Quantifying the glacial meltwater contribution to streams in 1 mountainous regions using highly resolved stable water isotope 2 measurements 3

This study aims to determine the contribution of glacial meltwater to streams in mountainous regions based on stable 11 water isotope measurements (d18O and d2H). For this purpose, three partially glaciated catchments were selected as the study 12 area in the central Swiss Alps being representative of catchments that are used for hydropower energy production in Alpine 13 regions. The glacial meltwater contribution to the catchments’ stream discharges was evaluated based on high-resolution d18O 14 and d2H measurements of the end-members that contribute to the stream discharge (ice, rain, snow) and of the discharging 15 streams. The glacial meltwater contribution to the stream discharges could be unequivocally quantified after the snowmelt in 16 August and September when most of the annual glacial meltwater discharge occurs. In August and September, the glacial 17 meltwater contribution to the stream discharges corresponds to up to 95±2% and to 28.7%±5% of the total annual discharge 18 in the evaluated catchments. The high glacial meltwater contribution demonstrates that the mountainous stream discharges in 19 August and September will probably strongly decrease in the future due to global warming-induced deglaciation, which will 20 be, however, likely compensated by higher discharge rates in winter and spring. Nevertheless, the changing mountainous 21 streamflow regimes in the future will pose a challenge for hydropower energy production in the mountainous areas. Overall, 22 this study provides a successful example of an Alpine catchment monitoring strategy to quantify the glacial meltwater 23 contribution to stream discharges based on stable isotope water data, which leads to a better validation of existing modelling 24 studies and which can be adapted to other mountainous regions. 25


Introduction 26
The current global energy production still heavily relies on fossil energy resources such as oil and gas, emitting large quantities 27 of greenhouses gases such as CO2 and methane into the atmosphere. Currently, 53 gigatons tons of greenhouse gases are 28 alteration of the sample. After October 2019, samples for stable water isotope analysis were taken manually since the increasing 153 snow cover prevented the continuous automatic monitoring by using the autosampler. 154

Stable water oxygen and hydrogen isotope measurements of end-members and stream 156
The stable oxygen and hydrogen isotope ratios of rain, snow, ice and stream discharge samples were analysed using a Picarro 157 L2120-I cavity ring down spectrometer (CRDS) with vaporization module V1102-I at the Institute of Geological Science, 158 University of Bern, Switzerland. The measured stable oxygen and hydrogen isotope ratios were expressed in the delta notation 159 (d = (R/RStd -1) · 1000 (‰)), where R and RStd are the isotope ratios of the sample and the standard, respectively. Raw d 18 O 160 and d 2 H values are obtained by a tenfold measurement of each sample followed by a post run-correction (memory and drift) 161 according to van

Discharge separation based on stable isotope measurements. 168
The contribution of the different end-members (ice, rain, snow) to the discharges of the three catchments was quantified based 169 on highly resolved stable water isotope ratio (d 18 O, d 2 H) measurements in the catchments' effluents. To quantify the end-170 member discharge contribution, the two end-members were considered that contributed predominately to the discharge using 171 a binary mixing approach: 172 173 IEffluent = X·IEnd-member1+ (1-X)·IEnd-member2 (1) 174 175 where IEffluent is the isotopic composition of the catchment's effluent, IEnd-member1 and IEnd-member2 are the isotopic compositions of 176 the end-members (snow, rain, ice) and X is the contribution of the end-members to the effluent. 177

