The hydrological cycle of the lake watershed was simulated by the rainfall-runoff model WetSpa, which was developed by Liu and Smedt [1]. The model was extended for water quality simulation by development of empirical flow - concentration relationships. Particulate P concentrations were computed with a power function of the discharge. Dissolved P concentrations were established, depending on the soil and landuse types of the corresponding sub-catchments. Calibration and validation of the models were carried out for the main effluent Zala River and 14 inflowing smaller streams around the lake. Measured daily river flows and mostly weekly water quality data were available for the selected water bodies. The rest of the watershed (streams without monitoring data) was modelled with the calibrated parameters set.

Phosphorus concentrations in urban runoff were derived from field studies. The equations were developed from our earlier site measurements executed in small residential sub-catchments located in the shoreline zone [2]. In order to continuously model phosphorus concentrations in the runoff, a water quality submodel was added to the dynamic stormwater flow model SWMM [3], which was applied to the entire direct catchment area.

The models were henceforward run for climate change scenario analysis. The results indicate that climate change would have stronger impact on the variation of annual pattern than the changes in the annual averages of total runoff and diffuse phosphorus load. Assuming moderate social-economic development, the forecast impact is slightly appreciable. Alterations appear as a shift of the seasonal values of both runoff and T P loads (an increase in winter and spring; but a decrease in summer and autumn). However, the temporal decrease of the runoff probably does not induce permanent negative water balance of the lake. T P loads significantly increase in wet years due to a strong increment of precipitation volume in winter and springtime producing higher soil loss rates and associated P loads. The possible uncertainties of the climate change predictions (e.g. probability of extreme events) are certainly higher than the forecast alterations in runoff and P load caused by climate changes. Therefore the calculations presented are possible rather than established statements.

1

Y. Liu, F. De Smedt, "A GIS-based Hydrological Model for Flood Prediction and Watershed Management", Documentation and User Manual, Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Brussels, Belgium, 2004.

2

P. Budai, A. Clement, "Estimation of nutrient load from urban diffuse sources: experiments with runoff sampling at pilot catchments of Lake Balaton, Hungary", Wat. Sci. Tech., 56(1), 295-302, 2007. doi:10.2166/wst.2007.464

3

L.A. Rossman, "Storm Water Management Model", User's Manual (EPA/600/R-05/040), National Risk Management Research Laboratory, Office of Research and Development, U.S. EPA, Cincinatti, Ohio, USA, 2005.

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