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Agronomy / Soil Physics

Soil Moisture Tensiometer & van Genuchten Model Simulator

Compute volumetric water content, field capacity, wilting point, available water content and plant-available depth in real time from the matric potential ψ measured by a tensiometer, through the van Genuchten model. Switch soil type and crop to see when to irrigate and how much.

Parameters
Soil type
Sets van Genuchten parameters θ_s, θ_r, α and n
Matric potential ψ
hPa
1 hPa ≈ 1 cm H₂O. 33 kPa = field capacity, 1500 kPa = wilting point
Crop
Sets the irrigation-trigger threshold
Root zone depth
cm
Vegetables 20-40 cm, orchards 80-150 cm
Irrigation amount
mm
Amount applied per event (rainfall-equivalent mm)
Results
—
Water content θ (m³/m³)
—
Field capacity FC (m³/m³)
—
Wilting point PWP (m³/m³)
—
Available water AWC (m³/m³)
—
Plant-available depth (mm)
—
Irrigation decision
—
Soil profile & tensiometer diagram

Soil profile, root zone and tensiometer (ceramic cup + vacuum gauge). Colour intensity shows water content, the needle points to the current ψ. The trigger lamp lights when ψ exceeds the crop threshold.

Soil water retention curve SWRC θ(ψ)
AWC comparison by soil type
Theory & Key Formulas

$$\theta(\psi) = \theta_r + \frac{\theta_s - \theta_r}{[1 + (\alpha|\psi|)^n]^{1-1/n}},\quad AWC = \theta_{FC} - \theta_{WP}$$

θ_s = saturated water content, θ_r = residual content, α and n = soil-specific parameters, FC at 33 kPa, WP at 1500 kPa. The van Genuchten (1980) equation.

$$D_{\text{avail}} = (\theta - \theta_{WP})\cdot Z_r,\quad D_{FC} = \theta_{FC}\cdot Z_r$$

Plant-available depth D_avail and field-capacity depth D_FC; Z_r is root depth. Convert with m³/m³ × cm × 10 = mm.

$$\text{Irrigate if } \psi \gt \psi_{\text{threshold}}(\text{crop})$$

Crop-specific irrigation thresholds (leafy 25 kPa, cereal 50 kPa, orchard 70 kPa, turf 20 kPa, dryland 150 kPa). 1 kPa = 10 hPa.

Soil moisture tensiometer and the van Genuchten model — irrigation design

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The tensiometer stuck in a field — what does it actually measure? Just the water content?
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Good question. It is not measuring "how much water" but "how strongly water is being held". The technical name is matric potential ψ. The drier the soil, the harder it grips water, and the vacuum gauge inside the tensiometer reads a larger negative pressure. Units are kPa or cm H₂O. On the left, push ψ up and watch the water content θ drop on the right. ψ is what the plant actually feels, so irrigation control based on ψ is more rational than chasing absolute water content.
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OK. But why does the conversion from ψ to θ use that scary "van Genuchten" equation?
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Because in 1980 van Genuchten managed to capture the S-shaped soil water retention curve with a single closed-form expression, and the agronomy world adopted it. In the formula α is the ψ scale at which the soil starts to release water, and n is how steep the curve is — basically how uniform the grain size is. Sand has uniform big pores, so α is large and n is large: steep curve. Clay has tiny pores of all sizes: small α, small n, gentle curve. Try switching the soil type to "Clay" — at the same ψ = 100 hPa the water content stays much higher; clay simply refuses to give water back easily.
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You're right, clay barely drops. So what are FC, PWP and AWC?
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These three are the bread and butter of irrigation design. Field capacity (FC) is the upper limit the soil holds once gravity has finished draining — conventionally θ at ψ = 33 kPa. Permanent wilting point (PWP) is the lower limit below which plants cannot extract water any more, at ψ = 1500 kPa. The difference AWC = FC − PWP is the "tank" the plant can actually use. Sand has FC ≈ 0.068 and PWP ≈ 0.046, so AWC is only about 0.022 — about 9 mm in a 40 cm root zone. That tank empties in a day or two, which is why sandy fields are irrigated little and often.
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There is an "irrigation decision" indicator at the bottom that changes when I switch crops — what's behind it?
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Each crop has its own "irrigate when drier than this" threshold. Leafy vegetables wilt early, so irrigate at ψ ≈ 25 kPa. Cereals around 50 kPa, orchards 70 kPa, and dryland crops like sorghum are intentionally pushed to 150 kPa to encourage deeper roots. So at ψ = 80 kPa the same field reads "irrigate" (red) for lettuce but "hold" (green) for an orchard. That is exactly what modern smart-irrigation sensors from Decagon, Sentek and Stevens HydraProbe automate in the field.
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One more — how do I choose the irrigation amount? Too much or too little both feel wrong.
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The rule of thumb is "refill to FC". Compute the gap between the current plant-available depth and the FC depth, and apply just that. Anything above FC drains below the root zone with the fertiliser dissolved in it (deep percolation loss). Anything well below FC leaves the lower roots dry. The "Irrigation amount" slider lets you compare your real application with the gap to FC — matching them is the starting point of deficit irrigation scheduling.

