Improving the GPR reflection method for estimating soil moisture and detection of capillary fringe and water table in a boreal agricultural field

The objective of this thesis was to monitor the soil moisture (SM) and water table depth (WTD) in an agricultural field using ground-penetrating radar (GPR). First, SM was estimated using hyperbola-fitting method (27-50 cm depth range) and compared with vertically installed 30 cm long Time Domain Reflectometry (TDR) probe data. TDR-measured and GPR-estimated SM were not significantly different, and the root mean square error (RMSE) was 0.03 m3 m-3. Second, the depth of the capillary fringe (DCF) was estimated distinguishing the reflections from the top of the capillary fringe in a GPR radargram. A site-specific strong linear relationship (R2 = 0.9778) of DCF and measured-WTD was developed. RMSE between GPR-based WTD and actual WTD was 0.194 m. Proposed average capillary height for the particular site throughout the growing season (0.741 m) agrees with the existing literature and would be beneficial for the agricultural water management in the region

Distinguishing Capillary Fringe Reflection in a GPR Profile for Precise Water Table Depth Estimation in a Boreal Podzolic Soil Field

Relative permittivity and soil moisture are highly correlated; therefore, the top boundary of saturated soil gives strong reflections in ground-penetrating radar (GPR) profiles. Conventionally in shallow groundwater systems, the first dominant reflection comes from the capillary fringe, followed by the actual water table. The objective of this study was to calibrate and validate a site-specific relationship between GPR-estimated depth to the capillary fringe (DCF) and measured water table depth (WTDm). Common midpoint (CMP) GPR surveys were carried out in order to estimate the average radar velocity, and common offset (CO) surveys were carried out to map the water table variability in the 2017 and 2018 growing seasons. Also, GPR sampling volume geometry with radar velocities in different soil layers was considered to support the CMP estimations. The regression model (R2 = 0.9778) between DCF and WTDm, developed for the site in 2017, was validated using data from 2018. A regression analysis between DCF and WTDm for the two growing seasons suggested an average capillary height of 0.741 m (R2 = 0.911, n = 16), which is compatible with the existing literature under similar soil conditions. The described method should be further developed over several growing seasons to encompass wider water table variability