Abstract
ABSTRACT This study assesses the extent to which silting increases water-scarcity risk, considering the temporal reduction of water availability and increased demand using land use and water-demand scenarios at the transition of Caatinga and Cerrado biomes of the Bocaina reservoir watershed (103hm4), in the Brazilian dry region. Methodological steps were: reservoir silting measured in-situ 20 years after dam construction; climate variables computed with the aid of a conventional station (2005-2014); soil erodibility assessed using 16 soil samples; and topography and land cover estimated based on 21 years of Landsat imagery. Three land use scenarios were generated (invariability, degradation and preservation) with the climate scenario derived from the semi-arid rainfall temporal variability; whereas two water-demand scenarios (invariability and higher efficiency) were a function of the efficiency of the irrigation systems. Water availability was calculated using the volume-yield elasticity (VYELAS) Model. The field results (1985-2015) showed a gross erosion rate of 13.5 Mg-ha-1 -yr-1 in the basin. The annual sediment yield (1.7 Mg-ha-1) and the decadal reservoir silting (1.0%) were below regional average due to the low sediment delivery ratio (12.6%) in the area. Scenario projections (2040) suggest water demand may double if irrigation methods do not improve, whereas siltation may cause water availability to decrease up to 10% in the period. In this case, the water-supply reliability will be below the recommended standard value (90%), regardless of the land use scenario. Nevertheless, simultaneous soil preservation and improved irrigation efficiency can reduce the decadal water-scarcity risk from 82% (worst scenario) to 17%.
Highlights
In dry environments, the principal aim of surface reservoirs is to improve water availability
In the Brazilian dry northeastern region, surface reservoirs provide more than 90% of the water supply (GAISER et al, 2003)
Water demand scenarios: we evaluated the following water demands associated with the Bocaina reservoir: irrigation, urban human consumption, animal consumption and ecological discharge
Summary
The principal aim of surface reservoirs is to improve water availability. In the Brazilian dry northeastern region, surface reservoirs provide more than 90% of the water supply (GAISER et al, 2003). Reservoirs are prone to silting, which reduces their storage capacity and changes their morphology towards a more open shape. These alterations increase evaporation and water outflow and reduce water availability (ARAÚJO; GÜNTNER; BRONSTERT, 2006), which is deleterious in water-scarce areas, such as in northeastern Brazil. Water demand expands with time and generates an increasingly unbalanced system. Better knowledge of silting impacts on reservoirs improves the efficiency of local water management, especially in waterscarce environments, where water grants should be judiciously conceded
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