Page - 392 - in Book of Full Papers - Symposium Hydro Engineering

5. ADJUSTMENT OF OPERATION Once the SPI (or indices in general) is calculated based on past and forecasted values, it can be used to adjust operation of reservoirs. Adjustment cannot be established in a general way but requires site-specific rules. However, what can be generalized is the way in which the results of indices are introduced. First, site-specific operation rules best suited for intervention must be identified. Second, threshold values for indices must be identified for when intervention should be triggered. Third, the aggregation period is highly dependent on the local situation and needs to be determined for each individual case. Reservoirs in West-Germany with catchment areas ranging from 250 to 600 km², operated with pool-based rules, could be improved by using a 9 to 12 month aggregation period and a threshold value of -1.5 (SPI + Evaporation based indices). When the index dropped below -1.5, release rules associated with the next lower pool were applied to counter an expected decrease of inflow. This meant a reduction of downstream releases or a different release pattern related to inflow and depending on the time of year and current storage volume. Not surprisingly, not all critical periods could be identified by applying this approach. However, two thirds of critical low flow conditions with corresponding low water levels in the reservoirs could be tackled in a timely way. The approach was used without forecasts prior to 2011 and as of 2011 with forecasts to make full use of the observation period with more than 100 years. In central Germany, a reservoir with a catchment area of less than 50 km² revealed a different pattern. Only aggregation periods longer than 18 months with a threshold value of -1.0 showed good results. Shorter aggregation periods or lower threshold values were either not consistent enough or started too late to result in counter measures that took effect. In this case, the target operation rules for intervention was water supply provision. Similar to a hedging rule, water provision was subjected to a quota of a rather small percentage as soon as the index dropped below -1.0 to prevent larger reductions later on. In doing so, the reservoir could be mostly kept above a water level that becomes critical from the viewpoint of water quality. The initial assumption that the size of a catchment area is a reasonable parameter for estimating aggregation periods could not be confirmed. The interplay between climate, the catchment area’s geology as an indicator for inertia and the reservoir itself seems more complex. As a result, each reservoir or reservoir system must be individually scrutinized to find the best set of aggregation periods and threshold values. ACKNOWLEDGEMENTS The project is funded by the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB). 392