Trade off relationships: incorporating ecosystem spatial scale and timing issues into the decision making process

In large hydropower-dominated river basins, reservoirs are mainly operated so as to maximize revenues fromenergy generation regardless of the consequences of reduced flow fluctuation on downstream ecosystems. Revenuefrom hydropower plants is essential to a country’s economy; however the impact on ecosystems downstream canbe large-scale. The timing of flow releases does not mimic natural systems, which has impacts over differenttemporal and physio-spatial scales to the ecosystem. To inform decision making often hydro-economic modelingis used and it is essential that the response of the system is understood and incorporated adequately into assessmentdesign, to allow for trade-offs to be identified. This requires issues of timing and spatial scale to be understoodand incorporated over different planning horizons. Nesting these issues of scale into decisions is complex; wheredecisions are made on timescales from hours to months and spatially within a basin depending on the operation ofthe system. Up-scaling the most critical interactions between flow, form and ecosystem into the decision makingprocess, for different time horizons or planning scales, is essential. A proposed framework is illustrated with theZambezi basin.