Influência do uso da terra nos parâmetros limnológicos da bacia hidrográfica do rio Itu - RS

Abstract

With a 2,809.61 km² area the Itu River Basin is placed in the western side of Rio Grande do Sul. This region, the so-called Campanha Gaúcha, is traditionally used for livestock as it presents plenty of native grasslands. However major changes have been noticed during the last decades in the area activities such as agriculture and forestry use of the fields instead. Sandy desertification and gully erosion have also taken place due to very friable geological substrates. So this work aims to analyze the spatial and temporal changes of the land use in five micro-basins around Itu River and their relationship with limnological variables. Descriptive analysis of the five micro-basins were performed from the slope, hypsometry, soils and land use. There were selected images captured by the Landsat TM satellite from January months of 1991, 1999 and 2010 in order to: 1) check the cover and land use evolution by using maps; 2) elaborate a synthesis map considering slope, cover and land use. The land use classes were defined as: forest, field, bare soil, agriculture and water. Hypsometry and slope maps were made from SRTM data while the soil types maps was elaborated with the shapefile downloaded from the IBGE website. The fieldwork was carried out in July 2013 in one sampling point per micro-basin. The water variables analyzed were temperature, pH, electrical conductivity, turbidity and total suspended solids (TSS). The bare soil is the kind of land use which stands out most among the micro-basins as it appears with high percentages. The micro-basins 3 and 4 are the ones which present most significant values for this land use class due to the low desertification in the areas where they are located in. The locations closer to the Itu River springs (most slopped areas) lead to the micro-basins 1 and 2 being the only ones which show a rising forest class. The presence or absence of riparian forest surrounding watercourses may have been influencing the temperature change as well as the turbidity and the amount of solids in suspension. For micro-basin 3 the latest can also have been influenced by the presence of sand on the riverbanks. In their turn pH and electrical conductivity are likely to have been affected by the red latosol which is the predominant soil of the region and presents an acid character (the pH is due to the type of soil where water flows through