In-situ determination of soil water retention curves in heterogeneous soil profiles with a novel dielectric tube sensor for measuring soil matric potential and water content

Techniques of determining soil water retention curve (SWRC) have been developed for several decades, but very few have considered the heterogeneity of SWRC at different depths caused by texture, structure and other constituents. In this study, we designed a novel dielectric tube sensor (DTS) moving in a pair of tubes installed in-situ to measure soil water content (SWC) and soil matric potential (SMP) in heterogeneous soil profiles for determining wetting-path SWRCs. A series of experiments were conducted to test the optimal size of DTS, calibrate the DTS for SWC measurement and SMP measurement, test the water proof function of the impermeable sheet and equilibrium rate of SMP tube and determine SWRCs in a layered and stony soil tank with a relatively slow drip irrigation. Commercial sensors were also employed in comparison with the DTS during the drip irrigation experiment. The results showed that when the distance of the two electrodes of the DTS was 5 mm, the measurement range, volume of sensitivity and coefficient of variation were optimal. The calibration results verified that the DTS was suitable to measure volumetric SWC (R-2 > 0.9914) and SMP (R-2 = 0.9992). The SMP tube exhibits good water-proof property among different layers, and the average equilibrium rate was 2.1 kPa.s(-1) in loamy soil and 7.3 kPa.s(-1) in pure sand. The dynamics of SWC and SMP measured by the DTS approximately agreed well with the data measured by the commercial sensors. The relatively reduced agreement between the DTS and TEROS21 for SMP measurements may attribute to their different volume of sensitivity and installed position. Based on these, the wetting-path SWRCs were determined in-situ in a soil tank containing four kinds of soil samples. Furthermore, the determined SWRCs in four soil samples verified that stones decreased soil water holding capacity.