Recent ground thermo-hydrological changes in a Tibetan endorheic catchment and implications for lake level changes

Climate change modifies the water and energy fluxes between the atmosphere and the surface in 2 mountainous regions. This is particularly true over the Qinghai-Tibet Plateau (QTP), a major headwater 3 region of the world, which has shown substantial hydrological changes over the last decades. Among 4 them, the rapid lake level variations observed throughout the plateau remain puzzling and much is still 5 to be understood regarding the spatial distribution of lake level trends (increase/decrease) and paces. 6 The ground across the QTP hosts either permafrost or seasonally frozen ground and both are affected 7 by climate change. In this environment, the ground thermal regime influences liquid water availability, 8 evaporation and runoff. Therefore, climate-driven modifications of the ground thermal regime may 9 contribute to lake level variations. For now, this hypothesis has been overlooked by modelers because 10 of the scarcity of field data and the difficulty to account for the spatial variability of the climate and its 11 influence on the ground thermo-hydrological regime in a numerical framework. 12 This study focuses on the cryo-hydrology of the catchment of Lake Paiku (Southern Tibet) for the 13 1980-2019 period. We use TopoSCALE and TopoSUB to downscale ERA5 data and capture the spatial 14 variability of the climate in our forcing data. We use a distributed setup of the CryoGrid community 15 model (version 1.0) to quantify thermo-hydrological changes in the ground during the period. Forcing 16 data and simulation outputs are validated with weather station data, surface temperature logger data and 17 the lake level variations. We show that both seasonal frozen ground and permafrost have warmed (1.7 18 °C per century 2 m deep), increasing the availability of liquid water in the ground and the duration of 19 seasonal thaw. Both phenomena promote evaporation and runoff but ground warming drives a strong 20 increase in subsurface runoff, so that the runoff/(evaporation + runoff) ratio increases over time. 21 Summer evaporation is an important energy sink and we find active layer deepening only where 22 evaporation is limited. The presence of permafrost is found to promote evaporation at the expense of 23 runoff, consistent with recent studies. Yet, this relationship seems to be climate dependent and we show 24 that a colder and wetter climate produces the opposite effect. This ambivalent influence of permafrost 25 may help to understand the contrasting lake level variations observed between the south and north of 26 the QTP, opening new perspectives for future investigations. remain scarce. They are however a powerful approach to tackle the question: how much 103 climate-driven ground thermal changes might affect the water cycle in high mountain headwater 104 regions? In this study, we use physical land surface modeling to quantify the ground thermo- 105 hydrological changes of an endorheic Tibetan catchment over the last 40 years as a response to climate 106 change. We show the interplay in the water and energy fluxes occurring between the atmosphere, the 107 surface and the subsurface and discuss their impact on the hydrology of the catchment and their 108 implication regarding lake level variations. which is sufficient to establish a stable temperature profile in the hydrologically active part of the ground (the 2 meters). without freezing (lowest locations), these areas were excluded from the averaging. In the end, the averaged results account for 89% of the catchment land area (i.e. excluding 401 glaciers and lake). The results show an increasing trend in the duration of the seasonal thaw of +46 days 402 per century (p=3×10 -4 ). When looking at the average start and stop days of the seasonal thaw in the 403 Julian calendar (day 150 is the 30 th of May and day 300 is the 27 th of October), we note that this increase 404 is mainly caused by a later ending date of the thaw season ( Stop date Fig. 7, +61 days per century, 405 p=8×10 -10 ) and not by an earlier starting date (+14 days per century, p=0.09). magnitude of our reconstruction is correct. Year to year comparison is less accurate we suggest that this is the consequence of the aforementioned limitations regarding data scarcity Beyond other physiographic of 2 m-deep