Frequently asked questions

A tensiometer measures the soil matric potential ψ (a negative pressure). When the ceramic cup is buried, drier soil pulls water out of the cup more strongly, and the vacuum gauge inside reads that suction in kPa or cm H₂O. 1 kPa = 10 cm H₂O = 10 hPa. Small ψ (a few kPa) means wet soil; ψ ≈ 33 kPa is the field capacity reference and ψ ≈ 1500 kPa is the permanent wilting point. A standard tensiometer works from 0 to about 80 kPa — beyond that, air enters the cup and it must be re-primed.
The van Genuchten (1980) equation θ(ψ) = θ_r + (θ_s − θ_r)/[1 + (α|ψ|)^n]^(1−1/n) is the global standard for relating volumetric water content θ to matric potential ψ. θ_s is the saturated water content, θ_r the residual content, α the inverse of the air-entry pressure (the ψ scale at which the soil starts to release water), and n the steepness of the curve (uniformity of grain size). Sand has large α and large n (steep curve); clay has small α and small n (gentle curve). HYDRUS, DSSAT and AquaCrop all use this equation as a core component.
FC (θ at ψ = 33 kPa) is the maximum water the soil holds once gravity drainage stops, PWP (ψ = 1500 kPa) is the lower limit below which plants cannot extract water, and AWC = FC − PWP is the pool plants can actually use. In design, multiply AWC by the rooting depth to obtain a plant-available depth in mm (e.g. AWC = 0.15, Zr = 30 cm → 45 mm) and irrigate after 50-70% of it has been depleted. Sandy soils have small AWC and dry fast — irrigate little and often. Clay soils have larger AWC and tolerate larger but less frequent irrigations.
Crops differ in root water-uptake strength and in leaf transpiration demand. Leafy vegetables (lettuce, cabbage) have shallow roots and high transpiration, so they wilt easily and lose market value — irrigate early at ψ ≈ 20-25 kPa. Cereals (wheat, maize) sit around 50 kPa; orchards have deep roots and stronger uptake, tolerating up to 70 kPa. Dryland crops such as sorghum and sunflower are deliberately stressed to 150 kPa to drive deeper roots. The tool toggles this threshold by crop and shows an irrigation decision (required / hold).

Real-world applications

Protected horticulture (strawberries, tomatoes, leafy vegetables): Greenhouse production routinely couples tensiometers to solenoid valves and drip lines for fully automated, little-and-often irrigation triggered at ψ ≈ 20-30 kPa. Substrate-specific van Genuchten parameters for coir or peat are measured in advance and embedded into PLC or cloud platforms (e.g. e-kakashi, AKISAI) that close the irrigation loop.

