Seite - 261 - in Book of Full Papers - Symposium Hydro Engineering

SUMMARY The Harz is the main water tower in central Germany and its water resources have been developed since the 16th century to support the mining industry and since the 19th century for public water supply. Today, nine multi-purpose dams in the Harz provide high quality raw water for public water supply. Neustadt dam is the Southernmost and smallest of these nine dams and is fulfilling its water supply function since 1905. At present, all safely available water is committed in a raw water supply contract and the buyer – a local water company – has requested to increase the water withdrawal in the future. On the other hand, with a fully committed water balance, any long-term or sudden changes in the dam’s inflow regime would have a direct negative impact on safe water withdrawal. In this paper, long-term records from the dam’s inflow gauging station, two nearby stream gauges, two met stations, three rain gauges and a phenological station were analysed for any shifts or changes. The investigation revealed significant changes in in the flow regime. Firstly, starting in the late 1980s, monthly maximum reservoir inflow shifted from April towards the beginning of the year. Secondly, mean summer runoff had decreased by 35 %, and in some months by up to 55 % in the last 30 years. A breakpoint analysis revealed 1987/88 as a statistically highly significant point in time when these changes occurred, this finding was independently confirmed by data from two nearby gauging stations. The observed changes in the flow regime could not be explained by precipitation data at the site, which showed neither any significant trends nor any breakpoints. This implies a change in the partitioning of precipitation into runoff, evapotranspiration and groundwater recharge. Because of the geological characteristics of the catchment area, both groundwater recharge and subsurface runoff are negligible. The working hypothesis is that a shift of runoff volume towards evapotranspiration has occurred after the 1987/88 breakpoint. This hypothesis is supported by the direct link between measured increases in air temperature and potential evapotranspiration. Furthermore, phenological observations and seasonal temperature sums provide clear evidence of an earlier onset and longer duration of the vegetation season at the site. Going forward, it is expected that future trends and sudden shifts of meteorological and hydrological parameters will potentially increase the demand and expectations of society on dams and their buffering role in the hydrological cycle. However, some of these functions and purposes are contrarian to each other, hence dam management will potentially become more challenging. Finally, several remedial adaptation strategies for dam operators with a fully committed water balance are discussed in general and the two approaches towards increasing safe water withdrawal from Neustadt dam are briefly outlined. 261