Driving Factors of the Hydrological Response of a Tropical Watershed: The Ankavia River Basin in Madagascar

Ramahaimandimby, Sandratra Zonirina [UCL] Randriamaherisoa, Alain Vanclooster, Marnik [UCL] Bielders, Charles [UCL]

Understanding the hydrological behavior of watersheds (WS) and their driving factors is crucial for sustainable water resources management. However, at large scales, this task remains challenging due to the spatial heterogeneity in landscapes, topography and morphology (T), land cover (LC), geology (G), and soil properties (S). In this context, the aim of this study was to identify the key factors that influence the hydrological signatures of four watersheds: Ankavia (WS1: 55% forest cover), Ankaviabe (WS2: 77% forest cover), Sahafihitry (WS3: 41% forest cover), and Antsahovy (WS4: 48% forest cover), over a 10-month study period. These catchments are located within the SAVA region of northeastern Madagascar and have a humid tropical climate. We investigated the relationship between selected catchment descriptors and hydrological signatures by using a Pearson coefficient-based correlation matrix. More specifically, catchment descriptors (extracted from T, LC, G, and S) were correlated with the following hydrological signatures: base flow index (BFI), mean runoff coefficient (rc), mean peak flow (Qp), mean runoff event time scales (ts), high flows (Q5), low flows (Q95), and mean discharge (q_mean). The analysis revealed that land cover, soil properties, and geology seem to be the best predictors for BFI and Q95, while soil properties mainly govern rc, Qp, Q5, ts, and q_mean. These findings provide valuable insights into the key drivers of hydrological behavior that can inform water resource management strategies. In particular, WS2 exhibits better flood buffering capacity but also experiences lower base flows in the dry season, potentially due to higher evapotranspiration. Conversely, WS3 and WS4 (and to a lesser extent WS1) have lower flood buffering capacity, but these watersheds encounter less pronounced low flows in the dry season due to higher BFIs, possibly attributable to lower evapotranspiration rates. The results underscore the importance of responsible land use practices and conservation efforts, which are essential for the sustainable development of the region. By incorporating these practices into water management strategies, we can help ensure a more stable and reliable water supply for communities and ecosystems within the region.