Spatio-temporal patterns of hydrological processes on non-floodplain wetlands in an upstream basin of Pampa Plain (Argentina) during present wet conditions

CELLO, PABLO; KRÖHLING, DANIELA M.; BRUNETTO, ERNESTO; ZALAZAR, MARÍA CECILIA; GARCÍA, REINALDO; NALESSO, MAURO; ARTIGAS, JACINTO; CÓRDOVA, JOSÉ R

Viena

Congreso; EGU General Assembly 2024 (EGU-European Geosciences Union); 2024

Copernicus GmbH

This work aims at deepening the knowledge of the mechanisms that govern the response of smalltemporary non-floodplain wetlands (NFWs) of neotectonic origin in the North Pampa under wetconditions. The study focuses on the Vila-Cululú upstream sub-basin (973 km2), a tributary of theSalado River belonging to the Paraná River basin. The Pampa Plain has been affected by morefrequent high-intensity rainfall events during the last five decades giving rise to a steady increasein the water table and a decrease in soil infiltration, leading to flood events that impact both ruraland urban environments. Under wet conditions, a flat landscape alters the surface runoff andfavors the development of temporary NFWs, increasing flood vulnerability and jeopardizinghuman activities. Structural depressions with polygonal patterns and a network of Late Pleistocene(ca. 100 ka. BP) parallel ENE-trending fluvial palaeochannels characterize the study area. Thesepalaeochannels were deactivated by neotectonics and covered by loess, Last Glacial Maximum inage. In some sectors, the palaeochannels intercept the small tectonic depressions and significantlyrestrict the present drainage network (low-order streams and artificial channels). The researchinvolved an integrated approach, including geomorphic and morphometric analyses based onremotely sensed satellite imagery in a GIS platform and fieldworks, and 2D hydrologic-hydraulicsimulations using HydroBID Flood (hydrobidlac.org) to capture the system behavior for anextraordinary rainfall event (December 2016-March 2017). Simulation results show that the modelrepresents hydrodynamics fairly well. The flooded areas were comparable to those obtained fromthe analysis of satellite images. The dendritic runoff pattern towards the tectonic depressions, thewater storage evolution, and the hydraulic connectivity were numerically replicated. In particular,the Vila-Cululú sub-basin points out a significant delay in the hydraulic response downstreamsince the system must first satisfy groundwater and surface water storage. Once storage capacityis exceeded, the hydraulic behavior results in a dynamic process that involves the spilling andmerging of ponds generated in small deflation hollows, generally nested within fluvial palaeochannels. Such a hierarchical structure controls surface runoff towards the shallow tectonicdepressions. This mechanism gives rise to the development of NFWs as simulation time evolves.Besides, the surface runoff flow pattern also highlights the poor capacity of both natural andartificial drainage networks, displaying highly lateral mobility and scarce connectivity downstream.However, these NFWs eventually might connect to a more hierarchical drainage networkdownstream at the final stage of the storm event. The dense network of artificial channels startedto develop in the 1940s to evacuate water excess to the outlet. Despite the anthropicinterventions, geomorphologic thresholds finally control hydrodynamics adding to surface waterstorage and limiting channel conveyance. This work is one of the first studies in North Pampa thatcombines hydrological and geomorphological data to explain the present hydrodynamics. Thesecould be applied to palaeoflood hydrology. Identifying critical geomorphological thresholds addsto the knowledge of different levels of hydrologic connectivity, providing a better assessment offlood hazards on large plains.