Modeling of non-floodplain wetlands in an upstream basin of the Pampa Plain, Argentina, during current hydro-climatic extreme conditions: Geomorphological thresholds controlling hydrodynamics

Abstract

This contribution analyzes the processes related to the development of small, temporary, non-floodplain wetlands (NFWs) of neotectonic origin in the loessic North Pampa Plain under wet conditions. The study focuses on the Vila-Cululú upstream sub-basin (973 km²), a tributary of the Salado River belonging to the Paraná River basin. Under wet conditions, the flat landscape influences the surface runoff and favors the development of temporary NFWs, increasing flood vulnerability and compromising human activities. Late Pleistocene fluvial palaeochannels intercept small tectonic depressions and significantly restrict the present drainage network (low-order streams and artificial channels). This research involves an integrated approach, including geomorphic and morphometric analyses based on remotely sensed satellite imagery in a GIS platform and fieldwork, and 2D hydrologic-hydraulic simulations using HydroBID Flood to capture the system behavior during a rainfall event that recently occurred over several weeks and caused extended flooded areas. Simulation results show that the model significantly captures the system's hydrodynamics. The resulting flooded areas were comparable to those obtained from the analysis of satellite images. The dendritic runoff pattern towards the tectonic depressions, the water storage evolution, and the hydraulic connectivity were numerically replicated. Results indicates the Vila-Cululú sub-basin shows a significant delay in the hydraulic response downstream since the system must first satisfy groundwater and surface water storage. Once storage capacity is exceeded, the hydraulic behavior results in a dynamic process that involves the spilling and merging of ponds generated in small deflation hollows, generally nested within fluvial palaeochannels. Such a hierarchical structure controls surface runoff towards the shallow tectonic depressions. This mechanism allows the development of NFWs as simulation time progresses. Additionally, the surface runoff flow pattern exhibits significant lateral mobility and low connectivity downstream, although this varies depending on the magnitude of the storm. Despite the dense network of artificial channels developed to evacuate water excess to the outlet, geomorphological thresholds further control hydrodynamics by adding to surface water storage and limiting channel conveyance. Identifying critical geomorphological thresholds add to the knowledge of different levels of hydrological connectivity, providing a better assessment of flood hazards. This work is one of the first studies in North Pampa in which hydrological and geomorphological data are combined to explain the present hydrodynamics and that could be applied to other fluvial basins of large plains of South America.