178
To quantify the contribution of each end-member to the catchment's effluent, equation 1 was resolved to X: 179 180 X = (IEffluent -IEnd-member2)/(IEnd-member1 -IEnd-member2) (2) 181 April, whereas the snow ablation period during which the snow cover becomes continuously thinner due to sublimation, 235 melting and redistribution processes was observed between May and October (Fig. 3). 2008). Similar to the accumulation period, no significant deviation from the LMWL was observed during the snow ablation 282 period, revealing that sublimation processes were not the predominant isotope fractionation process (Fig. 4). The more enriched 283 Wendenwasser catchment. The hydro-chemical measurements included discharge volumes, electrical conductivity and stable 319 water isotope ratios (d 18 O and d 2 H) as well as precipitation (Fig. 6). The stream discharges in the three investigated catchments were highest in the Steinwasser followed by the 363 Wendenwasser and Giglibach catchments (Fig. 6B) correlating to the different sizes of the catchments (Fig. 1). The stream 364 discharges in the Steinwasser and Wendenwasser catchments showed large temporal variations and can be divided into a high 365 (June -August 2019), intermediate (September -October 2019) and low discharge time period (November -March 2020) 366 (Fig. 6B). During the high discharge period, the Steinwasser and Wendenwasser stream discharges ranged between 3 and 10 367 m 3 /s and between 1 and 3 m 3 /s, respectively, with a few peak discharges of up to 12 m 3 /s and 9 m 3 /s, respectively during heavy 368 precipitation events (Figs. 6A and B). The intermediate discharge period was dominated by short discharges peaks in both the 369 Stein-and Wendenwasser catchment (up to 9 m 3 /s), which were also related to precipitation events (Fig. 6A) followed by 370 baseflow recessions to discharges of around 0.80 m 3 /s (Figs. 6A and B). At the beginning of the high discharge phase (June -371 mid-July 2019), only discharge data for the Steinwasser catchment is available. During this time, the stream discharge in the 372

Quantitative discharge separation based on stable isotope ratio in the catchment's effluents 404
The quantitative discharge separation in the three catchments was conducted based on the stable water isotopes measurements 405 and focused on the glacial meltwater contribution to the catchments' effluents between August and September 2019. This time 406 period is of special interest since the glacial meltwater contribution to the stream discharges is a) likely highest throughout the 407 year due to the combination of high temperatures and the absence of snowmelt and b) subject to disappearance in the future 408 due to climate change-induced deglaciation. Therefore, the quantification of the glacial meltwater contribution in August and 409 September is crucial to predict discharges of mountainous streams in future in the course of climate change. The absence of 410 snowmelt in September and August also provides the advantage that only two end-members (rain and ice) need to be taken 411 into account for the quantification of the glacial melt water contribution using stable water isotopes, which facilitates the data 412 interpretation. The glacial meltwater contribution between August and September 2019 was quantified using the temporal 413 occur earlier in the year (May/June) compared to today (June/July) due to the earlier occurrence of the snowmelt caused by 532 global warming. However, predictive discharge simulations for Alpine catchments suggest that the annual mountainous stream 533 discharge volumes will not significantly decrease despite of the ceasing glacial meltwater contributions as they will be 534 compensated by higher discharge volumes in winter and spring (Hydro-CH2018). Nevertheless, the changing flow regimes in 535 mountains streams caused by climate change will pose a challenge for hydropower energy production in Alpine regions. Hence, 536 the operation of hydropower energy production using artificially dammed lakes in alpine regions needs to adapt to these 537 changing flow regimes in mountainous streams caused by climate change. This is major importance for achieving a carbon 538 neutral energy production, as hydropower energy is the most important renewable energy resource and it is crucial that 539 hydropower energy is exploitable to the same extent in the future despite global warming to further reduce greenhouse gas 540 emissions. 541 Overall, this study demonstrates a successful monitoring strategy for three partially glaciated mountainous catchments 542 for quantifying the glacial meltwater contribution to stream discharges based on stable water isotope measurements. In 543 particular, the study showed that for a successful quantification of the glacial meltwater contribution based on stable water 544 isotopes a high temporal resolution of the end-members and catchment discharges is necessary, especially of snow and rain as 545 they vary strongly over time. Moreover, our results showed that a quantification of the glacial meltwater contribution is only 546 possible when snow meltwater is absent as the isotopic signature of snow and rain overlap. However, this is no major drawback 547 since the glacial meltwater contribution is only significant when no snowmelt is occurring. Additionally, the annual glacial 548 meltwater discharge volumes in three catchments showed an excellent power-law correlation with the catchment's glaciated 549 area. This correlation allows the estimation of the annual glacial meltwater discharge volume in other mountainous catchments 550 based on the glaciated area only. This is an advantage as the glaciated area is easier to determine than stable water isotope 551 measurements in mountainous streams and in the contributing end-members. Taken as whole, an implementation of the 552 developed sampling strategy in this study to other mountainous catchments will provide an improved validation of existing 553 mountainous catchment modelling studies for the quantification of the glacial meltwater contribution to streams in 554 mountainous regions. 555

Data availability 556
The raw the data for this study can be accessed in the Zenodo data repository through: https://doi.org/10.5281/zenodo.5571465 557