Conectividade de sedimentos: uma abordagem multiescalar em ambiente semiárido

Abstract

The transmission of matter and energy and the transport of water and sediments in the river system have been increasingly present in geomorphological research, constituting essential aspects of environmental management. The connectivity approach has been used to understand the processes and complexities of matter and energy transmission in river basins, water and sediment transport. In this sense, the present research seeks to understand the processes of sediment transmission and storage and the hydrological changes that have occurred in the Tigre Stream Basin - PB through the sediment connectivity approach at multiple scales. The methodological procedures were divided into stages of application of the Connectivity Index (CI) and connectivity mapping for the basin scale and the hillslope scale, both in dry and wet periods and finally, the validation stage through the Field Connectivity Index (FCI). For the analysis of connectivity at the basin scale, the digital elevation model used was NASADEM, with a spatial resolution of 30 m, and a total of fifty-one Landsat images were obtained from the area between 2014 and 2023 and from the result generated through the processing of Landsat images for the application of NDVI, the average values of each image allowed grouping into four vegetation scenarios: very low density, low density, medium density, and high density. The vegetation maps served as a basis for the C factor in mapping the structural connectivity of water and sediments, indicating a high variability of biomass density in each scenario and presenting different responses regarding accumulated antecedent rainfall. Connectivity maps already point to higher connectivity in very dry and dry scenarios, moderate connectivity in the medium scenario, and lower connectivity in the wet scenario. For the analysis of connectivity on hillslopes, three river sections (T1, T2, and T3) were selected, from which digital elevation model data with a resolution of 0.2 m and RGB images were obtained using a drone in three different periods (C1, C2, and C3), generating a total of nine orthophotos. The orthophotos served as a basis for applying VIgreen in all sections and periods analyzed, resulting in the elaboration of vegetation maps, which consequently were used in the C factor for applying CI. The results showed that in all sections - especially in T2 - C2 was the period with the highest vegetation cover density, as well as the period in which connectivity was lower, showing a strong correlation with rainfall, since in C2, the values of accumulated antecedent rainfall resulted in 249.2 mm for 90 days and 260.6 mm for 120 days, that is, values that are considered for a wet scenario in the region. Regarding validation, the values obtained through FCI at four selected points were compared with the CI values at the two analysis scales, where the values coincided in all indices for higher connectivity in points T1B and T2B and demonstrated coherence with the evidence observed in the field. In general, the analysis of sediment connectivity in the Tigre Stream Basin has shown to be lower in areas with dense vegetation cover, reducing the intensity of surface runoff and erosive processes, which is an essential aspect for preserving and maintaining semi-arid river environments. The data and information obtained in this research are relevant for understanding connectivity in different river environments, contributing to future assessments for the functional connectivity of the Tigre Stream Basin, essential for environmental management at multiple spatial and temporal scales, as well as for optimizing soil management and livelihoods in Brazilian semi-arid agricultural lands.