Hydro-climate study of the Orog lake - Tuin river basin, South Mongolia

Hydrology and climate research at the catchment scale plays a vital role in accurately identifying the characteristics of the hydrological cycle, climatic resources, and water balance components, as well as their interrelationships. This type of research provides a scientific foundation for the sustainable management of water resources, maintenance of ecosystem equilibrium, risk assessment related to climate change, and evidence-based policymaking. Accordingly, we selected the Orog Lake–Tuin River basin as a representative case of Mongolia’s semi-arid region and the Central Asian endorheic basins. Using open-access data from the United States Geological Survey (USGS), we analyzed changes in the water surface area of Orog Lake over the past 30 years (1991–2020), employing Landsat 5 and Landsat 8 satellite imagery. Additionally, high-resolution, spatiotemporal climate data from the ERA5 reanalysis dataset—developed by the European Centre for Medium-Range Weather Forecasts (ECMWF) and internationally recognized—was extracted across 1440 spatial layers for four key climate parameters. These were processed and analyzed to identify seasonal interactions during July–September, a period when atmospheric, surface, and hydrological processes are most dynamically interconnected. We established statistical relationships among lake area, river discharge, precipitation, and evapotranspiration, and mapped the spatial distribution of key climate indicators across the basin. These findings were evaluated in comparison with observed data. The study revealed that the surface area of Orog Lake has declined by approximately 70 km², while the average discharge of the Tuin River has decreased by 0.5 m³/s. Over the past 30 years, total annual precipitation has dropped by 23 mm, air temperature has risen by about 2°C, and total potential evapotranspiration has increased by 18 mm. A strong correlation (r = 0.7) was found between river discharge and precipitation, while the correlation between lake surface area and river discharge was moderate to weak (r =0.48). This suggests that a portion of the river's flow may be infiltrating into subsurface layers (soils and rocks) before reaching the lake.