Teaching
Soil hydraulic curves — a hands-on classroom guide
A short introduction for students: what water-retention and conductivity curves are, what each model parameter does, and a guided exercise you can run live in the browser. No installation, no maths prerequisites beyond the lecture.
1What are we looking at?
Soil holds water in its pores. Two curves describe how it behaves as it dries:
- Water-retention curve — θ(h): how much water (θ) the soil holds at a given suction (h). Wet soil at low suction holds a lot; as suction rises the soil dries out.
- Hydraulic-conductivity curve — K(h): how easily water still moves as the soil dries. Conductivity drops fast — often by many orders of magnitude.
Suction is shown as pF = log₁₀(suction in cm of water). A few anchors students should know:
| pF | Suction | Meaning |
|---|---|---|
| ~1.8 | ~60 cm | field capacity (after drainage) |
| ~2.5 | ~330 cm | still plant-available |
| 4.2 | ~15 000 cm | permanent wilting point |
| 6.8 | — | oven-dry |
2What each parameter does (van Genuchten)
The most common retention model has just a few parameters. In the app each input shows its typical range on hover, so you can feel the magnitude:
| Symbol | Name | What changing it does |
|---|---|---|
| θs | saturated water content | height of the curve at wet end (total porosity). |
| θr | residual water content | the “floor” the curve flattens to when dry. |
| α | air-entry (1/cm) | where the curve “breaks” — bigger α = drains at lower suction (sandier). |
| n | pore-size uniformity | steepness — bigger n = sharper drop (uniform pores, sand); small n = gentle (clay). |
| Ks | saturated conductivity | starting height of the K curve. |
| τ | tortuosity | how fast K falls as the soil dries. |
Rule of thumb: sand = high α, high n (drains early and sharply); clay = low α, low n (holds water, drains slowly).
3Guided exercise — Learner mode
Open the app and turn on 🎓 Learner mode (under the model matrix). This reveals a soil-type preset menu and a ▸ Preview button that draws the curve straight from your parameter values — no data needed.
- Make sure the model is van Genuchten (m=1−1/n), unimodal, original.
- In Learner mode, pick Soil type → Sand. The parameters fill with typical sand values. Press ▸ Preview and look at the steep θ(h).
- Now pick Clay and Preview again. Notice the curve holds water far longer and falls gently. Why? (Hint: α and n.)
- Go back to your own numbers: set n = 1.2, Preview; then n = 3.0, Preview. Watch the steepness change.
- Change α from 0.01 to 0.15 and Preview between each. Watch the air-entry point slide.
- Switch the variant to PDI and Preview — see the dry end bend toward θ = 0 (adsorptive water).
Fit vs. guess. Upload measured data (or the example file), hand-tune the parameters until your Preview matches the dots, then press Fit. Compare your “eyeballed” numbers with the algorithm’s in the parameter table — a great way to build intuition.
4Worksheet (printable)
Soil texture & the retention curve
- Preview Sand, Loam, and Clay. Sketch the three θ(h) curves on one axis.
- Which soil has the highest plant-available water (θ between pF 1.8 and 4.2)? Read the values in the Statistical analysis box after fitting.
- Set α = 0.02 vs 0.10 (keep everything else). Describe in one sentence what α controls.
- Set n = 1.2 vs 2.5. Describe what n controls.
- For the conductivity curve K(h): which parameter sets where it starts, and which sets how fast it falls?
- Upload the example data and fit. Are your hand-tuned parameters within the algorithm’s 2.5–97.5% interval?
5Where to go next
- Full user manual — every model, metric, and option, with more worked examples.
- Try the other retention shapes (Brooks–Corey, Kosugi) and the Ctrl+click model comparison.
- Try bimodal on a structured soil to represent two pore systems.