Hydrological Impact of a High-Density Reservoir Network in Semiarid Northeastern Brazil

Dense reservoir networks, with thousand of small dams, can be increasingly found throughout the world, especially in water-scarce environments, such as the Brazilian northeastern region. Although the effect of individual small dams might be negligible, their joint effect has proved to be relevant on water and sediment connectivity. Literature, however, is scarce concerning the effect of such networks on water availability and/or sustainability of mesoscale or large-scale basins. This research intended both to assess the effect of the dense reservoir network of the semiarid Upper Jaguaribe Basin (UJB; 24,200 km2 , in Brazil) for a 45-year period (1961–2005) and to investigate a network arrangement that maximized its hydrologic sustainability. Imagery of the years 1970 and 2002 was analyzed to assess temporal evolution of the network. The Water Availability in Semiarid Environments (WASA) model, which proved valid for the Upper Jaguaribe Basin, was used to assess its sustainability for almost 100 different arrangements. The optimization process was performed using the Simplex-MSX algorithm. Results showed that the hydrologic sustainability of the basin increased throughout the analysis period. Nonetheless, since the late 1990s, the system showed clear signs of saturation. The optimization process identified that the best network arrangement should have residence time of approximately 3 years, with volumetric density of 0.2 hm3·km-2 , and the importance of the reservoir size continuity to obtain effective hydrologic performance.