ИСТИНА

A long-term streamflow forecast is crucial for large-scale water reservoirs operation, as water management plans are designed for several months ahead in order to distribute the available resources between stakeholders. Water regime in the European Russia is characterized by high snowmelt-driven spring freshet followed by long summer low-flow periods hence adequate tools to assess the available water resources during these periods are required. An approach to seasonal ensemble forecast of unregulated water inflow into a large reservoir has been developed involving a physically-based semi-distributed hydrological model ECOMAG driven by ensembles of future weather scenarios for a specified lead-time of the forecast. Case study was carried out for the Cheboksary reservoir (cathment area is 374 000 km2) on the middle Volga River. Initial watershed conditions on the date of the forecast (1st of March for spring period and 1st of June for summer) were simulated by the hydrological model forced by daily meteorological observations several months prior to the forecast date. To assign ensemble of weather scenarios for the lead-time of the forecast (3 months ahead in this study), two approaches were applied: (1) the historical, observed daily weather patterns, which assumed to be representative of possible future weather conditions; and (2) the artificial daily weather patterns simulated by a stochastic weather generator. Hindcasts were produced for spring/summer seasons beginning from the filling of the reservoir in 1982. Statistics of the hydrological extreme indicators (volume, duration, and severity of floods and droughts) were derived from the forecasted hydrograph ensembles and evaluated against the corresponding indicators obtained from observations.