Land use change analysis of the flooded area in the Guaiba Hydrographic Region in southern Brazil 2024

Abstract. The impact of extreme weather events is largely influenced by land cover, as demonstrated by the catastrophic flood in Rio Grande do Sul (RS), Brazil, in May 2024. Over 200mm of rain fell daily in numerous municipalities, displacing 2.3 million people. Although forests cannot entirely mitigate such extreme rainfall, they can help reduce runoff and related damages. We conducted a geospatial analysis to assess land use changes from 1985 to 2022 in the Guaíba Hydrographic Region, applying a three-step raster analysis using GIS tools. We classified pixels as natural or anthropogenic to monitor vegetation changes over four periods. Data processing efficiency improved significantly with a PostgreSQL approach, reducing query time from 20 hours to five minutes after a lenghthy initial pre-processing. Our findings indicated a higher long-term anthropogenic influence in flooded areas, with vegetation loss in Pampa Grasslands (PP) at 33.2%, compared to 18.1% for the Atlantic Forest (AF) and 16.8% in flooded areas. Between 1985 and 2022, we observed a fluctuating conversion rate of natural forests, with an overall loss in grasslands at an increasing annual rate. Soybean cover rose dramatically during these years, growing 430% until 2022, diminishing natural pastures in the Pampa biome. Our analysis emphasizes the effectiveness of forest protection policies while revealing that grassland areas remain poorly managed despite their crucial role in mitigating flood impacts.