 cm stored | −100 cm | Cannot recover history |
| Full record | Heads plus reversals | −100 cm | Path can be modelled |
CHAPTER 6 OF 12 · TRANSFER THE METHOD
6. Carry hysteresis into conductivity and flow
Hydraulic conductivity can also be path dependent because connected water-filled pathways differ between wetting and drying. Using only a hysteretic retention curve with a non-hysteretic conductivity relation may still miss observed flux behaviour.
The practical effect depends on the forcing. Strong infiltration-drainage cycles, irrigation pulses and shallow fluctuating water tables may expose it more than nearly monotonic drying over a narrow range.
CHAPTER 7 OF 12 · TRANSFER THE METHOD
7. Connect the established soil-water routes
The Richards-equation guide shows where storage and conductivity enter the forward balance. The hydraulic-conductivity guide separates conductivity from gradient assumptions.
Use the inverse-modelling guide for parameter correlations and experiment design, and the dual-permeability guide when preferential domains, rather than hysteresis alone, control the response.
CHAPTER 8 OF 12 · TRANSFER THE METHOD
8. Design measurements that retain memory
Record continuous or sufficiently dense pairs of h and θ through complete wetting, drying and reversal sequences. Synchronise sensors, document temperature and density corrections, and distinguish equilibrium steps from dynamic lag.
Replicate cycles to see whether the loop stabilises or changes through swelling, aggregate breakdown or trapped-air redistribution. A loop measured in a repacked core may not transfer directly to field structure.
CHAPTER 9 OF 12 · CHECK LIMITS AND EVIDENCE
9. Know the limits and common misconceptions
Every wetting–drying difference is not hydraulic hysteresis. Sensor lag, non-equilibrium, temperature effects, spatial mismatch and evaporation can create apparent loops. Conversely, fitting one retention curve to both branches can hide systematic residuals.
Scanning curves are not arbitrary lines between boundary curves. Their geometry follows a chosen hysteresis model and reversal bookkeeping. More parameters can improve fit while weakening identifiability.
CHAPTER 10 OF 12 · CHECK LIMITS AND EVIDENCE
10. Use an evidence and source routine
Record soil structure and preparation, initial saturation, h and θ conventions, boundary wetting and drying data, reversal points, scanning rule, entrapped-air treatment, conductivity hysteresis, equilibration time, sensor response, cycle repeatability, parameter uncertainty and validation under changed forcing.
Šimůnek and colleagues’ USDA-hosted study estimates hysteresis in soil water retention from permeameter data. The HYDRUS-1D technical manual documents boundary wetting and drying curves and scaled scanning-curve assumptions.
CHAPTER 11 OF 12 · ASK AND CONTINUE
11. Frequently asked questions
Can I use one θ(h) curve for all simulations?
Sometimes as an approximation, but test it when the forcing includes reversals and storage or flux errors matter.
Does the wetting curve always lie below the drying curve?
That is common under a stated convention and range, but verify the data, definitions and model rather than assuming a universal ordering.
Is a larger loop always more realistic?
No. Loop size must be supported by measurements and must transfer to changed cycles without unstable reversal behaviour.
CHAPTER 12 OF 12 · ASK AND CONTINUE
12. Continue with a reversal-history transfer check
Create two head sequences with the same final h but different reversal points. Sketch the main wetting and drying boundaries, place each reversal and trace the scanning paths. Then identify which observations would distinguish hysteresis from sensor lag.
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