Frequency Domain Reflectometry Probes Research Articles

Mandarin irrigation scheduling by means of frequency domain reflectometry soil moisture monitoring

The accurate estimation of plant water needs is the first step for achieving high crop water productivity. The main objective of the work was to develop an irrigation scheduling procedure for mandarin orchards under Mediterranean conditions based on replacing the amount of consumed water using reference values of soil moisture according to different phenological periods. The proposed methodology includes a definition part where the threshold values were established relating the trees’ stem water potential and the volumetric soil water content measured with Frequency Domain Reflectometry probes. A second part includes the steps for standardizing measurements from capacitance probes by using the LEACHM hydrological simulation model to take into account the sensor-to-sensor variations. Finally, an extrapolation procedure based on soil water retention curves was used for adapting critical soil water content thresholds to different soil conditions. Field evaluations were made in a citrus orchard located in eastern Spain during two seasons. Standardize critical soil water contents were: i) 24 % vol. for post-harvest, bloom - fruit set and phase III of fruit growth; ii) 27 % vol. for phase I of fruit growth, and iii) 29 % vol. for phase II of fruit growth with average daily air vapour pressure deficit values ranging between 0.2 - 0.4; 0.9–1.1 and 1.1–1.3 kPa, respectively. When implemented in the orchard, the sensor-based strategy resulted in water saving of 26 % respect to a control treatment, irrigated using the standard FAO-56 approach, without significant differences in yield and increasing the crop water productivity by 33 %. In conclusion, we suggest that the determination and use of the critical soil water content is a useful tool for scheduling irrigation. The proposed standardization and extrapolation methodology allows the irrigation strategy to be applied to other mandarin orchards under similar climatic conditions.

Comparison of water consumption estimates for tropical and winter forages by FDR probes and weighing lysimeters

Soil moisture determination is essential for a good use of available water resources. In this regard, the use of frequency domain reflectometry (FDR) probes has as advantages mobility and practicality in relation to lysimeters. The experiment was carried out between April and June 2016 at the Luiz de Queiroz College of Agriculture (ESALQ/USP), located in Piracicaba, SP at the geographical coordinates 22°42?14.6? S and 47°37?24.1? W and altitude of 546 m. This study aimed to assess these FDR probes to estimate water consumption in comparison to measurements by weighing lysimeters (ETcLys) and reference evapotranspiration (ETo) in Mombaça and Bermuda grass pastures under single cultivation and overseeded with oat and ryegrass. Soil moisture was assessed daily by FDR probes by estimating crop evapotranspiration (ETcFDR probe) from soil water balance calculation, which was correlated with ETcLys and ETo (Penman-Monteith, FAO 56). For all treatments, FDR probes presented the highest water consumptions when compared to the other two evapotranspiration methods, with accumulations of 126.5 and 125.6 mm for single and overseeded Mombaça grass, respectively. For Bermuda grass, water consumption was 123.4 mm in the single cultivation and 128.5 mm when overseeded. The method of estimating evapotranspiration by FDR probes showed good correlations with ETo and ETcLys.

Simulation of municipal solid waste degradation in aerobic and anaerobic bioreactor landfills.

Municipal solid waste generation is huge in growing cities of developing nations such as India, owing to the rapid industrial and population growth. In addition to various methods for treatment and disposal of municipal solid waste (landfills, composting, bio-methanation, incineration and pyrolysis), aerobic/anaerobic bioreactor landfills are gaining popularity for economical and effective disposal of municipal solid waste. However, efficiency of municipal solid waste bioreactor landfills primarily depends on the municipal solid waste decomposition rate, which can be accelerated through monitoring moisture content and temperature by using the frequency domain reflectometry probe and thermocouples, respectively. The present study demonstrates that these landfill physical properties of the heterogeneous municipal solid waste mass can be monitored using these instruments, which facilitates proper scheduling of the leachate recirculation for accelerating the decomposition rate of municipal solid waste.

Observations of stem water storage in trees of opposing hydraulic strategies

Hydraulic capacitance and water storage form a critical buffer against cavitation and loss of conductivity within the xylem system. Withdrawal from water storage in leaves, branches, stems, and roots significantly impacts sap flow, stomatal conductance, and transpiration. Storage quantities differ based on soil water availability, tree size, wood anatomy and density, drought tolerance, and hydraulic strategy (anisohydric or isohydric). However, the majority of studies focus on the measurement of storage in conifers or tropical tree species. We demonstrate a novel methodology using frequency domain reflectometry (FDR) to make continuous, direct measurements of wood water content in two hardwood species in a forest in Michigan. We present results of a two month study comparing the water storage dynamics between a mature red oak and red maple, two species with differing wood densities, hydraulic architecture, and hydraulic strategy. We also include results pertaining to the use of different probe lengths to sample water content only within the active sapwood and over the entire conductive sapwood and the outer portion of heartwood in red oak. Both species studied exhibited diurnal cycles of storage that aligned well with the dynamics of sap flux. Red maple, a diffuse porous, relatively isohydric species showed a strong dependence on stored water during both wet and dry periods. Red oak, a ring porous relatively anisohydric species, was less reliant on storage, and did not demonstrate a dependence on soil water potential. Comparison between long and short FDR probes in the oak revealed that oaks may utilize water stored in the innermost layers of the xylem when soil moisture conditions are limiting. We found the FDR probes to be a reliable, functional means for continuous automated measurement of wood water content in hardwoods at a fast time scale. Application of FDR technology for the measurement of tree water storage will benefit forest ecologists as well as the modeling community as we improve our understanding and simulations of plant hydrodynamic processes on a large scale.

