Flood Seasonality Mechanisms in Brazil

A coincidence in the timing of floods and their drivers can be used as a proxy for the causality of flood generation. By investigating flood generation mechanisms, we can better understand how runoff is generated and its predominant flow paths. However, so far, no study has explored the drivers of flood seasonality on a large scale in Brazil, particularly the roles of intense rainfall and soil moisture. Here, we investigate the relationship between the seasonality of floods, maximum annual rainfall, and maximum annual soil moisture data of 886 basins in Brazil for 1980-2015 to shed light on flood generation mechanisms. We analyze circular correlation of the variables’ timing and compare their mean dates of occurrence. Floods generally occur around February in central Brazil, April in Amazonia’s southern tributaries, June in Amazonia’s northern tributaries, and between austral autumn and spring in southern Brazil. On average across Brazil, floods tend to occur at the same time of year as soil moisture peaks and lag behind rainfall peaks by three weeks. In Amazonia, central and northern Brazil, flood timing is more highly correlated with the timing of soil moisture peaks than with that of rainfall peaks. In these regions, rainfall usually peaks early or mid-wet season even though floods and soil moisture peaks usually occur at the end of the wet season. We suggest that such delays between rainfall peaks and floods are associated with high subsurface water storage capacities. On the other hand, in southern and southeastern Brazil, flood timing is highly correlated with the timing of both soil moisture and rainfall peaks. Intense rainfall quickly saturates the soil and generates floods, indicating a predominance of low subsurface water storage capacities. These findings give a large-scale indication of how floods and runoff are generated in Brazil, supporting flood forecasting and climate-change impact studies.