PNNL

Abstract

Considerable research effort has recently been directed at improving and operationalising ensemble seasonal streamflow forecasts. Whilst this creates new opportunities for improving the performance of water resources systems, there may also be associated risks. Here we explore these potential risks by examining the sensitivity of forecast value (improvement in system performance brought about by adopting forecasts) to changes in the forecast skill for a range of hypothetical reservoir designs with contrasting operating objectives. Forecast-informed operations are simulated using rolling-horizon, adaptive control and then benchmarked against optimised control rules to assess performance improvements. Results show that there exists a strong relationship between forecast skill and value for systems operated to maintain a target water level. But this relationship breaks down when the reservoir is operated to satisfy a target demand for water; good forecast accuracy does not necessarily translate into performance improvement. We show that the primary cause of this surprising behaviour is the buffering role played by storage in water supply reservoirs, which renders the forecast superfluous for long periods of the operation. The main driver of system performance is the forecast accuracy at the timing during which critical decisions are made—namely during severe drought. Our results emphasise the importance of forecast skill consistency and highlight a need for sensitivity assessment in value-of-forecast studies involving reservoirs with supply objectives.