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3.3. RUNOFF AND RESERVOIR INFLOW The time series of 105 years streamflow data at Neustadt 1 did show a small but insignificant decrease of seasonal and annual runoff (Table 2), a finding that is in line with observed trends in the Western Harz mountains [5]. At Ilfeld and Ellrich gauging stations, the decreases in summer runoff were significant and at Ellrich, even the decreasing annual runoff trend was statistically significant. Table 2 Monthly and seasonal runoff totals at stream gauging stations Neustadt 1 (1 GY), Ilfeld (0 GY) and Ellrich (0 GY). Data source: TLUG However, the latter stations record started in 1970 rather than the Neustadt 1911 commencement date, hinting that record length might have impacted the results in some way. The observed long-term changes in seasonal and annual runoff at all three gauging cannot be explained by the long-term trends in precipitation, thus indicating that runoff changes were influenced by other processes. Looking at individual months, we find that at the dams inflow stream gauge Neustadt 1, nine out of twelve months registered significant decreases following the 1987/88 breakpoint (Figure 5). While streamflow in individual months decreased by 50 % or more, seasonal summer inflow decrease was a staggering 36 %, a finding well confirmed at the other two nearby gauging stations of Ilfeld and Ellrich as well as findings in the Eastern part of lower Saxony [6]. Average annual reservoir inflow at Neustadt dam had decreased from 2.56 MCM (1912- 1987) to 2.15 MCM (1988-2016). During the 1988-2016 sub-period, March had replaced April as the month with the highest mean runoff as compared to preceding Neustadt 1/Krebsbach Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Wi Su Year Mean 1912-2016 [MCM] 0.1 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.0 0.0 0.0 0.1 1.0 0.3 1.4 Trend 1912-2016 [Tm 3 a -1 ] 0.0 0.1 -0.4 -0.2 0.1 -0.5 -0.2 0.0 -0.1 -0.1 0.0 -0.1 -1.5 -0.7 -2.2 Breakpoint in year 1995 1954 1938 1928 2010 1988 1987 1990 1988 1988 1994 1989 1995 1987 1988 Mean 1988-2016 [MCM] 0.1 0.15 0.21 0.16 0.21 0.13 0.07 0.04 0.03 0.02 0.03 0.05 0.97 0.23 1.2 Change after 1988 [%] -7 -3 1 -6 3 -39 -33 -39 -47 -55 -31 -18 -8 -36 -15 Trend 1988-2016 [Tm 3 a -1 ] -0.7 -2.2 -0.4 -0.6 -7.0 -1.3 0.4 -0.1 0.0 -0.1 -0.2 -0.1 -12 -0.3 -14 Ilfeld/Beere Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Wi Su Year Mean 1952-2015 [MCM] 2.1 3.8 4.5 3.4 4.6 3.9 1.7 1.3 1.0 0.7 0.7 1.2 22 7 29 Trend 1952-2015 [Tm 3 a -1 ] 1 1 32 8 8 -23 -4 -8 -7 -7 -3 -3 2 -68 -51 Breakpoint in year 1977 1977 1973 1979 1977 1988 1987 1988 1988 1987 1958 1958 1977 1987 1995 Mean 1988-2015 [MCM] 2.26 3.57 5.0 3.55 4.9 2.85 1.37 0.78 0.62 0.43 0.66 1.08 22 4.95 27.1 Change after 1988 [%] 11 -10 25 10 14 -39 -28 -54 -50 -51 -14 -20 0 -37 -9.6 Trend 1988-2015 [Tm 3 a -1 ] -14 -47 4 -24 -124 -15 16 -2 -4 4 2 0.5 -150 7 -157 Ellrich/Zorge Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Wi Su Year Mean 1970-2015 [MCM] 2.2 3.1 3.8 2.6 3.5 2.7 1.4 1.1 0.8 0.7 0.7 1.2 18 6 24 Trend 1970-2015 [Tm 3 a -1 ] -9 -4 7 -4 -21 -57 -16 -21 -10 -7 -4 -8.5 -128 -77 -248 Breakpoint in year 1997 1998 1980 2003 2008 1995 1986 1987 1988 1988 1993 2000 2000 1986 2000 Mean 1988-2015 [MCM] 2.19 2.92 3.8 2.62 3.51 1.95 1.13 0.68 0.53 0.51 0.68 1.1 17 4.58 21.6 Change after 1988 [%] 0 -13 2 4.9 0 -51 -35 -58 -58 -44 -15 -28 -12 -41 -20 Trend 1988-2015 [Tm 3 a -1 ] -42 -79 -40 -50 -161 -77 -3 -1 -5 -2 -9 -7 -437 -35 -600 Level of significance of trends and breakpoints: p > 0,1 (standard); p < 0,1 (italic) ; p < 0,05 (bold); p < 0,01 (bold italic) Color codes of changes [%] after 1988: < -50 > -50 > -40 > -30 > -20 > -10 > -5 > 5 > 10 > 20 > 30 257