A New Fast in-Situ Soil Hydraulic Characterization Method Combining 2D Soil Water Flow Modelling and Time-Domain Reflectometry

Soil hydraulic behavior is fundamental for determining water flow dynamics within the soil–plant–atmosphere continuum. Soil Hydraulic Properties, SHP, govern essential processes, including soil water storage within the root zone and the entire soil profile, evapotranspiration, plant water and nutrient uptake, runoff generation, deep percolation, groundwater recharge, as well as the transport of solutes and contaminants. There are several laboratory and field methods to characterize SHP. Laboratory measurements are generally more straightforward than field tests. However, their reliability depends on the selection of sample sizes that adequately represent the present heterogeneity in natural soils. In-situ methods for determining SHP are often labor-intensive and time-consuming due to the need for detailed spatial and temporal data. Because SHP exhibit significant spatial variability, many measurements are required to accurately characterize the SHP. This variability highlights the need for faster and more efficient methods to characterize SHP across multiple sites. This study proposes a fast in-situ method for SHP characterization called TDR-2D. It combines Time-Domain Reflectometry (TDR) with soil water modelling in a wetted bulb under a dripper. The TDR-2D method was simultaneously applied to multiple sites across an experimental field to estimate their SHP. The same sites were characterized using the Tension Infiltrometer Method, TIM. The soil hydraulic parameters estimated by TDR-2D were evaluated by comparing them to those obtained by TIM. Parameter correlation matrices were employed to assess uncertainty in parameter estimation. An additional sensitivity analysis was conducted to evaluate the influence of different dripper nominal flow rates (2, 4, and 6 l/h) on parameter estimation. The results indicate that the TDR-2D method reliably estimates soil water retention parameters across all tested flow rates. Estimation of the saturated hydraulic conductivity (K₀) was particularly accurate at flow rates of 2 and 4 l/h whereas accuracy declined at 6 l/h. Furthermore, model output sensitivity to soil hydraulic parameters decreased with increasing dripper flow rate. Overall, for the soils investigated, the findings suggest that the TDR-2D method performs optimally at a nominal flow rate of 4 l/h, providing accurate SHP estimates while minimizing parameter uncertainty. Since the TDR-2D and TIM methods yield Russo–Gardner (RG) and van Genuchten–Mualem (vGM) parameters, respectively, direct comparison required conversion between the two parameter sets. The results demonstrate that conversion from vGM to RG parameters is generally feasible, whereas the reverse conversion is less straightforward and should be approached with caution.