Spatial Mapping of Soil Moisture Using a Rover-Mounted CRNS System in Semi-Arid Agricultural Landscapes

Project Description: 

Soil moisture is a fundamental state variable in the terrestrial water cycle, controlling the partitioning of rainfall between infiltration and runoff, driving evapotranspiration, and regulating plant water availability throughout the growing season. In semi-arid agricultural regions such as the North West Province of South Africa, where rainfall is highly variable and irrigation resources are limited, accurate knowledge of soil water content at field scale is critical for sustainable water management, crop yield prediction, and drought monitoring. Despite this importance, existing measurement approaches struggle to provide the combination of spatial coverage, temporal frequency, and measurement depth that operational agricultural monitoring requires. Point-scale in-situ sensors offer high temporal resolution but are spatially unrepresentative, while satellite remote sensing products provide broad spatial coverage but are limited in their sensitivity to subsurface moisture and their ability to resolve field-scale variability. Cosmic Ray Neutron Sensing (CRNS) addresses this observational gap by measuring the flux of secondary cosmic ray neutrons at the land surface, which is inversely modulated by the hydrogen content of the surrounding environment. In its rover-mounted configuration, a CRNS detector is transported along survey transects by a vehicle, continuously recording neutron count rates that can be converted into spatially distributed soil moisture estimates. Each measurement integrates neutron flux over a horizontal footprint of approximately 150–300 metres and a soil depth of 20–40 centimetres, producing observations that are inherently representative of field-scale conditions rather than single points. Repeated rover surveys over the same area at different times allow temporal changes in the spatial pattern of soil moisture to be tracked, capturing the effects of rainfall events, irrigation cycles, and seasonal drying in a spatially explicit manner. A key challenge in rover-based CRNS is that the detected neutron signal is sensitive not only to soil water but to all hydrogen pools in the measurement environment, including water stored in above-ground vegetation biomass. As crops grow through their seasonal development stages, the biomass water equivalent — the column of water represented by plant material within the sensing footprint — changes substantially and must be accounted for in the soil moisture retrieval if systematic biases are to be avoided. This correction is particularly important in agricultural landscapes where biomass accumulation is rapid and spatially variable. Monte Carlo neutron transport simulations, such as those produced by the URANOS modelling framework, provide a physically rigorous basis for quantifying these biomass effects and developing correction factors appropriate to specific crop types and growth stages. Validation of rover-derived moisture maps against independent observations — including point-scale in-situ sensors and satellite remote sensing products such as Sentinel-1 synthetic aperture radar and SMOS/SMAP passive microwave retrievals — is essential for establishing the accuracy and utility of the spatial moisture products. This project will design and execute a programme of repeated rover-mounted CRNS surveys across contrasting land-use types in the North West Province study area, encompassing dryland cropping, irrigated agriculture, and natural semi-arid vegetation. The student will develop and apply biomass water equivalent corrections appropriate to the dominant vegetation types encountered, produce gridded soil moisture maps using spatial interpolation techniques, and conduct a systematic validation against in-situ and satellite-based reference datasets. Processed and validated soil moisture maps will be delivered to the group's ICT4V collaborators for integration into regional hydrological models. The project will produce outputs suitable for peer-reviewed publication and will strengthen the operational capacity of the group's rover-based CRNS programme in southern African agricultural environments.(NWU)

Supervisor

North-West University (NWU)

Co-Supervisor