The Upper Madeira River sub-basin suspended sediment connectivity system. A brief hydro-climatological approach assessment.

Abstract

With a contribution of ~ 50% of the total suspended sediment load to the Amazon River, the Madeira River has a key role in sustaining the major fluvial system. This high contribution has its origin in the complex Andes geology, and the interaction with different erosive factors. Particularly, the rainfall plays an important role in the sediment dynamic. Hence, this work presents the analysis that was done in the Upper Madeira sub-basin relating the spatiotemporal rainfall variability with the water discharge and the suspended sediment concentration. Through the assessment of the fine fraction of the suspended sediment at the outlet of the principal tributaries -Beni and Mamoré rivers-, it was found that a decrease of suspended sediment concentration, downstream the Hydroelectric Madeira Complex (Jiraú and Santo Antonio dams) at the station of Porto Velho in Brazil is partially related to a decrease of sediment, upstream the dams in the Beni River. Yet, the existence of different hypothesis related to the decrease, as well as the limited data available at the Andean Madre de Dios River and middle sub-basin, made unable to conclude with confidence the main reason for the decrease. One of the hypothesis, was the change in the spatiotemporal rainfall variability that could limited the sediment in the river. Therefore, considering that the station of Rurrenabaque in the Andean Beni River, represents nearly 60% of the suspended sediment at the outlet of the river (Vauchel et al. 2017), the rainfall, water discharge and suspended sediment concentration relationships were evaluated. The main cause related to the decrease was also not well described only by the assessment at Rurrenabaque, however, the evaluation between the surface and base flow with the suspended sediment concentration, showed the dual role rainfall has on the suspended sediment dynamics, firstly promoting solid material detachment and mobilization it to the river channel by the surface flow and, secondly through the groundwater recharge it promotes the dilution of the sediment concentration. The former allowed the estimation of suspended sediment concentration based on a water discharge seasonal empirical function that was further compare with a probabilistic estimation (Copula functions). As described above the key role the rainfall has on the suspended sediment dynamic and to be able to have a rainfall validated data set to assess variable relationships, the satellite-based precipitation product CHIRP in its second version was validated according the main modes of rainfall variability in the sub-basin, particularly at its Andean region.