SAR-based monitoring of maize phenology in North West province of South Africa

Maize is a vital crop in South Africa, but its production in the semi-arid North West province is vulnerable to low and variable rainfall. This study assessed the effectiveness of Sentinel-1 Synthetic Aperture Radar (SAR) data at monitoring maize phenology and its relationship with yield in this region from 2018 to 2021. SAR effectively monitored key phenological stages. Increases in VV (vertical transmit－vertical receive polarization) and VH (vertical transmit－horizontal receive polarization) gamma naught were linked to rainfall amounts during the sowing period. A rise in the VH/VV cross-polarization ratio signaled the emergence period, as validated by a reference NDVI value of 0.15. The peak in VH gamma naught corresponded with the heading stage, and the peak in VH/VV cross-polarization ratio suggested the maturity stage, 84－144 days after emergence. Interestingly, the VH gamma naught peak had a negative correlation with yield in the overall data. This inverse relationship is attributable to the low plant density and drought stress during the 2018－19 cropping season, which reduced growth and thus increased volume scattering within the maize canopy structure. We conclude that SAR is an effective tool with which to detect phenological stages, and forecast yield in the North West province.