Assessing the variability of 40K measurements using a portable gamma-ray spectroscopy in an irrigated agricultural field (Spain)

Understanding the temporal and spatial distribution of soil water content (SWC) is critical for efficient water resource management in agriculture. However, the variability of SWC over time and space presents challenges in obtaining accurate values at field scale using conventional methods. Proximal gamma-ray spectroscopy (PGRS), supported by adequate calibration and biomass corrections, emerge as promising methods for monitoring SWC. The inverse correlation between the gamma counts of the radioisotope ⁴⁰K (1461 KeV) and volumetric SWC (m³/m³) demonstrates potential for reliable soil moisture estimation in agricultural and hydrological applications. This contribution examines the potential application of a portable sodium iodide (NaI) scintillation detector (PGRS) for estimating SWC in an irrigated wheat field. We explore the sensitivity of the ⁴⁰K variations to changes in soil moisture and detector height. Over the last two months of the growing season, several one-hour manual monitoring surveys were conducted to capture the effect on ⁴⁰K signal of irrigation and soil moisture status before and after the harvesting. In each survey, total counts of ⁴⁰K were recorded using a NaI detector positioned at different elevations above the ground in the middle of a wheat field. Preliminary results indicate a general correlation between ⁴⁰K (cps) and SWC throughout the study period, suggesting the sensitivity of the PGRS detector to SWC variations. Our findings show a slight increase in ⁴⁰K counts by decreasing the detector height for all the field surveys conducted. In addition, we observed that the lowest counts of ⁴⁰K were recorded during the survey with the highest soil water content after irrigation. We can conclude that ⁴⁰K signal is sensitive to both changes in SWC and the height position of the detector. Furthermore, this detector offers a significant advantage, as it not only captures data on the ⁴⁰K peak but also analyses the full gamma spectrum.