A soil water content monitoring in Longzhong semi-arid region by high-density electrical resistivity tomography

Soil water content (SWC) is a key factor influencing crop growth in semi-arid regions. Monitoring the variations in SWC is critical for agriculture. In recent years, geophysical survey has been used in the study of high-resolution detection of water infiltration and it is hugely popular because of its zero-damage to soil micro-structure and easy measurement. With a maize field in Longzhong semi-arid region as the case study, we used high-density electrical resistivity tomography (ERT) to measure (before and after the precipitation) two-dimensional electrical resistivity of the column of soil below an electrode placed on the soil surface. Then SWC and electrical resistivity were monitored in two-dimensional soil profile in order to explain the variations in SWC under different conditions and to determine the correlation between electrical resistivity of each soil horizon and its water content in Longzhong semi-arid region. Soil temperature was also discussed in the ERT data interpretation. The results revealed the potential of ERT to improve soil and agronomic studies. Vertical distributions of two-dimensional electrical resistivity image inverted from measured data were different. On the whole, natural rainfall infiltration reduced the trend in two-dimensional electrical resistivity. The trend of local electrical resistivity of the inverted ERT images was one of “high-low-high”, quite consistent with the “dry-wet-dry” cycle of the precipitation process. Two- dimensional sections of SWC calculated using ERT showed a reliable linear correlation (R₂ = 0.651 8, n = 96) between the estimated and measured SWC in the root-zone horizon, with a slope approximately equal to 1. Within the depth range of 02.0 m, the precision of the calculated specific SWC quantified by the root mean square error (RMSE) was 2.64%, with a bias corresponding to an overestimation of 0.74%. The densely distributed SWC detectors installed in the H₁ horizon (00.5 m) enhanced precise data collection, resulting in better measurement accuracy than in the H₂ horizon (0.52.0 m). The study also discussed the factors responsible for the deviation between measured SWC and estimated SWC. The development and adoption of precision farming and rational irrigation required detail knowledge of soil and crop. The method used in this study was useful in the research and description of high-resolution soil spatial variability and hydric characteristics. Additional field calibration was required for applying the method practical on routine field application. The use of general petro-physical relationship between soil electrical resistivity and its moisture, if appropriate, could bring the method a step closer to practical field application for the purpose of improving irrigation management.