ИСТИНА

An approach has been proposed to analyze hydrological extreme simulation uncertainty originating from the internal variability of the atmosphere (“climate noise”), which is inherent to the climate system and considered as the lowest level of uncertainty achievable in climate impact studies. Importantly, the role of the climate noise is dominating over spatial-temporal scales of water management in large river basins. To assess the climate noise effect, numerical experiments were made with climate model ECHAM5 and hydrological model ECOMAG. The case study was carried out to Northern Dvina River basin (catchment area is 360,000 km2), whose hydrological regime is characterised by extreme freshets during spring-summer snowmelt period. The climate noise was reproduced by repeating ECHAM5-runs, when only initial conditions are changed by small perturbations; external forcing parameters were corresponded to the modern climate conditions and did not vary. Totally, 45 model runs were made under the different initial conditions. An ensemble of the ECHAM5-outputs for the period of 1979-2012 was used (after bias correction post-processing) as the hydrological model inputs, and the corresponding ensemble of 45 multi-year hydrographs was simulated. Mean values of flood volume and peak discharge, as well as inter-annual variance of these characteristics, were derived from the simulated ensemble of hydrographs. Uncertainty of the derived statistics was estimated, and these statistics were compared with the corresponding ones obtained from the observed streamflow series. We found that uncertainties of the extreme flood statistics are sensitive to the climate noise, with the uncertainty of the flood characteristic variance is much higher than the uncertainties of the mean value.