Disentangling Thermal Artifacts: A Novel Approach to Accurate Soil Moisture Measurement
- Professor Balthazar
- Mar 22, 2020
- 4 min read
Updated: Mar 24
Abstract
Soil temperature fluctuations introduce significant errors into conventional soil moisture readings, compromising data reliability. This article elucidates the mechanisms through which temperature affects sensor performance, from manufacturing calibration to in-field measurements, and presents a novel, real-time temperature compensation algorithm developed by the author.
Introduction
"Necessity is the mother of invention," a maxim that resonates deeply with my experience. As a nascent researcher, the paucity of reliable and affordable soil moisture sensors presented a formidable challenge to my academic endeavors and the practical needs of agricultural practitioners. Dissatisfied with the performance of commercially available sensors, I embarked on a mission to design a sensor that would meet the rigorous standards of my first research group (IDRG, founded in 2006) and the broader agricultural community.
This endeavor, while intellectually stimulating, was fraught with technical complexities. This article focuses on a critical aspect of this journey: the temperature sensitivity of soil moisture sensors and the development of a solution to mitigate its adverse effects.
The Pervasive Influence of Temperature on Soil Moisture Measurements
Soil temperature (Ts) is a fundamental parameter in crop management, yet its variability introduces a confounding factor in soil moisture measurement. As demonstrated by Wraith and Or (1999) and Seyfried and Grant (2007), Ts fluctuations can induce significant errors in volumetric water content (VWC) readings, leading to paradoxical outcomes where drying soil appears to exhibit increased moisture content, and vice versa.
Mechanisms of Temperature-Induced Errors
Temperature fluctuations impact soil moisture measurements across three critical stages:
Manufacturing Calibration:
Inadequate temperature control during sensor assembly and calibration can lead to unreliable calibration coefficients. The adoption of temperature chambers, while effective, is often deemed prohibitively expensive by manufacturers.
Soil-Specific Calibration:
While soil-specific calibration is widely recognized as essential for accuracy, the influence of temperature fluctuations on this process is often overlooked. Variability in Ts during calibration can undermine its efficacy, rendering the resulting data unreliable.
In-Field Measurements:
In situ measurements are inherently susceptible to temperature variations, which can obscure the true dynamics of plant water use. The resulting data may exhibit a stronger correlation with Ts than with actual moisture content, particularly at resolutions below 24 hours.
The Dual Nature of Temperature Effects
The impact of temperature on soil moisture readings manifests through two primary mechanisms:
Effects on Sensor Electronics:
The electronic components within soil moisture sensors exhibit temperature-dependent properties. While a linear relationship between temperature and sensor output is often assumed, the reality is more complex. The impedance of electronic circuits, comprising both resistance and reactance, yields a non-linear response to temperature variations. This non-linearity, exacerbated by component tolerances in low-cost sensors, necessitates a nuanced approach to temperature compensation.
Effects on Soil Electrical Properties:
Soil, acting as an extension of the sensor's electronics, influences dielectric permittivity, a parameter directly correlated with temperature. Dielectric-based sensors, particularly those employing steel electrodes, are susceptible to temperature-induced variations in electrical conductivity and moisture readings. The unpredictable and often opposing correlations between these parameters and temperature further complicate data interpretation.
Addressing the Temperature Challenge: A Novel Solution
Traditional approaches, such as multiple regression analysis and averaging (Saito et al., 2013; Kapilaratne and Lu, 2017), offer limited efficacy in mitigating temperature-induced errors. These methods, while valuable, lack the real-time, automated capabilities required for precise soil moisture monitoring.
My sensor design (see: "APAS T1 soil moisture sensor") incorporates several key innovations:
An integrated, high-accuracy temperature sensor.
The avoidance of steel electrode arrays.
The employment of a high measurement frequency.
Optimized circuit design to minimize temperature effects.
A comprehensive experimental protocol involving a large sensor sample size across a wide temperature range.
A temperature-compensation algorithm derived from the experimental data, utilizing multiple regression analysis. The algorithm normalizes sensor readings to 25 °C, effectively compensating for temperature-induced fluctuations.
The sensor stores sensor-specific parameters in its memory. A dedicated reader or data logger executes the temperature-compensation algorithm, providing real-time, temperature-independent moisture readings.
Conclusion
The development of this temperature-compensated soil moisture sensor represents a significant advancement in precision agriculture. By addressing the inherent temperature sensitivity of conventional sensors, this innovation enables accurate, real-time monitoring of soil moisture, thereby empowering researchers and practitioners with reliable data for informed decision-making.
Citation
Osroosh, Y., 2020. Disentangling Thermal Artifacts: A Novel Approach to Accurate Soil Moisture Measurement. EnviTronics Lab, March 22.
References
Kapilaratne, R.G.C.J., Lu, M., 2017. Automated general temperature correction method for dielectric soil moisture sensors. Journal of Hydrology, 551:203-216. https://doi.org/10.1016/j.jhydrol.2017.05.050
Saito, T., Fujimaki, H., Yasuda, H., Inosako, K., Inoue, M., 2012. Calibration of temperature effect on dielectric probes using time series field data. Vadose Zone Journal, 12(8)http://dx.doi.org/10.2136/vzj2012.0184.
Seyfried, M.S., Grant, L.E., 2007. Temperature effects on soil dielectric properties measured at 50 MHz. Vadose Zone J., 6: 759–765.https://www.stevenswater.com/resources/documentation/hydraprobe/Seyfried3.pdf
Wraith, J.M., Or, D., 1999. Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: Experimental evidence and hypothesis development. Water Resources Research, 35: 361–369.https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/1998WR900006
Comments