**5. Conclusions**

The detailed spatio-temporal distribution of snow has not been previously studied in Mongolia. In this study we examined snow cover in the Khentii Mountains, which is an important headwater for Lake Baikal and is the origin of the Selenga River. The use of multi-source data provided us essential information for understanding the timing and spatial characteristics of snow in the Sugnugur catchment within a semi-arid climate where snow plays an important role in the regional water resources and surface energy balance.

The snow ground observation at the hydro-climatic station demonstrated strong variability in the timing of snow, especially during the intermittent snow cover period, showing a duration of 6–21 days/year, while persistent SCD was relatively stable with 132–141 days. Daily mean surface temperature of below −1 ◦C with continuation of at least 14 days could give a good proxy of seasonal persistent snow cover. The manually measured SD at different elevations and land-cover types did not indicate any variability during the two consecutive snow field measurements.

The combined MODIS, Landsat, and Sentinel retrievals with various spatial and temporal resolutions gave us an opportunity to map snow cover and its duration in the mountainous region with 30 m resolution by applying a series of adjustments, including temporal gap-filling and conditional adjustments. The derived SCD rate is similar to that found in other Central Asian Mountains. The derived annual SCD shows high variations, but no significant trend since 2000. Overall, the combination of snow ground observation, field measurements, and high resolution open source optical remote sensing images with high temporal resolutions can be an option for understanding snow distribution and duration in mid-latitude mountain regions.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3263/9/1/53/s1, Table S1. The acquisition dates of the satellite images that were used for producing the spatial distribution of snow. All selected images were taken on cloud-free conditions. Table S2. The overview of snow field measurements which were conducted on March 1, 2017 and 2018. Figure S1. Development of SCA for winter 2016-2017 from the combination of Landsat and Sentinel retrievals. The light-blue color indicates snow cover. Figure S2. Typical snow distribution in the Sugnugur catchment during the end of seasonal persistent snow cover period. Photographs were taken using a normal digital camera with automatic time-lapse setting. Figure S3. Anomalies of the estimated SCD for Terra and the combined version compared to the observed total SCD at the hydro-climatic station site for winters 2012/2013 to 2016/2017.

**Author Contributions:** M.M. analyzed the in situ and Landsat data, proposed the methodology, and wrote the manuscript. S.G. analyzed the MODIS and Sentinel data and contributed to the methodology and review. C.G.P. provided crucial suggestions and improved the manuscript writing. G.Y. and J.Y. carried out the snow field measurements. L.M. provided climatic data, supported the fieldwork, and supervised the study.

**Funding:** This research received no external funding.

**Acknowledgments:** Munkhdavaa Munkhjargal received funding from the Federal Ministry of Education and Research (BMBF) through the German Academic Exchange Service (DAAD) during the preparation of this manuscript. We acknowledge financial support by Deutsche Forschungsgemeinschaft within the funding programme Open Access Publishing, by the Baden-Württemberg Ministry of Science, Research and the Arts and by Ruprecht-Karls-Universität Heidelberg.

**Conflicts of Interest:** The authors declare no conflict of interest.
