[AGE31-P06] Simultaneous measurement of soil matric potential and water content with dual probe heat pulse
Keywords:matric potential, soil water content, dual probe heat pulse, thermal conductivity, volumetric heat capacity, porous media
We developed a sensor measuring Ψm and θ simultaneously by improving the sensor design of a Ψm sensor made of porous media in conjunction with dual probe heat pulse (DPHP) technique developed by Kojima et al. (2017). DPHP is a method to simultaneously measure volumetric heat capacity (C) and thermal conductivity (λ) of soil. Kojima et al. (2017) applied DPHP to a porous medium made of Kaolinite and equilibrated with surrounded soil. They measured C and λ of the porous medium and converted them into Ψm by the preliminary obtained relationship. We extended the DPHP probe in the porous medium from 40 mm to 80 mm, and made them projecting into soil. By this change, the sensor can measure thermal property in two places, i.e., in porous media and in soil. Ψm is estimated from the thermal properties of porous media, and θ is estimated from C of soil.
An experiment to obtain relationships between thermal properties of porous media and Ψm, and to evaluate the accuracy of θ estimation by the developed sensor (hereafter, DPHP Ψm-θ sensor). A cylindrical container (46 cm diameter and 9 cm height) was filled with soil (Andisol) with a bulk density of 0.84 Mg cm-3. Five DPHP Ψm-θ sensors, two 5TEs (METER Group, Inc.) which measure θ, two MPS-6s (METER Group, Inc.) which measure Ψm, and one tensiometer were inserted at 4.5 cm depth. The soil in the container was left in a constant temperature room (20°C) after it was saturated by tap water. The soil was naturally dried by evaporation for 30 days. The relationship between thermal properties of porous media measured with DPHP Ψm-θ sensor and Ψm measured with tensiometer and MPS-6 was evaluated. And the accuracy of θ estimation with DPHP Ψm-θ sensor was evaluated by comparing the estimated θ to that measured with 5TE.
The relationships between Ψm and C or λ of porous media could be expressed by a modified water characteristics curve model. Accuracies of Ψm estimation from C were 13%-18%, and those from λ were 13-15%. Thus, Ψm estimation from λ is more accurate than that from C. In addition, the measurable range of Ψm with λ was wider than that with C. While using λ to estimate Ψm had advantages in sensor performance, using C to estimate Ψm was more practical because the relationship between Ψm and C did not show significant inter-individual differences and temperature dependence. Each thermal property has advantages so that we can use both properties to accurately determine Ψm. The accuracy of θ estimation with DPHP Ψm-θ sensor was 6% which is sufficiently high.
In this study, a DPHP based sensor measuring Ψm and θ simultaneously was developed. The developed sensor could estimate Ψm and θ accurately, thus, it can be a powerful tool for variety of studies in vadose zone.
Citation: Kojima, Y., K. Noborio, M. Mizoguchi, and Y. Kawahara. 2017. Matric potential sensor using dual probe heat pulse technique. Agricultural Information Research. 26(4):77-85