Pequena açudagem e sustentabilidade hidrológica em grandes bacias semi-áridas: estudo de caso da bacia do açude Orós

Abstract

One of the central challenges of researchers and water managers in large-scale watersheds is to evaluate the water availability in real conditions. In watersheds of the Brazilian semi-arid region, one of the main challenges consists of considering the effect of the proliferation of small reservoirs. This research has as the main objective to search the optimum arrangement for the system of accumulation in large watersheds in the semi-arid environment. The objective function to be optimized is the ASCE hydrological sustainability index (SI) of the watershed. In order to achieve the considered objective, the case of Orós dam (Ceará, Brazil) watershed (ca. 24000 km²) was studied. This watershed has presently more than four thousand small reservoirs, and it is strategic for the water supplying in the state. The WASA model (‘Water Availability in Semi-Arid Environments’) was applied to the watershed for the period from 1961 to 2005 for validation purposes. The robustness of the WASA model in predicting water availability in the large-scale watershed, considering the effect of thousands of small reservoirs, was proven. Using the optimization method of Simplex-MSX, 60 different arrangements for the small reservoirs in the watershed were simulated for a 45-year period with daily steps. For each arrangement the respective hydrological efficiency (assessed by the sustainability index) was estimated. The optimum arrangement allowed the establishment of parameters capable to guide the policy for small reservoirs in large semi-arid watersheds. The main results of the research showed that the maximum sustainability index was obtained for average contribution area of 7 km² per dam; and storage volume of the basin such that its time of residence is in the order of 3 years