Changes in lake area and water level in response to hydroclimate variations in the source area of the Yellow River: a case study from Lake Ngoring

In the north-eastern Qinghai-Tibet Plateau (QTP), the source area of the Yellow River (SAYR) has been experiencing significant changes in climatic and environmental conditions in recent decades. To date, few studies have combined modern hydrological conditions with paleoclimate records to explore the mechanism(s) of these changes. This study seeks to improve understanding of hydrological variability on decadal and centennial timescales in the SAYR and to identify its general cause. We first determined annual fluctuations in the surface area of Lake Ngoring from 1985 to 2020 using multi-temporal Landsat images. The results show that lake surface area changes were generally consistent with variations in precipitation, streamflow and the regional dry-wet index in the SAYR, suggesting that the water balance of the Lake Ngoring area is closely associated with regional hydroclimate changes. These records are also comparable to the stalagmite δ¹⁸O monsoon record, as well fluctuations in the Southern Oscillation Index (SOI). Moreover, an association of high TSI (total solar insolation) anomalies and sunspot numbers with the expansion of Lake Ngoring surface area is observed, implying that solar activity is the key driving factor for hydrologic variability in the SAYR on a decadal timescale. Following this line of reasoning, we compared the δ¹³C_org-based lake level fluctuations of Lake Ngoring for the last millennium, as previously reported, with the hydroclimatic history and the reconstructed TSI record. We conclude that the hydrological regime of Lake Ngoring has been mainly controlled by centennial fluctuations in precipitation for the last millennium, which is also dominated by solar activity. In general, it appears that solar activity has exerted a dominant influence on the hydrological regime of the SAYR on both decadal and centennial timescales, which is clearly manifested in the variations of lake area and water level of Lake Ngoring.