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Water availability reduction in reservoirs for different sediment yield scenarios in a semiarid watershed

IWRA World Water Congress 2008 Montpellier France
1. Water availability, use and management
Author(s): José Carlos de Araújo
Pedro Henrique Augusto Medeiros
José Carlos de Araújo Dept. Agricultural Engineering Federal University of Ceará Pedro Henrique Augusto Medeiros Graduate Student Federal University of Ceará

Keyword(s): Water availability, reference scenarios, reservoir silting, semiarid, Brazil

AbstractThe impact of anthropogenic actions on water availability is especially important in semiarid regions, where conflict for water is already a reality. In the State of Ceará, Brazil, home of 8 million inhabitants, recurrent droughts have been dealt with historically by construction of dams. Nonetheless, this approach has proven to be only a partial solution: there are about 30,000 dams in the 150,000 km2 State, but the problem of water scarcity still prevails. This research evaluates the impact of different reference scenarios on reservoir silting and, therefore, on water availability in the Salgado river watershed (12,200 km2 and 23 municipalities), South of Ceará. Three reference scenarios are described and their impacts assessed for the 15 largest dams in the watershed, for a five-decade period. The water yield used in the research refers to 90% annual reliability (Q90). Reference Scenario I assumes that sediment yield remains, for the next 50 years, the same as observed in the State for the last 50 years (Araújo and Knight, 2004), i.e., the land-use policy would constrain the use of new areas to compensation environmental practices. Reference Scenarios II and III assume that sediment yield increases proportionally to the increase of the irrigation area in the State according to IPCC scenarios A (RSA) and B (RSB), as presented by Doell and Hauschild (2002). RSA refers to “Coastal Boom and Cash Crops” and admits increasing agricultural production not only for local, but also for external markets; enhancement of tourism along the coast; and rapid GDP growth of the metropolitan area of Fortaleza. RSB, which corresponds to “Decentralization and Integrated Rural Development”, assumes strengthening of medium-sized towns, extension of small-scale agro-industry, and State autonomy in relation to the Brazilian South. The water yield versus reliability simulations were performed using the VYELAS model (Araújo, Güntner and Bronstert, 2006), based on Monte- Carlo method applied to reservoir water budget. Results for Scenarios I, II and III indicate Q90 reduction, in the research period, of 4.5%, 14.2% and 5.5%, respectively. The difference of water availability, for a five-decade horizon, considering the most optimistic (I) and the most pessimistic (II) scenarios is 9.7 Mm3.yr-1, which is enough to supply water to 88 thousand inhabitants, considering per capita withdrawal of 300 litres per day. The results show that planning and implementation of a conservationist land-use policy (scenario I against scenario II) can save up to 10% of surface water yield on the long run. The research also presented the possibilities of joint use of scenarios and hydrological modelling for environmental planning.
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