Assessment of the potential of sub-diurnal in situ C-band radar data for monitoring maize crop in semi-arid regions

This study aims to investigate the potential of sub-diurnal C-band radar data for maize crop monitoring. To this end, a tower-based experiment equipped with a C-band radar antenna (15-min acquisition interval) was conducted in a maize field in 2021. In situ measurements of soil and vegetation were carried out between September and December 2021. To analyze the radar response to plant water stress, a period of induced stress was implemented around the development peak. Both the temporal coherence ( ρ ) and radar backscattering coefficient ( σ 0 ) are analyzed. The analysis shows that the temporal coherence exhibits a marked daily cycle. More specifically, a morning drop ( Δ|ρ| morning ) was observed prior to the onset of wind. Previous studies on wheat crops and olive orchards have attributed this drop to the onset of the plant's physiological activity at dawn. The daily mean σ 0 values across all three polarizations were strongly correlated with both evapotranspiration (ETR) and evaporative fraction during both the growth and maturity stages ( r > 0.60). This observed correlation is due to structural changes in the canopy as vegetation grows, associated with increased transpiration. Despite significant differences in acquisition configurations between tower-based radar measurements and Sentinel-1, a remarkable overall agreement is observed between both datasets. In contrast to ρ , Sentinel-1 σ 0 detects the water stress period, with a decrease of about 2 dB coinciding with intensifying water stress. These findings emphasize the importance of disentangling physiological drivers from structural indicators when interpreting σ 0 dynamics in relation to plant–water interactions. • We used sub-diurnal radar data to monitor maize in a semi-arid region. • Coherence dropped at dawn, suggesting links with maize physiological activity. • Diurnal backscatter changes were related to maize crop water status. • Backscatter decreased during stress, likely from water deficit and cooling. • Our measurements matched Sentinel-1 coherence (VV,VH) and backscatter, esp. VH.