Description: An approach is proposed to assess hydrological simulation uncertainty originating from internal atmospheric variability. The latter is one of three major factors contributing to uncertainty of simulated climate change projections (along with so-called "forcing" and "climate model" uncertainties). Importantly, the role of internal atmospheric variability is most visible over spatio-temporal scales of water management in large river basins. Internal atmospheric variability is represented by large ensemble simulations (45 members) with the ECHAM5 atmospheric general circulation model. Ensemble simulations are performed using identical prescribed lower boundary conditions (observed sea surface temperature, SST, and sea ice concentration, SIC, for 1979–2012) and constant external forcing parameters but different initial conditions of the atmosphere. The ensemble of bias-corrected ECHAM5 outputs and ensemble averaged ECHAM5 output are used as a distributed input for the ECOMAG and SWAP hydrological models. The corresponding ensembles of runoff hydrographs are calculated for two large rivers of the Arctic basin: the Lena and Northern Dvina rivers. A number of runoff statistics including the mean and the standard deviation of annual, monthly and daily runoff, as well as annual runoff trend, are assessed. Uncertainties of runoff statistics caused by internal atmospheric variability are estimated. It is found that uncertainty of the mean and the standard deviation of runoff has a significant seasonal dependence on the maximum during the periods of spring–summer snowmelt and summer–autumn rainfall floods. Noticeable nonlinearity of the hydrological models' results in the ensemble ECHAM5 output is found most strongly expressed for the Northern Dvina River basin. It is shown that the averaging over ensemble members effectively filters the stochastic term related to internal atmospheric variability. Simulated discharge trends are close to normally distributed around the ensemble mean value, which fits well to empirical estimates and, for the Lena River, indicates that a considerable portion of the observed trend can be externally driven.

Description: With global warming expected to be more pronounced in the Arctic regions, soil erosion and associated sediment input to river systems is subsequently subject to increase here, too. This study attempts to understand complex processes involved in runoff generation, and sediment transport in the Lena River basin, and to quantify their respective fluxes at the river outlet. To assess the rate of soil erosion in the Lena River basin we augmented the regional distributed hydrological model ECOMAG with a soil erosion calculation routine MUSLE model. The ECOMAG model is used to generate runoff from a distributed network of elementary catchments, each having a unique distribution of hydrological response units accounting for various soil, vegetation and groundwater properties, as well as seasonal thawing of permafrost. For each elementary catchment the MUSLE routine was calculated on a daily timestep to assess the amount of sediment entering the stream network during snowmelt and rainfall. The model was calibrated using the available measurement data of streamflow and sediment runoff from the river gauging network. The selected approach allowed for correct simulation of the observed streamflow discharge and sediment load time-series and is applicable for the impact studies in this region. The study was conducted under RSF project 21-17-00181.

Description: An approach to reducing uncertainties in modeling of river runoff sources by expanding the information support of the hydrological model with hydrochemical data is presented. The object of the study is the upper reaches of the Moskva River with an area of 1360 km〈sup〉2〈/sup〉. The physically based ECOMAG model was used to simulate hydrological processes in the catchment area. Numerical experiments demonstrated a strong equifinality effect, in which several different sets of the model calibrated parameters produce equally good results of simulation of runoff hydrograph, but contribution of the water sources (quick flow, interflow and baseflow) to the total runoff significantly differ. In order to reduce the equifinality effect, the proposed method allows simultaneous use of conjugate calibration by the total hydrographs of the daily runoff and additional calibration by the contribution of the base flow determined by hydrochemical data. Specific marker compounds of environmental tracers of the water sources were used to support the delineation of source contributions to the mixed-signal observed in the river: specifically chemical oxygen demand (permanganate, dichromate), specific electrical conductivity, the concentrations of Na+ and K+ ions and total phosphorus. Using the statistical approach and the mass balance method, the contributions of various sources to the river runoff were determined. The proposed approach usually ensures the modeling accuracy of the runoff sources in different phases of the water regime in the order of 10-20%. This study was carried out under Governmental Order FMWZ-2022-0003.