Respostas Hidrológicas a Diferentes Cenários de Cobertura da Terra em Áreas de Captação Hierarquizadas de uma Bacia Hidrográfica do Extremo Sul do Brasil

Abstract

Land cover is an important conditioning factor for the hydrological dynamics of watersheds, just as the drainage hierarchy corresponds to one of its main morphometric characteristics. The present study is based on the simulation of hydrological responses to different land cover scenarios, considering the influence of the hierarchization of the catchment areas of the Cadeia river watershed (CRW), in the extreme south of Brazil. The main objective is to evaluate the influence of hierarchization of catchment areas in the analysis of the hydrological behavior of this watershed, in response to different coverage scenarios. CRW occupies an area of 121.3 km2 , has undulating relief, soils of the argisol class, drainage density equal to 1.83 km/km2, and river hierarchy of 5th order. The current land cover corresponds to a mosaic of forest, grassland, agriculture and exposed soil, unlike the natural cover, characterized by the wide predominance of forest. To support the intended simulations, 19 land cover scenarios were created, organized into two groups: I) full coverage scenarios, in which the entire watershed has only one coverage class, and the reference scenario is the current coverage; and II) coverage scenarios by hierarchical catchment area, which assume the exposed soil as reference, presenting the other classes restricted to catchment areas of the same hierarchy level. In the modeling stage, the Limburg Soil Erosion Model (LISEM) was used, due to its ability to simulate surface runoff taking into account the variability of soil cover in the watershed. With regard to rainfall data, three rainfall events were selected as representative of the late, intermediate and early patterns. The results of the simulations involving the full coverage scenarios revealed the highest hydrological responses (direct surface runoff, peak discharge and peak time) for the exposed soil scenario. The other scenarios showed an attenuation of responses, and the magnitude of this attenuation varied significantly in each scenario, resulting in the sequence forest > grassland > current coverage > agriculture. For the coverage scenarios by hierarchical catchment area, the simulations of the forest, grassland and agriculture scenarios point, as progress is made in the hierarchy, to a clear tendency towards an increase in the attenuating impact of the hydrological responses in relation to the exposed soil scenario. This behavior pattern is consistent with the recommendation to prioritize areas whose runoff directly supplies 5th order segments in strategies that seek, for example, reforestation as a measure to control floods associated with heavy rain events in the BHAC. Further studies on this subject are recommended.