Imaging soil water dynamics with spectral induced polarization in Vineyards

Summary Spectral Induced Polarization (SIP) has gained recognition as an advanced geophysical method for monitoring soil water content. SIP’s ability to simultaneously assess soil texture and water content makes it particularly valuable for studying soil dynamics under varying environmental conditions. However, its application in complex field environments has been hindered by issues such as capacitive and inductive coupling, which affect the quality of measurements. In this study, we combined water monitoring in soil and plant (field and lab SIP measurements, sap flow and soil moisture monitoring) to characterize soil heterogeneity and evaluate vine water availability in a Médoc vineyard during the summer drought of 2023. Different SIP field acquisition strategies relying on multiwire cables, fully coaxial cables array or hybrid coaxial/multiwire were tested. The acquisition setup was shown to strongly affect data quality depending on soil moisture conditions. Lab and field SIP measurements confirmed a strong correlation between the quadrature conductivity at 0.25 Hz and soil volumetric water content (VWC) as well as a linear relationship between phase shift at 0.25 Hz and VWC. The real and imaginary parts of the conductivity was used to infer VWC dynamics based on empirical petrophysical relationships established in situ. A mechanistic model based on the Dynamic Stern Layer model was also applied to high-quality SIP data for the same purpose. We found that imaginary conductivity was much less sensitive to soil water conductivity than real conductivity. Thus, in vineyard soils subject to soil amendments and resulting variations in soil water salinity, we hypothesized that SIP monitoring provide more reliable estimates of changes in soil moisture content than standard electrical resistivity tomography. We showed that SIP monitoring effectively captured soil drying dynamics down to a depth of 1 m during the growing season. The SIP method combined with soil moisture probes could thus provide simultaneous access to both soil moisture dynamics and the spatial distribution of soil texture, opening up new perspectives for mapping soil moisture dynamics in the field, even in case of potentially large soil water salinity fluctuations. In our case, SIP indicated a decrease in soil water storage from 150 to 50 mmH20 during the summer drought of 2023. By combining SIP and vine sapflow monitoring, Vine water availability, defined as total transpirable soil water could also be estimated at 98±8 mm H20 for the vines equipped with sapflow sensor, which is of great interest for culture water management. Finally, the distinct responses of the real and imaginary conductivity components underscore the value of SIP for soil moisture assessment in viticultural environments subject to variable salinity inputs. This work is the first to attempt a quantitative estimation of soil water storage in commercial vineyards using SIP methods. It extends previous applications limited to other agricultural settings and broaden the applicability of mechanistic models (Dynamic Stern Layer model) for predicting volumetric water content based on multi-frequency complex conductivity measurements under field conditions.