Orchards and vineyards (precision irrigation): Californian and Chilean wine grape growers practise "Regulated Deficit Irrigation (RDI)", deliberately stressing vines to ψ = 50-80 kPa to concentrate sugars. Dozens of tensiometers per block are interpolated through the SWRC model and combined with satellite NDVI for whole-block management.

Paddy-to-upland conversion fields: When soybean or wheat follow rice on heavy clay, both matric potential and water table elevation must be monitored. Clay has FC ≈ 0.40 and PWP ≈ 0.20 — a large AWC that buffers droughts, but also a high risk of waterlogging. Tensiometers and piezometers used together watch for both extremes.

Soil physics research and modelling: HYDRUS-1D/2D, DSSAT, AquaCrop and SWAP all embed the van Genuchten equation (and the related Mualem-van Genuchten hydraulic conductivity model). Fitting α and n from laboratory or pressure-plate SWRC measurements is the first step in any serious model calibration.

Common misconceptions and pitfalls

The first trap is treating the tensiometer reading as a water content. As this tool makes explicit, converting ψ to θ requires the van Genuchten parameters (α, n, θ_s, θ_r) and these vary wildly between soils — sand and clay can differ by a factor of three to five at the same ψ. Generic statements like "30 kPa means about X% water" are unsafe; either measure the SWRC of the actual field or use a Pedotransfer Function (e.g. ROSETTA) based on the soil texture.

Second, do not assume that a tensiometer reading above 80 kPa means a broken sensor. That is just cavitation: air has entered the ceramic cup and the unit needs re-priming. If that happens frequently, the soil simply spends too much time outside the instrument's range. For the dry side (80-1500 kPa) switch to granular matrix sensors (Watermark) or TDR/FDR probes (Decagon EC-5, Aquaspy). No single sensor covers the full range, so combining instruments is standard practice.

Third, over-irrigation is never "playing it safe". Anything above FC drains below the root zone with nitrate and other nutrients dissolved in it, which is a leading cause of groundwater contamination and eutrophication of closed water bodies. Both the EU Nitrates Directive and Japan's groundwater nitrate standards target this loss. Keep the "Irrigation amount" slider at or below the gap to FC, and use rainfall forecasts to pre-empt irrigation — an ESG-relevant practice as well as an agronomic one.

How to Use

  1. Enter matric potential in hPa (range: -10 to -1500 hPa) measured from your tensiometer at root zone depth.
  2. Input root depth in cm and irrigation depth in mm to define the active soil profile.
  3. The simulator applies van Genuchten parameters (typically θs=0.45, θr=0.08 for loam) to compute volumetric water content, then calculates field capacity (-33 hPa), permanent wilting point (-1500 hPa), and available water capacity (AWC = FC − PWP).
  4. Review plant-available depth (mm) and irrigation decision: if current AWC < 50% threshold, irrigate to field capacity.

Worked Example

Loam soil with matric potential −80 hPa at 45 cm root depth, 40 mm irrigation depth. Van Genuchten model yields θ at −80 hPa ≈ 0.28 m³/m³. Field capacity (−33 hPa) = 0.32 m³/m³; PWP (−1500 hPa) = 0.10 m³/m³; AWC = 0.22 m³/m³. Plant-available depth = 0.22 × 450 mm = 99 mm. Current moisture at −80 hPa represents 63% depletion; irrigation decision recommends 35 mm application to restore field capacity.

Practical Notes

  1. Tensiometer readings >−30 hPa indicate near-saturation; drainage may be impeded in clay loam (Ks <10 mm/day).
  2. For sandy loam (θs=0.40, θr=0.05), AWC ≈ 0.18 m³/m³; schedule irrigation when matric potential drops below −60 hPa to avoid plant stress in water-sensitive crops (lettuce, carrot).
  3. In structured clay soils, macropore bypass can cause matric potential to remain >−100 hPa despite volumetric saturation; combine tensiometer data with soil water characteristic curves from lab consolidation tests.