Feasibility of irrigation monitoring with cosmic-ray neutron sensors

Abstract

Abstract. Accurate soil moisture (SM) monitoring is key in irrigation as it can greatly improve water use efficiency. Recently, cosmic-ray neutron sensors (CRNSs) have been recognized as a promising tool in SM monitoring due to their large footprint of several hectares. CRNSs also have great potential for irrigation applications, but few studies have investigated whether irrigation monitoring with CRNSs is feasible, especially for irrigated fields with a size smaller than the CRNS footprint. Therefore, the aim of this study is to use Monte Carlo simulations to investigate the feasibility of monitoring irrigation with CRNSs. This was achieved by simulating irrigation scenarios with different field dimensions (from 0.5 to 8 ha) and SM variations between 0.05 and 0.50 cm3 cm−3. Moreover, the energy-dependent response functions of eight moderators with different high-density polyethylene (HDPE) thickness or additional gadolinium thermal shielding were investigated. It was found that a considerable part of the neutrons that contribute to the CRNS footprint can originate outside an irrigated field, which is a challenge for irrigation monitoring with CRNSs. The use of thin HDPE moderators (e.g. 5 mm) generally resulted in a smaller footprint and thus stronger contributions from the irrigated area. However, a thicker 25 mm HDPE moderator with gadolinium shielding improved SM monitoring in irrigated fields due to a higher sensitivity of neutron counts with changing SM. This moderator and shielding set-up provided the highest chance of detecting irrigation events, especially when the initial SM was relatively low. However, variations in SM outside a 0.5 or 1 ha irrigated field (e.g. due to irrigation of neighbouring fields) can affect the count rate more than SM variations due to irrigation. This suggests the importance of retrieving SM data from the surrounding of a target field to obtain more meaningful information for supporting irrigation management, especially for small irrigated fields.

Field Testing of Gamma-Spectroscopy Method for Soil Water Content Estimation in an Agricultural Field

“…CRNS estimates SWC for a circumference with a radius around 200 m depending on installation height and a moisture-dependent sensing depth of 12 to 70 cm [49] or 15 to 83 cm [50]. The CRNS is a well-established and accurate method and senses a deeper signal than GPS [33], but challenges arise for fields that are smaller than the CRNS footprint, especially when irrigation is occurring or when SWC is changing in distant areas of the footprint [51,52]. Fortunately, the gSMS is equipped to describe the subfield scale at greater sensing depth than GPS and match the 30 m scale more closely than CRNS.…”

Evaluating Cosmic Ray Neutron Sensor Estimates of Snow Water Equivalent in a Prairie Environment Using UAV Lidar

“…The DRFs were provided through ASCII-files containing energies and weights and are readily available at the URANOS GitLab repository (https://gitlab.com/mkoehli/uranos). To calculate the neutron counts from the model outputs, we applied a method from Brogi et al (2022) to interpolate the DRF for the 25 mm thick H P D E (HPDE) with a cubic spline to create a continuous curve. We assigned a weight to each simulated neutron depending on the energy during detection, and then summed to obtain the total neutron count (Brogi et al, 2022).…”

“…We used a freely available neutron transport model, the URANOS v1.14 (Köhli et al, 2015(Köhli et al, , 2023, available at https://gitlab.com/mkoehli/uranos), to model the neutron flux under conditions observed at our study area. URANOS utilizes a Monte Carlo approach to simulate the neutrons and has been specifically developed for CRNS applications (Köhli et al, 2023), including monitoring soil moisture changes through irrigation (Brogi et al, 2022) and monitoring SWE (Schattan et al, 2019). In URANOS, millions of neutrons are generated from randomly distributed point sources within a user-defined area, while tracking each neutrons' paths and interactions from its source to the point of detection through a ray casting algorithm (Köhli et al, 2023).…”

Three years of soil moisture observations by a dense cosmic-ray neutron sensing cluster at an agricultural research site in north-east Germany

“…This becomes even more evident for the second and third irrigation events. Prospective research will show whether it is, despite this low signal to noise ratio, possible to resolve the subfootprint heterogeneity introduced by irrigation (see also Brogi et al, 2022).…”