Land use/land cover changes and hydrological response in the Piracicaba river basin This study aimed at to evaluate the effect of changes in land use and land cover over the last 37 years (1972 to 2008) on the hydrological response in the Piracicaba river basin, SP. The main changes in the land use and land cover in the basin over that period were the increase in the urban area, decrease in the pasture area, and increase in the sugarcane plantations. Moreover, the basin has a peculiarity, which is the presence of the Cantareira System, which diverts water to the metropolitan region of São Paulo (RSMP), since 1974. We used a hydrological model of Great Basin (MGB), which has the advantage of evaluating in the distributed form, the variation of the land use/land cover, besides the flow propagation. The model was adjusted to the Piracicaba river basin (12,245 Km 2 in area) to simulate daily streamflow. Daily data, from 1972 to 2008, were used for the process of model fitting, from 12 streamflow stations, 61 rainfall and 20 meteorological, digital terrain model, maps of soils and land use and land cover (for the years 1978, 1983, 1985, 1997 and 2003), and parameters that are important to differentiate soil types and uses and land cover. The process of simulation of the streamflow was carried out in two stages: calibration of parameters (from 1972 to 1990) and verification (1991 to 2008). To assess the quality of fitting it was considered the visual analysis of the hydrographs (observed and calculated streamflows) and the values of objective functions (R2, Rlog and ∆V). We performed a sensitivity analysis of each parameter, in order to verify the influence of each one. The results showed a good fitting between the daily observed and calculated streamflows. The best results of the objective functions were obtained for the sub-basins of larger drainage area. The Cantareira System strongly influences the simulation of daily streamflow. Sensitivity analysis of parameters revealed that the simulation results of the model are extremely sensitive to variations of the parameters: soil water storage capacity (Wm), form parameter of the relationship between storage and saturation (b), parameters of drainage underground (K BAS) and subsurface (K INT), albedo and surface resistance (r S). Alternative scenarios for changes in the land use and land cover were tested and compared the simulation of the scenario actual of Piracicaba river basin in the period between 1972 and 2008. Comparing the scenario with no changes in the basin since 1972 to the actual scenario, a reduction in streamflow was detected: maximum of 2,5 m 3 /s, minimum of 2,1 m 3 /s, and an average of 1,9 m 3 /s. Other scenarios were tested and compared to the actual scenario, and it was detected an increase of the mean streamflow of 17,2 m 3 /s for scenario 100% of agriculture, and a decrease of the mean streamflow of 3,9 m 3 /s for the scenario of conversion from agriculture into pasture.
