A novel frequency-domain integrated sensor for in-situ estimating unsaturated soil hydraulic conductivity

Unsaturated soil hydraulic conductivity (K-u) plays a significant role in agricultural, hydrological and ecological applications. However, the existing measurement technologies are failed to consecutively measure soil hydraulic conductivity in-situ, high cost or destructive to soil structure. Here we designed a novel frequency-domain integrated sensor that is combined with Richards' equation to in-situ estimate K-u. The novel sensor has three perforated cylinder coaxial (PCC-1, PCC-2, PCC-3) probes and a stainless steel pin to measure soil water potentials at up (PCC-1), middle (PCC-2) and bottom (PCC-3) layers and soil water content at middle layer, respectively. After sensor calibration, drying experiments were conducted with sandy loam and clay loam. The Richards' equation was used to in-situ estimate dynamics of K(u )combining finite difference method with measurements of the novel sensor under certain assumptions, and thus neither additional parameters nor boundary condition was required. In order to assess the accuracy of the in-situ estimated K-u, the actual K(u )were determined in laboratory using van Genuchten and Campbell hydraulic models as references. Then, a field experiment was conducted at an experimental farm to test the performance of the novel integrated sensor. The results showed that the in-situ estimated K(u )in unsaturated range had a good agreement with the model calculated K(u )in laboratory. The RMSEs (in log10) were 0.707 cm h-1 (van Genuchten) and 0.426 cm h(-1) (Campbell) for sandy loam and 0.749 cm h-1 (van Genuchten) and 0.587 cm h(-1 )(Campbell) for clay loam. In near-saturated range, the in situ estimated K(u )were somewhat underestimated in comparison with the model calculated K-u. This may attribute to the limitation of accuracy of soil water potential sensor (TEROS 21) used for calibration, the insensitivity of drying method to the estimation of K(u )or the effect of structural macropores or discontinuity of soil intrinsic hydraulic characteristics in near-saturated range. The overall trend of in-situ estimated K(u )in the field is similar to that in laboratory. The consistent between the in-situ estimated and model calculated K(u )in laboratory verified the availability and feasibility of the novel integrated sensor and the estimation of K-u in the field was achieved. In addition, the novel integrated sensor is low cost and reduces sensor-to-sensor uncertainties when multiple sensors were used, which has distinct advantages and potential application under field conditions.