Five years of soil water potential data: what was learnt?

Over a half-decade of continuous monitoring, the study by researchers from Northern Arizona University, USA, provides a rare, high-temporal-resolution benchmark for root-zone soil water potential (ψ, in kPa). The study is of particular interest to Australian scientists because of its focus on semi-arid ecosystems and ecological regeneration. The study’s dataset, drawn from 126 soil water potential sensors, reveals nuanced temporal behaviour, treatment effects, and drought resilience signals that advance our empirical and conceptual understanding of soil–plant–atmosphere water relations in semi-arid forest ecosystems.

background and motivation

Soil water potential (sometimes called matric potential or soil suction) expresses the thermodynamic energy state of water in the unsaturated soil, determining how tightly water is held by the soil matrix and driving movement along energy gradients. While volumetric water content (θ) quantifies how much water is present, ψ tells us whether that water is energetically available to roots, and whether water will move into or out of soil zones under given gradients. This dual importance has long motivated efforts to measure ψ in situ, but high-quality, multi-year measurements remain relatively scarce, particularly across forested sites.

In Australia, there has been a long research focus on soil water potential dynamics from a range of applications from semi-arid ecosystems, mine-site rehabilitation, civil engineering, and hydrologic processes. Often, Australian scientists combine soil water potential with soil water content measurement to gain a complete understanding of the soil water characteristics or retention.

One challenge is that many traditional sensors (for example, tensiometers) cannot sustain long-term deployment in drier soils (cavitation) or over a wide dynamic range. More modern dielectric or ceramic matric sensors overcome some of these limitations, and the Tatum et al. study capitalizes on one such device, the TEROS 21 soil water potential sensor (METER Group). By embedding such sensors at 25–100 cm depth (the effective root zone), and capturing hourly data for 5 years post‐thinning, the authors ask: how do soil water potential trajectories differ under thinning vs. non-thinning? Do thinning treatments impart greater resilience to drought? And what can the fine-scale temporal patterns teach us about onset and duration of plant-stress thresholds?

the TEROS 21 sensor: how it works and strengths

The TEROS 21 soil water potential sensor is a dielectric (capacitive) matric potential sensor that couples a porous ceramic matrix to an electric measurement (essentially measuring dielectric permittivity of a ceramic disc in equilibrium with soil water) to infer water potential.

In practice, the sensor comprises a calibrated ceramic element whose moisture characteristic curve is known; as the ceramic equilibrates with the surrounding soil, its moisture content (hence dielectric permittivity) is read and converted to an estimate of soil water potential. The instrument also includes an integrated thermistor for temperature measurement, allowing compensations for thermal effects.

Notably, the TEROS 21 is factory-calibrated (six-point calibration), packaged in an epoxy casing, and requires minimal field maintenance (no refilling, no periodic recalibration) — features well suited to long-term deployments. Its measurement range spans from near saturation (0 kPa) down to extremely low values (–100,000 kPa nominal), though in practice accuracy is assured in the range –9 to –100 kPa (±(10 % + 2 kPa)). In Tatum et al.’s experimental field, the authors specifically cite hourly measurements in kPa from TEROS 21 sensors, noting their zero recalibration requirement post-factory.

Because TEROS 21 sensors respond to matric potential (not total potential), they do not capture osmotic or hydrostatic contributions, but for many field ecosystems and at typical soil salinities, the matric term dominates unsaturated water behaviour.

Thus, the TEROS 21 is ideally suited to continuous, multiyear monitoring of ψ in contexts where maintenance and stability are key.

Recently, a new model of the TEROS 21 was released, the TEROS 22. Like the TEROS 21, the new TEROS 22 model has a different shape. Previously, the TEROS 21 was known as the MPS-6 sensor.

empirical findings: what the five-year record reveals

Now we understand the soil water potential sensor, let's focus on the paper by researchers from Northern Arizona University. The research paper was titled "Five Years of Hourly Soil Water Potential Monitoring Demonstrates Forest Thinning Benefits in the North American Southwest", published in Ecohydrology, volume 18, 2025, by Tatum et al.:

https://doi.org/10.1002/eco.70104

thinning increases mean ψ and reduces time in stress

Across all years and depths, thinned forest zones exhibited consistently less negative soil water potentials (i.e. wetter conditions) than dense, non-thinned zones, indicating that thinning promotes better soil water availability in the root zone. Moreover, thinned plots spent significantly fewer days under a critical drying threshold (a ψ beyond which ponderosa pine experiences drought stress) compared to dense and boundary (edge) plots.

