Prediction of Water Quality in the Danube River Under extreme Hydrological and Temperature ConditionsOne of the requirements imposed by the Water Framework Directive (WFD, 2000/60/EC) is to analyze and predict how quality of surface waters will evolve in the future. In assessing the development of a stream's pollution one must consider all sources of pollution and understand how water quality evolves over time. Flow and water temperature regime of a stream or river are the main factors controlling the extent to which deterioration of a stream's water quality can propagate under constant input from pollution sources. In addition, there is ever increasing public concern about the state of the aquatic environment. Decision makers and scientists involved in water management call for studies proposing simulation models of water quality under extreme natural hydrologic and climatic scenarios. Also, human impact on water resources remain an issue for discussion, especially when it comes to sustainability of water resources with respect to water quality and ecosystem health. In the present study we investigate the long-term trends in water quality variables of the Danube River at Bratislava, Slovakia (Chl-a, Ca, EC, SO2-, Cl-, O2, BOD5, N-tot, PO4-P, NO3-N, NO2-N, etc.), for the period 1991-2005. Several SARIMA models were tested for the long-term prediction of selected pollutant concentrations under various flow and water temperature conditions. In order to create scenarios of selected water quality variables with prediction for 12 months ahead, three types of possible hydrologic and water temperature conditions were defined: i) average conditions - median flows and water temperature; ii) low flows and high water temperature; and iii) high flows and low water temperature. These conditions were derived for each month using daily observations of water temperature and daily discharge readings taken in the Danube at Bratislava over the period 1931-2005 in the form of percentiles (1th-percentile, median, 99th-percentile). Once having derived these extreme-case scenarios, we used selected Box-Jenkins models (with two regressors - discharge and water temperature) to simulate the extreme monthly water quality variables. The impact of natural and man-made changes in a stream's hydrology on water quality can be readily well simulated by means of autoregressive models.

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The aim of this study is to look at the impacts of land use and climate change on extreme runoff regimes in selected catchments of Slovakia, with an emphasis on selected characteristics of hydrological regimes, catchment runoff, and, especially extreme runoff. Changing climate conditions, characterized especially by changes in precipitation, air temperatures, and potential evapotranspiration in future decades, have been predicted by recent outputs of the KNMI and MPI regional climate change models and the A1B emission scenario. The land use changes were characterized by various future land use scenarios. Assuming these scenarios are accurate, the hydrological regime characteristics were simulated by the WetSpa distributed rainfall-runoff model, which was parameterized for the selected river basins with a daily time step until 2100. Changes in the total runoff and its components (the maximum and design discharges), as well as changes in soil moisture and actual evapotranspiration, compared to the current state, confirm the assumption of an increase in the extremes of the hydrological regimes during periods of flood events. The results of the study showed a need for a reevaluation of design discharge values for future designs of water management structures.

In order to estimate possible changes in the flood regime in the mountainous regions of Slovakia, a simple physically-based concept for climate change-induced changes in extreme 5-day precipitation totals is proposed in the paper. It utilizes regionally downscaled scenarios of the long-term monthly means of the air temperature, specific air humidity and precipitation projected for Central Slovakia by two regional (RCM) and two global circulation models (GCM). A simplified physically-based model for the calculation of short-term precipitation totals over the course of changing air temperatures, which is used to drive a conceptual rainfall-runoff model, was proposed. In the paper a case study of this approach in the upper Hron river basin in Central Slovakia is presented. From the 1981–2010 period, 20 events of the basin’s most extreme average of 5-day precipitation totals were selected. Only events with continual precipitation during 5 days were considered. These 5-day precipitation totals were modified according to the RCM and GCM-based scenarios for the future time horizons of 2025, 2050 and 2075. For modelling runoff under changed 5-day precipitation totals, a conceptual rainfall-runoff model developed at the Slovak University of Technology was used. Changes in extreme mean daily discharges due to climate change were compared with the original flood events and discussed.

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This paper analyses the projected changes in short-term rainfall events during the warm season (April-October) in an ensemble of 30 regional climate model (RCM) simulations. The seasonality analysis was done for the Hurbanovo, Bratislava, Oravska Lesna, and Myjava stations in Slovakia. The characteristics of maximum rainfall events were analysed for two scenario periods, one past and one future (1960-2000 and 2070-2100) and compared to the characteristics of the actual observed events. The main findings from the analysis show that short-term events of 60 minutes appear to have stronger seasonality than daily events that show a rather high variability. The seasonality concentration index calculated for the 60 min events averages to 0.77, while that of daily events averaged to 0.65. The differences between the dates of the occurrence of past and future events are not significant in the lowland areas, while in the mountainous areas the future events have been found to occur earlier than past ones.

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