## 2 Materials and Methods  
### Regional Setting  
Due to limited groundwater availability, the majority of German reservoirs are located in low mountain regions in the federal states of North Rhine-Westphalia (NRW) and Savony (SN) [2]. The low mountain regions of both federal states are mainly part of the European Variscides. In NRW, the Variscides are represented by the geologic unit of the Renohercynian Zone, whereas in Savony they are represented by the Savothuringian Zone [21; 22] (Figure 1). In order to achieve a high representativity, the selection of the study areas was focused on both these geological units, where most of the German reservoirs are located. All six investigated reservoirs are located in low mountain ranges, which include areas of the Rhenish Slate Mountains (Bitburg Reservoir, Steinbach Reservoir, Breitenbach Reservoir, Obernau Reservoir) and of the Ore Mountains (Einsiedel Reservoir, Lehnmule Reservoir). The sites of the Rhenish Slate Mountains are characterized by sub-oceanic climatic conditions and a dominance of Late to Middle Devonian rocks. The sites of the Ore Mountains reservoirs are characterized by the gradual transition to a more continental climate and late Proterozoic rocks (Table 1). The Bitburg, Breitenbach, and Obernau reservoirs are characterized by a geology largely comprising altering layers of sand, silt, claystone, greywacke, and slate, whereby limestone is restricted to a few small parts of the Bitburg Reservoir catchment. Although also situated within the Rhenish Slate Mountains, the catchment of the Steinbach Reservoir comprises mainly quartzite and slate as geologic basement [23]. Both reservoirs situated within the Ore Mountains are characterized by a dominance of metamorphic rocks, which mainly comprise phyllite (Einsiedel Reservoir) and gneiss (Lehnmuhle Reservoir) [24; 25]. Due to a widespread occurrence of silicate rocks, forestry, and a humid climate, the catchments of all reservoirs are characterized by soilscapes dominated by Cambisols and Stagnosols [26; 27; 28]. The reservoirs are mainly used for drinking water supply. The only exception is the Bitburg Reservoir that was, although used for water regulation and energy production, included in the study because of mainly similar catchment characteristics. Although the study objects clearly differ in age and storage volume, all reservoirs are characterized \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Reservoir** & **Einsiedel** & **Lehmmhle** & **Steinbach** & **Bitburg** & **Breitenbach** & **Oberau** \\ \hline
**Constr. (years)** & 1891–1894 & 1927–1931 & 1963–1966 & 1970–1972 & 1953–1956 & 1967–1972 \\
**Vol. (Mill. m3)** & 0.3 & 21.9 & 4.5 & 1.2 & 7.8 & 14.8 \\
**Prec. (mm)** & 701 a  & 664 b  & 517 c  & 861 d  & 783 e  \\
**Temp. (c*C)** & 8.5 a  & 9.4 b  & 9.5 c  & 9.0 d  & 9.2 e  \\
**Elev. (m a.s.l.)** & 384 & 518 & 465 & 256 & 370 & 370 \\
**Geology** & phyllite & gneiss & shale, & shale, & shale, & shale, \\  & & quartzite & sand-, limest. & sandst. & sandst. \\
**Size (km2)** & 2.7 & 60.4 & 14.6 & 330 & 4.1 & 11.3 \\ \hline \multicolumn{6}{c}{Monitoring stations: a Chemnitz (WeWa) (1980–2010) [28]; b Dresden Klotz: (1980–2010) [29]; c Bad Kreuznach (1961–1990) [30]; d Olsdorf (1980–2010) [29]; e Siegen Klarnalge (1980–2010) [29].} \\ \end{tabular}
\end{table} 
Table 1: Properties of the investigated reservoirs and their respective catchments (Constr. : period of construction; Vol. : Storage volume; Prec. : mean annual precipitation; Temp. : mean annual temperature; Elev. : elevation above sea level; Size: size of the respective catchment of a reservoir). Figure 1: Study areas within the geologic units of the German Variscides. A: Bitburg Reservoir; B: Steinbach Reservoir; C: Breitenbach Reservoir; D: Obernau Reservoir; E: Einsiedel Reservoir; F: Lehhmänhle Reservoir. by catchment areas with forestry being the dominant land use and the impounding of relatively small watercourses.