Measurement of Soil Water Content with Dielectric Dispersion Frequency

Frequency domain reflectometry (FDR) is an inexpensive and attractive methodology for repeated measurements of soil water content (θ). Although there are some known measurement limitations for dry soil and sand, a fixed‐frequency method is commonly used with commercially available FDR probes. The purpose of our study was to determine if the soil dielectric spectrum could be used to measure changes in θ. A multifrequency FDR probe was constructed with a 6‐mm diameter, and a soil dielectric spectrum was obtained. Using the dielectric spectrum, the dielectric dispersion frequency (fd) was determined. It was discovered that changes in fd were highly correlated with changes in θ, and a third‐order polynomial equation (R2 = 0.96) was developed describing the relationship. The effectiveness of fd for θ measurement was evaluated for three soils and a sand across a range of θ. The effects of soil temperature and soil salinity were also evaluated. Accurate measurements of θ were obtained even in dry soil and sand. The root mean square error of the θ estimated by the fd measurement was 0.021. The soil temperature and soil salinity had no measureable effects on θ determination. The use of fd for θ determination should be an effective and accurate methodology, especially when dry soils, soil temperature, and/or soil salinity could potentially cause problems with the θ measurements.

Improved Calibration of Frequency Domain Reflectometry Probes for Volumetric Water Content Measurements of Three Media

The frequency domain reflectometry (FDR) meter is a convenient sensor used to measure the water content of soils and substrates. An adequate calibration equation evaluated by statistical techniques is important to ensure the performance of the FDR meter. In this study, the calibration equation was used to express the relationship between the volumetric water contents of substrates measured with the gravimetric method then calculated with bulk density and the values read from the FDR meter for three kinds of substrates. Two types of FDR probes were applied to illustrate the improvement with calibration equations for the volumetric water contents of three kinds of substrates. Seven calibration equations were selected to evaluate the fitting agreement. No unique FDR calibration equation could be proposed to fit all FDR probes and substrates. Each FDR probe had a specific calibration equation for the different substrates. The accuracy of an FDR probe could be improved significantly with the selection of an adequate calibration equation.

A Generalized Frequency Domain Reflectometry Modeling Technique for Soil Electrical Properties Determination

We have developed a generalized frequency domain reflectometry (FDR) technique for soil characterization that is based on an electromagnetic model decoupling the cable and probe head from the ground using frequency‐dependent reflection and transmission transfer functions. The FDR model represents an exact solution of Maxwell's equations for wave propagation in one‐dimensional multilayered media. The benefit of the decoupling is that the FDR probe can be fully described by its characteristic transfer functions, which are determined using only a few measurements. The soil properties are retrieved after removing the probe effects from the raw FDR data by iteratively inverting a global reflection coefficient. The proposed method was validated under laboratory conditions for measurements in water with different salt concentrations and sand with different water contents. For the salt water, inversions of the data led to dielectric permittivity and electrical conductivity values very close to the expected theoretical or measured values. In the frequency range for which the probe is efficient, a good agreement was obtained between measured, inverted and theoretically predicted signals. For the sand, results were consistent with the different water contents and also in close agreement with traditional time domain reflectometry measurements. The proposed method offers great promise for accurate soil electrical characterization because it inherently permits maximization of the information that can be retrieved from the FDR data and shows a high practicability.

Étalonnage de sondes FDR (« frequency domain reflectometry ») sur les cinq premiers centimètres des sols et des couverts de bryophytes de deux tourbières minérotrophes du milieu boréal Québécois (Canada)

To map the water content of Quebec’s northern boreal soils using remote sensing, it is necessary to calibrate the radar signal for fens (minerotrophic peatlands), which represent a high percentage of the land cover. Frequency domain reflectometry (FDR) probes were inserted in mosses and upper horizons (not presented here) of two representative minerotrophic peatlands, the “L’Abeille” (54°06′51.81″N, 72°30′03.73″O) and “La Chenille” (54°07′00.34″N, 72°30′15.43″O) fens, near Hydro-Quebec’s “La Forge” dam. The FDR probes were installed in the main environments located on both peatlands: plateaus (ridges, lawn plateaus), hollows (wet hollows, lower lawns, shallow pool margins), and hummocks (sphagnum hummocks and Pleurozium schreberi hummocks). However, the default calibrations of the FDR probes, performed for mineral soils, are not adapted to the materials analysed in this study. The main objective of this article was to establish specific relationships between apparent dielectric constants (Ka) and volumetric moisture contents (θv) for the organic soils and vegetative covers found in the studied peatlands. Hence, microenvironments (blanket mosses and upper peat horizons) were sampled and brought back into the laboratory. Several successive stable periods of pressure head (h) were applied to determine equivalent θv and Ka values. For all samples, a general equation Ka–θv was determined. The θv values measured in the laboratory were compared with values of volumetric moisture contents obtained with the default calibration (θFDR). The comparison confirmed that it is required to make specific FDR probe calibrations for organic soils and moss blankets, because default calibrations consistently underestimated actual volumetric moisture contents.