Drought stress due to sparse rainfall is one of the main factors determining plant species composition in semi-arid ecosystems. The tolerance of two dominant perennial grasses of northeastern Asia, Leymus chinensis and Stipa krylovii, to continuously changing drought stress was compared. Their responses to the stress were eval-uated in terms of the parameter values in Feddes model. The results indicated that transpiration in S. krylovii began to decrease at a higher matric potential (−102 cm) than in L. chinensis. L. chinensis has higher performance in tran-spiration rate at a higher matric potential. In contrast, it seemed that greater tolerance of L. chinensis for drought stress is inconsistent at lower matric potentials (−104 cm). Thus, the tolerance for drought stress was ambiguous but depended on the degree of stress. Furthermore, S. krylovii maintained a low ranspiration rate under a lower ma-tric potential, implying that this species is well-adapted to drought stress because it can continue transpiration, even under severe drought stress conditions. Notably, the es-timated values for root water uptake according to Feddes revealed strategic responses in both plants for survival in semi-arid regions.
ECH2O 5TE is an useful dielectric soil mois-ture sensor for a field soil. However, it is reported that the sensor outputs are affected by the electrical conductivity (EC) of the soil and the soil temperature (T). The objec-tive of this study was to evaluate effects of the EC and T on the sensor outputs based on a laboratory EC calibra-tion and a temperature calibration according to field data measured in a sand dune field. Result of the laboratory ex-periment showed that a single calibration equation could be adapted to the data for the soil water EC (ECw) < 10 dS m−1 with the soil bulk EC (ECb) < 0.50 dS m−1. As-suming a linear relation between the sensor output change(ΔRAW) and the temperature change (ΔT) under a constant soil moisture condition as α = ΔRAW/ΔT, α values were estimated based on the field data. Whereas estimated αwas 0.8 ◦C−1 ∼ 1.2 ◦C−1 for ECw ∼ 0.1 dS m−1, it signifi-cantly increased from 1.0 ◦C−1 to 2.2 ◦C−1 for ECw > 2.5 dS m−1.
The ECH2O GS3 soil moisture sensor (GS3, Decagon Devices, Inc.,) is suitable for measuring volumet-ric water content (θ), temperature, and bulk EC for agricul-tural materials. Two types of calibration equations are pro-vided for the GS3 water content measurements: a standard equation (θST) and an agricultural material equation (θAM). In this study, these two equations were evaluated based on the dry weight method θ (θM) for agricultural materials (Rock wool, Vermiculite, Peat moss, and Strawberry soil) and soils (Toyoura sand, Andisol and Kanto loam). The relative error of θST was 10 % – 14 % for agricultural ma-terials, 3 % for the sand, and 14 % for the Andisol and Kanto loam. The θST prediction was reasonably accurate for Toyoura sand, but practically not applicable for agri-cultural materials, Andisol, and Kanto loam. The relative error of θAM was 8 % – 10 %. The accuracy of θAM was slightly higher than for θST regardless of samples. The θST was corrected based on the linear regression line for the θM and θST relationship. The relative error for the cor-rected θST became 1 % – 5 % for all samples. Furthermore, the linear regression line was made according to three sat-urated, intermediate, and dried samples (3-point method). The relative error for the θST corrected with the 3-point method was 2 % – 6 %, which was much smaller than for the original θST. In conclusion, we recommend using the 3-point method to correct θST for the GS2 measurements for soils snf agricultural materials.
The Drill & Drop Probe (DDP) newly devel-oped by Sentek Pty. Ltd. is easy to install directly into a soil without using an access tube because the tapered probe matches auger well. The DDP is suited to situations where a monitoring probe may need to be moved in short term. In this study, a 120 cm length DDP was installed in a loamy field to confirm the installation procedures and propose a simple in-situ correction method for the volumetric water content, θ, measurement. Excavating a smaller diameter hole before using a tapered auger could reduce gaps be-tween the probe and surrounding soil. The DDP overes-timated θ by about 0.057 m3 m−3 relative to gravimetri-cally determined θ at the end of measurements for θ rang-ing from 0.26 to 0.37 m3 m−3. Subtracting the averaged error for stable eight data in ten measurements from the DDP measured value could improve the θ measurement. Since the observed frequency is normalized using air and water frequencies to minimize the sensor difference, this simple correction method can improve the θ measurement for a uniform textured soil. Judging from the continuous monitoring data, there were two types of erroneous mea-surements depending on the connection between the probe and soil: stable but constant error and fluctuating error. It would be possible to interpolate these data using adja-cent data sets in case of the DDP measurement at multiple depths in a soil profile.