This implies that thinning can buffer trees against extended periods of hydraulic stress by moderating the descent of ψ during dry spells.

more consistent water dynamics & dampened interannual variability

A key insight is that in thinned plots, year-to-year variation in mean ψ and in days of critical stress was lower than in unthinned plots. That is, thinning not only raises the “baseline” water potential but also stabilizes it across variable precipitation years. This dampening of extremes suggests a resilience enhancement: thinning may reduce the occurrence of extremely dry conditions that push the system into critical thresholds.

timing of drying and spatial gradients

By analysing onset of drying in the spring (i.e., date when ψ starts its sustained negative descent), the authors observed that south-facing, non-thinned edge plots dried earlier than either thinned or deep-forest plots. This highlights that microtopographic and aspect-driven solar forcing can accentuate stress at edges, particularly in dense vegetation that limits lateral water recharge.

Moreover, the hourly record permits detection of diel fluctuations (e.g. daytime depletion and nocturnal recovery) in ψ profiles, which can reflect nocturnal hydraulic redistribution, root water release, or rewetting via capillary infill.

insights into drought physiology and management

From a forest-management standpoint, the results suggest that restoration thinning is a viable tool for enhancing drought resilience by raising root-zone water energy states, constraining time spent under critical ψ, and reducing sensitivity to interannual precipitation swings. Given climate projections indicating increased drought frequency and intensity, leveraging such hydraulic buffering may be an important adaptive strategy in semi-arid forest systems.

From a hydropedological and ecohydrological view, the high-resolution, long-term ψ record provides a benchmark for soil–water models (e.g. unsaturated transport, root uptake kinetics) and for calibrating water stress thresholds in plant hydraulic models.

An example dataset of soil water potential from TEROS 21 sensors. This figure is data from a two-year period, in a related study by the same authors from Northern Arizona University, demonstrating soil water potential measurements from a TEROS 21 sensor. Image source: Figure 5, Belmonte et al. 2022, Ecohydrology, doi: https://doi.org/10.1002/eco.2406.

what was learnt — lessons and caveats

Long-term ψ datasets are invaluable but rare
The five-year continuous record from multiple depths and treatments is among the most extensive in a forest ecosystem, enabling inference of seasonal, interannual, and treatment-driven differences in soil hydraulic behaviour.
ψ is more integrative than θ
The ability to follow energy gradients (rather than just volumetric content) allows direct estimation of functional stress periods and movement direction, bridging soil physics and plant physiology more tightly.
Thinning confers hydraulic resilience
The data show clearly how reduction in stand density improves and stabilises root-zone ψ, reducing physiological drought exposure — a quantifiable mechanism by which thinning helps mitigate water stress.
Diel signal and hydraulic redistribution
The temporal resolution reveals subdaily swings in ψ likely tied to root conductance and redistribution, phenomena that are often invisible in coarser sampling schemes.
Sensor limitations remain
Even advanced dielectric matric sensors like TEROS 21 have limits: accuracy degrades at extreme dryness, and they omit osmotic/hydrostatic components. Moreover, hysteresis, sensor–soil contact, calibration drift, or salt effects require careful deployment and interpretation.
Modelling and empirical integration
Such datasets can provide validation and constraint for hydrological and ecohydrological models, improving predictions of drought response, infiltration dynamics, and soil–root coupling under changing climate.

In conclusion, the five-year record of hourly soil water potential measurements in a ponderosa pine system — enabled by TEROS 21 sensors — offers rare empirical clarity into how management (thinning) reshapes hydraulic regimes, how soil energy states evolve seasonally and over years, and how those dynamics project into plant drought exposure. As the hydrological and ecological communities strive for more mechanistic links between soil physics and vegetation function, such datasets set the benchmark for the next generation of integrative modelling and field evaluation of water stress in terrestrial systems.