Résumé
In Asia, many dams are currently managed to satisfy society's immediate needs: power generation and food production. Environmental criteria are often overlooked and traditional reservoir management often leads to alteration of the natural flow regimes, in turn leading to degradation of river-dependant ecosystems. However, the idea of maintaining a flow that resembles the natural flow regime in rivers is increasingly recognized as important, but it potentially conflicts with food and power production objectives. Therefore, a tradeoff analysis among these three objectives could help generating efficient use of the resource and preserving ecosystem services. The management of Kinda dam, in the Panlaung River basin (central part of Myanmar) is taken as a case study. Completed in 1990, it was designed as a multipurpose dam. Its catchment area is about 2,240 km2, with a power generation capacity 165 GWh/year. It can potentially deliver water to an irrigable area of 70,000 ha. This research first estimates the environmental flow (EF) requirements in the river using a set of Indicators of Hydrologic Alteration (IHA) to characterize the natural and altered flow regimes using IHA parameters grouped into five categories, i.e. magnitude, duration, timing, frequency and rate of change. IHA software was used to evaluate the alteration in flow regimes using the daily flow data series from 1961 to 1981 for the pre-dam period and from 2002 to 2011 for the post-dam period. We then simulate the operation of the reservoir under alternative objectives using HEC-ResSim model. The impact of water availability and management in the irrigated areas are evaluated with the help of CROPWAT, based on available climatic data and cropping patterns. Contrasted water management scenarios were evaluated in order to quantify those tradeoffs: (a) existing reservoir management rules and irrigation demand, (b) modified reservoir management rules in order to optimize power generation, and unaltered irrigation management, (c) scenario (b) with an additional constraint to maintain EF; (d) modified reservoir management rules in order to maximize food production; (e) scenario (d) with an additional constrain to maintain EF levels at 25, 50, 75 and 100% required EF ; Imposing EF constraints result in differentiated impacts on the two main sectors: negative for the rice production and positive for power generation (as compared to current rules that gives priority to irrigation). At basin level, lost in agricultural revenues could partially be compensated by higher electricity production. However, the net effect is negative. This correspond to a loss of 64 USD/ha of irrigated plots (i.e. a loss of around 1,000,000 USD/year) while the power generation could increase generating some additional revenues (around 400,000 USD/year). To conclude, this method allows estimating the opportunity cost of EF in Panlaung River basin. However, it does not allow comparing these costs to their potential benefits (biodiversity and diverse use values such as fisheries). Besides, with potential gainers and losers, policy makers still need to decide who would have to bear these costs (possibly via negotiated monetary compensations across sectors). (Texte integral)