Funções das áreas hidrologicamente frágeis na formação e modelagem do escoamento superficial na bacia experimental do Arroio Lajeado Ferreira, Arvorezinha - RS

Abstract

Physically based and distributed erosion models like the Limburg Soil Erosion Model (LISEM) are complex and require high-quality information, equations aligned with the processes, and accurate spatial representation of controlling factors. However, the equifinality of parameter fitting, combined with simplifications in spatial representation, introduces uncertainties in the portrayal of certain processes. Initial soil water content (SWC) and saturated hydraulic conductivity (Ksat), which are related to the infiltration process and formation of surface runoff, are the most sensitive parameters in the hydrological calibration of the model. Studies indicate the need to force a reduction in conductivity values beyond their physical context on slopes, and there is a tendency to overestimate sediment production by the model in this catchment. This study tests the hypothesis that slope lines, variable source areas, riparian zones, and roads generate a significant proportion of surface runoff, and their representation is crucial for describing water and sediment connectivity in the basin. These landscape features result from the characteristics of topography and soil (impermeable surfaces, low Ksat, concentrated surface flow, and profile saturation). In the model, they served as calibration areas for SWC, Ksat and soil cohesion values. They were defined based on high-resolution digital terrain models, on which the topographic wetness index (TWI) was applied to determine regions susceptible to soil moisture conduction, concentration, and saturation. The model's performance was evaluated by simulating 150 rainfall events of varying magnitudes from 2014 to 2022. In the results, 98 events showed a Nash-Sutcliffe Efficiency (NSE) indicator greater than 0.5, indicating good to very good performance. It was concluded that the LISEM model demonstrates structural and functional capacity to represent landscape features that have a significant impact on hydrological and erosive processes. Through robust calibration using monitored events, the model maintained appropriate values of hydraulic conductivity estimates on slopes and calibrated soil moisture values consistent with physical reality. However, regarding sediment behavior, the model's representation of hydrologically fragile areas still carries uncertainties in simulating suspended sediment concentration and sediment yield. Nevertheless, it provided a better estimation of surface runoff formation on agricultural slopes, where concentrated flow causes disaggregation. The representation of concentrated flows on an expanded drainage network improved the response of runoff and the representation of sediment depletion in preferential pathways.