Mongolia’s Selenga sub-basin of the Lake Baikal basin is spatially extensive, with pronounced environmental gradients driven primarily by precipitation and air temperature on broad scales. Therefore, it is an ideal region to examine the dynamics of the climate and the hydrological system. This study investigated the annual precipitation, air temperature, and river discharge variability at five selected stations of the sub-basin by using Mann-Kendall (MK), Innovative trend analysis method (ITAM), and Sen’s slope estimator test. The result showed that the trend of annual precipitation was slightly increasing in Ulaanbaatar (Z = 0.71), Erdenet (Z = 0.13), and Tsetserleg (Z = 0.26) stations. Whereas Murun (Z = 2.45) and Sukhbaatar (Z = 1.06) stations showed a significant increasing trend. And also, the trend of annual air temperature in Ulaanbaatar (Z = 5.88), Erdenet (Z = 3.87), Tsetserleg (Z = 4.38), Murun (Z = 4.77), and Sukhbaatar (Z = 2.85) was sharply increased. The average air temperature has significantly increased by 1.4 °C in the past 38 years. This is very high in the semi-arid zone of central Asia. The river discharge showed a significantly decreasing trend during the study period years. It has been apparent since 1995. The findings of this paper could help researchers to understand the annual variability of precipitation, air temperature, and river discharge over the study region and, therefore, become a foundation for further studies.

The Ugii Nuur Lake is not only one of the small hydrologically closed lakes located in the Orkhon River Basin in Central Mongolia but also the most vulnerable area for global climate change. Therefore, this study aims to investigate the impacts of recent global climate change on the water surface area. The data we analyzed were various measured hydro-meteorological variables of the lake basin and the lake surface area, which was estimated from Landsat series satellite data from 1986 to 2018. The methods we used were Mann-Kendall (MK), Innovative trend analysis method (ITAM), Sen’s slope estimator test, correlation, and regression analysis. The variation of lake water surface area has a strong positive correlation with the change of the lake water level (r = 0.95). The Mann-Kendall trend analysis has indicated that under a significant decrease in total annual precipitation ( $\mathrm{Z}$ = −0.902) and inflow river discharge ( $\mathrm{Z}$ = −5.392) and a considerable increase in total annual evaporation ( $\mathrm{Z}$ = 4.385) and annual average air temperature ( $\mathrm{Z}$ = 4.595), the surface area of the Ugii Nuur Lake has decreased sharply ( $\mathrm{Z}$ = −6.021). The total annual evaporation (r = −0.64) and inflow river discharge (r = 0.67) were the essential hydro-meteorological factors affecting the surface area of the Ugii Nuur Lake. The lake surface area decreased by 13.5% in 2018 compared with 1986. In the near future, it is vital to conduct scientific studies considering the volume of lake water, groundwater, and the anthropogenic impact.

The Tuul River Basin is the most important socioeconomic and political base area of Mongolia. Therefore, studying the interrelationships between changes in the ecohydrological processes of this basin and its land cover is of great importance for maintaining sustainability and the environment. This study investigated the annual average air temperature, total annual precipitation, and river discharge variability, and land cover changes at selected stations of the basin by using the hydrometeorological analysis, satellite analysis, and land cover determination statistical analysis. During the study period, the average annual air temperature rose from −1.5 °C to +0.3 °C (1.8 °C 361 °C). The average annual precipitation exhibits relatively low change during this period. River discharge varied during the study period. A significant decreasing trend in river discharge was observed at the Terelj (φ = −2.72) and Ulaanbaatar (φ = −5.63) stations, whereas the other stations, Altanbulag, Lun, and Orkhontuul, showed a significant increasing trend. During the study period, changes in land cover were directly related to main hydrometeorological parameters. Between 2000 and 2020, the amount of grassland decreased by 319.67 km², while the area of water bodies increased by 28.36 km². In the study area, mainly water bodies and sensitive areas of the land cover types were changed due to changes in precipitation. Studies in the arid and semiarid regions of Central Asia show that changes of ecohydrological processes have a significant impact on land cover changes.

Data on the origin and morphology of lake depressions in Mongolia are relatively poor. Bayan Lake in Zavkhan Province, Western Mongolia, has freshwater, supplied by groundwater emanating from a tectonic fault-oriented northeast to southwest, defining the southeastern side of the Bayan Lake. The depression of Bayan Lake has a tectonic origin, related with the fault which on its southern side, which is covered by sand dunes. The results of morphometric analysis, satellite mapping, geomagnetic anomaly assessment, and other relevant methods all support that the southern margin of the lake is defined by a large fault. The depression morphology of Bayan Lake shows that it was formed by subsidence accommodated by the fault along the southwestern side of the lake. Bayan Lake depression has the origin tectonic, but the recent barrier in the south is the dunefield on the bedrock.

There is the need to divide Mongolia's lake research into trends of development stages and to provide a detailed analysis of modern lake research. The aim of this study is to identify the development stages of lake research in Mongolia and to analyse lake research development. This study provides a comparative analysis of the development trends of lake research carried out in the country. The historical development of lake research in Mongolia was divided into 4 stages in chronological order, highlighting the current research direction, scope, and contribution to geography. Since 1990, researchers from many countries have been able to conduct extensive research in Mongolia, and especially since the 2000s, lake research intensified owing to global warming and environmental change. Studies, in particular, focus on paleogeography, paleoclimate changes, lake water regimes and water level fluctuation. Mongolian lake-based studies have tended to identify environmental evolution from the Pleistocene to the Holocene. The investigated areas were primarily Khuvsgul Lake and Darkhad Basin, Lakes Valley and Gobi Lakes, the Great Lake Depression, Mongolian Altai and Khangai Mountain lakes. Since 1990, scientists from Russia, Mongolia, Germany, China, Japan, USA and South Korea have been increasingly focusing on research into lakes in Mongolia. Paleogeographic and paleoclimatic reconstruction, based on the lakes of Central Asia, is being conducted in an innovative manner. Dividing Mongolia's lake research into development stages and identifying research trends will help determine which areas of research can be developed in the future.