Instantaneous profile method - the impact of experimental procedure on measurement results

Unsaturated soil hydraulic conductivity (UHC) plays a crucial role in natural systems, as it governs the flow of water in soil under partial saturation conditions, which are typical for most environmental processes. One of the laboratory methods used for UHC determination is  Instantaneous Profile Method (IPM). IPM requires the measurement of soil water flux at least at one end of a soil sample, alongside the measurements of soil water content (SWC) and soil water potential (SWP) at various points along the height of the soil sample.

This study analyzes the accuracy of UHC determination using IPM, based on simulated desaturation experiments for 460 types of soils. The parameters of soils used in the analysis were sourced form the Global Database of Soil Hydraulic Properties (GSHP) to reflect the realistic properties of the particular soils. The desaturation of a 5 cm-high soil sample is simulated using Richard’s equation. Simulations were performed for various experimental scenarios using axis translation method, where overpressure is used for enforcing the particular boundary conditions. Different overpressure causes different drying rates. Following boundary conditions were applied at the bottom of the sample: constant soil water potential of -10 mH₂O over 8 days, a linear decrease in soil water potential from 0 mH₂O to -10 mH₂O over 2.5 days, followed by maintenance at -10 mH₂O until the 8th day, and a linear decrease in soil water potential to -10 mH₂O over 5 days, followed by maintenance at this level until the 8th day of simulation.

IPM is based on measurements of SWC (using TDR method) and SWP (using microtesiometers) in subsequent layers in the soil core. The influence of the height of the layer (from 1 cm to 2.33 cm) on the accuracy of UHC was analyzed. Time interval between subsequent measurements was also examined in the context of the accuracy. The TDR measurements naturally introduces noise. Two denoising methods were evaluated: the Bézier curve method and the B-spline method.

In conclusion, selecting an appropriate denoising method is critical for accurate UHC determination. This study provides a comparative analysis of the effectiveness of different denoising techniques for TDR data. Results indicate that thinner soil layers allow for better estimation of UHC. However, the determination of UHC in subsequent layers introduces greater errors due to the propagation of flux estimation errors from earlier layers. The most accurate UHC value is determined for the first layer. Moreover, the experiment setup in which the boundary condition is changing slowly allows for better estimation of UHC in comparison with  the other scenarios. These findings underscore the importance of careful experimental design and the selection of suitable methods to minimize cumulative errors in UHC determination.

Acknowledgments:

Research was founded by the National Science Centre within the contract 2021/43/B/ST10/03143