# Evaluating CHIRPS and ERA5 for Long-Term Runoff Modelling with SWAT in Alpine Headwaters
## 2 Materials and Methods
### Long-Term Climate and Runoff Variations  
Our intention was to assess the performance of GPPs in a period with relatively constant variations in climate and runoff and without significant perturbations in their trends. The analysis of variations in climate and runoff indices in the longer-term period (1951-2018) enabled the identification of perturbations in the trends and the selection of a period for subsequent assessment of GPPs. In the 1951-2018 period, daily observations of mean air temperature (_TG_), precipitation (_P_) and snow depth (_SD_) were available at four gauges at different elevations: Sonnblick (SO) at 3106 m a.s.l., Zugspitze (ZU) at 2964 m a.s.l., Hohenpeissenberg (HO) at 977 m a.s.l. and Innoglstadt (IN) at 577 m a.s.l. Long-term discharge time series (_Q_) were available at five stations: at the Puig (PUI) and Innsbruck (INN) stations on the Sill River (b1 catchment), at the Rabland (RAB) station on the Drava River (b2 catchment), and at the Bruhl (BRU) and Lienz (LIE) stations on the Isel River (b3 catchment). The locations of the stations are shown in Figure 1.  
The average annual climate indices (_TG_, \(P\), _SD_) show a clear orographic separation in the pilot area. The _TG_ decreases from +9.8 \({}^{\circ}\)C in the valleys to \(-\)5.2 \({}^{\circ}\)C on the ridge tops, while the annual \(P\) increases from 886 mm to 2006 mm as well as the annual _SD_ from 0 cm to 343 cm. The fluctuations and trends of the climate indices are consistent across the area and with the orographic separation (Table 7, Figures 2 and 3). The _TG_ increase is very strong through the period 1951-2018 (statistically significant at \(p\) < 0.001 level at all stations). In the 70-year period, _TG_ increased by +2.1 \({}^{\circ}\)C, which is consistent with other findings [12; 13; 14]. The long-term annual _TG_ variations in the period 1951-2018 show a clear break around 1980 (Figure 3), followed by a continuous increase in _TG_ at all altitudes (all stations). After 1981, the _TG_ trends are even stronger and most pronounced at lower elevations (+0.44 \({}^{\circ}\)C 10y\({}^{-1}\) at station HO), as previously shown [13]. The long-term annual \(P\) generally fluctuates around the mean, without obvious breaks. At high elevations, however, a positive \(P\) trend is evident, with an increase in long-term means of 12% and 5% (+202 mm and +97 mm at gauges SO and ZU) in the post-1991 period. The long-term negative trends in annual _SD_ are found at all elevations but are stronger in the valleys (statistically significant at \(p\) < 0.05 level). The long-term _SD_ fluctuations at lower elevations clearly follow the _TG_ pattern, with a single break in the 1980s and a continuous decline after 1990. After 1981, the long-term annual _SD_ in the valleys decreased by \(-\)22% (\(-\)5 cm at gauge HO). The _SD_ variations at higher elevations (gauges SO and ZU) resemble the \(P\) pattern with several breaks: a decline in the 1960s and 1970s was followed by a brief recovery until the 1980s and a continuous decline after 1990. In the post-1981 period, the mean _SD_ at the mountain tops decreased by \(-\)10%, although the mean \(P\) increased by +8%. These results confirm a strong impact from the _TG_ increase in the area from previous findings with a shift in precipitation type (from snow to rain) and a strong decrease in snow cover [16; 17]. The 1951-2018 long-term trends for the annual \(Q\) show alternating variations between the catchments. In catchment b2 Drava, there is a weak negative trend in annual \(Q\), with a clear break around the 1980s (RAPS plots in Figure 3), while in catchments b1 Sill and b3 Isel, there is a weak positive trend without an obvious break. However, after 1981, the annual \(Q\) shows a positive trend across the pilot area. After examining the variations and perturbations in the climate and runoff indices in the period 1951-2018, the period after 1991, in which there were no obvious breaks in the \(TG\), \(P\), \(SD\) and \(Q\) trends, was selected for the evaluation of the GPPs.