Local Calibration of TDR Measurements for Determining Water and Organic Carbon Contents of Peaty Soils

Abstract

Time domain reflectometry (TDR) measurements of the volumetric water content (θ) of soils are based on the dielectric permittivity (ε), relating ε to θ, using an empirical calibration function. Accurate determination of θ for peaty soils is vital but complicated by the complexity of organic soils and the lack of a general calibration model. Site-specific calibration models were developed to determine θ from TDR measurements for a heterogenous peatland across gradients of peat decomposition and organic carbon (OC) content; derived by soil organic matter conversion. The possibility of predicting OC contents based on the corrected θ (θcor); ε; electrical impedance (Ζ); and a categorical predictor variable was explored. The application of plot-specific and local area calibration models resulted in similar results. Compared to common calibrations, the threshold for accurate determination of θ was at ε = 5; with higher ε underestimating θ by up to 25%. Including the von Post degree of peat humification as a bioindicator, the OC content could be modelled across the area and the full range of θ with an accuracy of ±1.2% for 496 measurements. In conclusion, a strong indication was found for determining OC in peatlands in situ using TDR and a site-specific calibration model for θ together with indices of peat decomposition.

Evaluation of the Calibration Curve of Water Content Sensors in Iron Ore Tailings

“…In this way, the water content that the system infers is volumetric. The gravimetric moisture content (w) (ratio between the mass of water and the mass of solids) can be converted into volumetric moisture content (θ) which relates to the void ratio (e) and dry unit weight (Gs), through the following relationship: Several authors present alternative calibration curves to those suggested in the literature or those provided by manufacturers [10,16,17]. The present study verifies the calibration curve of two models of sensors (RK510-01 and RK520-01) manufactured by Rika and presents a specific calibration curve for iron ore tailings from a real reservoir located in Minas Gerais.…”

“…Thus, the measurement of water content becomes a more objective way of evaluating variations in the presence of water in porous materials. Sensors for measuring water content found commercially (TDR, FDR, capacitive, among others) use principles that are dependent on the mineral in which the sensor is inserted [10][11][12]. Robinson et al [13] presented results demonstrating that the presence of iron minerals in the soil, in particular magnetite, can influence soil dielectric constant significantly.…”

Recent Advances in Dielectric Properties-Based Soil Water Content Measurements

“…There has been variation in the robustness, precision, and usefulness of other models. It is important to gather and categorize mathematical models of TDR because previous research has focused on restricted numbers and kinds of them [17,42]. The effectiveness of TDR mathematical models in predicting soil water content has also been reported in earlier research; however, these studies only examined a small number of models and soils [11,42].…”

“…For example, it is challenging to create precise models because of the intricate link between soil moisture content and dielectric permittivity. The dielectric permittivity of the soil can be influenced by several factors, such as salinity, temperature, and texture, which can result in inaccurate estimations of soil moisture [17,82]. Due to the attenuation of electromagnetic radiation, the penetration depth of remote sensing methods is restricted, and their spatial resolution might not be adequate for applications that require precise information on small-scale variations in soil moisture content.…”

“…As the water content increases, the dissipation factor decreases, indicating less energy loss in the soil [14,15]. Time-domain reflectometry (TDR), frequency-domain reflectometry (FDR), and remote sensing (RS) can all benefit from understanding the link between dielectric characteristics and SWC measurement [12,16,17]. Additionally, the dielectric properties of SWC can be estimated at a distance with remote sensing to maintain optimal conditions for plant growth and development [14,15,18,19].…”