Monitoring Riparian Vegetation and Its Influence on Morphological and Hydrological Processes in Non-Permanent Rivers

The role of riparian vegetation in shaping river morphology is widely recognized. The interaction between vegetation growth and channel evolution is characterized by complex, non-linear feedbacks, which complicate direct estimates of the role of key elements in the morphological evolution of non-permanent rivers. Riparian vegetation in non-permanent rivers exhibits growth patterns directly influenced by the flow timing, which controls water availability. Thus, monitoring riparian vegetation is an alternative approach to understanding the ecohydrological complexity and its seasonality in the bio-geomorphological influence of non-permanent rivers. This study evaluated how eco-hydrological and bio-geomorphological seasonality affects the morphology of non-permanent sandy channels in Northeast Brazil. DEMs and orthomosaics, generated from five UAV surveys of a representative section of the basin between October 2021 and August 2022, were analyzed, allowing the study of morphological changes and vegetation cover. Simultaneously, field quadrants were set up to monitor the cover and growth of some riparian plant species in various geomorphic units of the studied stretches. These data were correlated with measurements from six rainfall stations. Riparian vegetation was monitored in two distinct areas, the bar and the channel bed, over three periods. In the bar area, vegetation was sparse in December 2021, followed by a significant increase in April 2022 due to rainfall events, which led to accumulated precipitation between 90 mm and 117 mm. There was a slight decrease in July 2022, but the coverage remained higher than in December 2021. In the channel bed, vegetation was almost nonexistent in December 2021, with considerable recovery in April 2022. Vegetation cover in the bar area was strongly influenced by precipitation, showing peaks after rain events. In the channel bed, vegetation showed little response to precipitation variations, suggesting that the species present are more drought-resistant or receive less water, even during rainfall periods. Vegetation in the bar area varied over time, with peaks in January and April 2022, while cover in the channel bed remained consistently low, ranging from 0% to 10%.
Regarding species, Jatropha mollissima was predominant in both areas throughout the monitored period. Jatropha ribifolia and Vitex gardneriana were also in the bar but had a lower recurrence. In the channel bed, Prosopis juliflora was more common at first, but its presence diminished over time. These results highlight the importance of monitoring riparian vegetation in different bio-geomorphological settings, demonstrating that the vegetation composition in the bar area is more stable than in the riverbed. At the same time, in the channel bed, there is greater variation over time.