As mudanças no uso da terra no Bioma Pampa, nas últimas décadas, principalmente relacionadas a transformação de pastagens nativas (campos) em áreas de cultivo, como o plantio de soja, arroz ou árvores (especialmente eucalipto), levantam questionamentos sobre os impactos e potenciais mudanças na hidrologia. Algumas bacias hidrográficas já registraram cerca de 25% de suas pastagens transformadas em outros usos desde 2010, e essas transformações impactam na dinâmica hídrica. Este estudo analisa a vazão na bacia hidrográfica do Rio Santa Maria, no sul do Brasil, que abrange 14.900 km² e representa cerca de 8% da área do Bioma Pampa brasileiro, em um cenário de mudança de uso do solo no período de 2010 a 2019 com modelagem hidrológica usando o Soil and Water Assessment Tool (SWAT). Os resultados demonstram que a mudança de uso do solo altera tanto o balanço hídrico quanto a vazão da bacia. A vazão média anual aumentou 17,7%. Os picos de vazão de água são mais frequentes (16% mais para os picos com vazão média diária acima de 500 m3.d-1) e o pico máximo aumenta em 17,6%. Isso se explica pela expansão das lavouras de soja, principalmente nos solos mais sensíveis ao escoamento superficial. Os resultados sugerem que a substituição de áreas de pastagens nativas por culturas anuais aumenta o risco de inundações.Palavras-chave: Modelagem hidrológica; Terras agrícolas; Pastagem; SWAT; Bioma Pampa Flow rate and water balance related to land use changes in a medium-sized watershed in the Pampa Biome of Brazil Abstract: Changes in land use in the Pampa Biome, especially in recent decades, with the transformation of native rangeland into cropland, such as the soybeans, rice, or trees plantation (especially eucalyptus), raise questions about the impacts and potential changes in local hydrology. Some watersheds have already registered about 25% of their rangeland transformed into other uses since 2010, and these transformations impacts the water dynamics. This study analyses the flow rate in the Santa Maria River watershed, in southern Brazil, which covers 14,900 km² and represents about 8% of the area of the Brazilian Pampa Biome, in a scenario of land use change in the period from 2010 to 2019 through hydrological modeling using the Soil and Water Assessment Tool (SWAT). The results demonstrate that the land use change alters both the water balance and the discharge of the watershed. The mean annual flow increases of 17.7%. The water flow peaks are more frequent (more 16% for the peaks with a mean daily flow over 500 m3.d-1) and the maximum peak increases by 17.6%. It is explained by the expansion of soybean crops, particularly on the soils more sensitive to superficial runoff. The results suggest that replacing native rangeland areas by annual crops increase the risk of flooding.Keywords: hydrological modeling; cropland; rangeland; SWAT; Pampa Biome

Changes in land use in the Pampa Biome, especially in recent decades, with the transformation of native rangeland into cropland, such as the soybeans, rice, or trees plantation (especially eucalyptus), raise questions about the impacts and potential changes in local hydrology. Some watersheds have already registered about 25% of their rangeland transformed into other uses since 2010, and these transformations impacts the water dynamics. This study analyses the flow rate in the Santa Maria River watershed, in southern Brazil, which covers 14,900 km² and represents about 8% of the area of the Brazilian Pampa Biome, in a scenario of land use change in the period from 2010 to 2019 through hydrological modeling using the Soil and Water Assessment Tool (SWAT). The results demonstrate that the land use change alters both the water balance and the discharge of the watershed. The mean annual flow increases of 17.7%. The water flow peaks are more frequent (more 16% for the peaks with a mean daily flow over 500 m3.d-1) and the maximum peak increases by 17.6%. It is explained by the expansion of soybean crops, particularly on the soils more sensitive to superficial runoff. The results suggest that replacing native rangeland areas by annual crops increase the risk of flooding.