Monitoring spatiotemporal soil moisture variability in the unsaturated zone of a mixed forest using electrical resistivity tomography

Abstract

European forests are suffering considerably from the consequences of the droughts of recent years, and the exact reasons and influencing factors for this are still not fully understood. This study was conducted to characterize the changes and dynamics of soil moisture in a mixed forest in northern Bavaria within 1 year. Since electrical resistivity correlates well with soil water content, we used two‐dimensional electrical resistivity tomography (ERT) monitoring and time‐lapse analyses to supplement punctual measurements by sensors and soil analyses to show soil moisture changes throughout a whole year (2020–2021). While the topsoil dries out significantly from summer to autumn down to a depth of about 3 m, a clear increase in soil water content and a decrease in resistivity below 3 m can be observed during winter period. Anomalies in the topsoil (0–1 m) showing lower resistivities than the surrounding substrate could be related to tree positions by additional terrestrial laser scans. A significant relationship could be found between tree crown projection area and resistivity in 1–2 m depth. We found a trend that mean resistivity below pine is lower as below beech. ERT data were also used to estimate the soil water content via Archie's law and the results correlate strongly with the measured values, but the degree of correlation varies depending on the depth level. ERT as a noninvasive method, in combination with additional data, for example, on the vitality status of individual trees, could help to better understand root water uptake and water supply to trees, especially during periods of drought.

Estimating soil water content in a thorny forest ecosystem by time‐lapse electrical resistivity tomography (ERT) and HYDRUS 2D/3D simulations

“…In Chen et al (2019) andPeddinti et al (2020), an RMSE between ERT and soil water probe ranged from 0.02 to 0.027 (depth dependent) and 0.03 cm 3 cm À3 , respectively. Moreover, the Archie law parameters obtained through calibration and field conditions (as presented in Table 1) are consistent with those found in previous research conducted by Nijland et al (2010) and Rieder and Kneisel (2023). This alignment with prior studies adds confidence to the validity of our ERT-based θ determination and reinforces the reliability of the results obtained in this study.…”

“…The calibration results demonstrated high performance with an RMSE of 0.017 cm 3 cm À3 (Figure 8), and the validation RMSE was only slightly higher at 0.021 cm 3 cm À3 . These results surpass the literature's standards, as previous studies by Rieder and Kneisel (2023) reported depth-dependent RMSE values ranging from 0.01 to 0.02 cm 3 cm À3 . In Chen et al (2019) andPeddinti et al (2020), an RMSE between ERT and soil water probe ranged from 0.02 to 0.027 (depth dependent) and 0.03 cm 3 cm À3 , respectively.…”

“…Therefore, non‐invasive techniques such as geophysical methods (Turesson, 2006; Fustos et al, 2017) and mechanistic simulations (Šimunek et al, 2012) have gained the scientific community's interest in estimating . Such techniques can complement or replace FDR in forestry ecosystems for determination (Fäth et al, 2022; Rieder & Kneisel, 2023).…”

Variations in Bedrock and Vegetation Cover Modulate Subsurface Water Flow Dynamics of a Mountainous Hillslope

“…While during snowmelt the decrease in resistivity related to groundwater upwelling extended toward the end of the transect, and hence the stream, in late summer, this feature ends when entering the forested area at x = 220 m. We interpret this as a reduction in soil moisture due to water uptake by the trees at this location (Figure 8f). Significant reduction in soil moisture within forested areas have also been observed in other studies with soil and bedrock conditions comparable to the eastern part of the transect (Peskett et al, 2020;Rieder & Kneisel, 2023).…”

“…Their results show that forest strips have a significant impact on the hillslope hydrology by lowering the groundwater table and reducing soil moisture, compared to adjacent grassland. Electrical resistivity tomography (ERT) monitoring also allows to assess soil moisture dynamics below single trees in detail (Fäth et al, 2022), and Rieder and Kneisel (2023) show that individual tree species show different subsurface water demands and dynamics.…”