Abstract:

Drought as a major disaster has hit south and central China hard over the last decade. Seasonal hydrological droughts raise many concerns on the largest retention lakes in central Yangtze River, which has received wide attention due to the possible negative impact from the impoundment of Three Gorges Dam (TGD). The hydrological drought analysis at Dongting Lake is important for clarifying some of the most complex hydrological issues in relation to the intertwined interactions of a lakeriverreservoir system from the operation of Three Gorges Reservoir (TGR) located upstream of the central Yangtze River. The assessment metrics for a hydrological drought were established according to the exposing of wetlands at Dongting wetlands. These metrics were used to determine the characteristic water level in relation to the occurrence and duration (severity) of the hydrologic drought at varying lake areas during the dry seasons. The causal effects of a hydrological drought were analyzed based on the hydrological regimes and seasonality across lake areas. The drought impact on the lake water due to the water storage in the TGR was evaluated by using a neural network model. Such an analysis confirms that: （1) the frequency, severity and causes of hydrological droughts varied for different lake areas and seasons due to the specific basin morphology and the deviation of water regimes. The severity of a hydrological drought became more intensify after 2000, starting with the most severe impact at West Dongting Lake, followed by the East Dongting Lake, and then the South Dongting Lake; （2) the occurrence of a hydrological drought varied for different lake areas and seasons. The autumn drought at East Dongting Lake was caused by a runoff reduction from the Yangtze River whereas the autumn droughts at West and South Dongting Lake were caused by a runoff reduction from Yangtze River and Dongting Lake basin simultaneously. Yet the spring drought covering the entire lake was caused by a runoff reduction from Dongting Lake basin itself; （3) the water storage in the Three Gorges Reservoir has advanced the exposed time of wetlands, and has prolonged the autumn drought by approximately 30%. However, the modeling also reveals that the regular operation of the TGD did not change the natural drought trends at Dongting Lake, and it is not deemed as the primary cause of recent hydrological droughts. The river flows are less homogeneous over a large region, thus, flow regulation has the basis to balance low flow downstream. However, the TGDs operation has aggravated seasonal hydrological droughts by prolonging the exposed time of wetlands in autumn. Although the analysis reveals that the regular operation of the TGD did not change the general drought trends at Dongting Lake, and it is not deemed as the primary cause of recent hydrological droughts. There are still many challenges to accomplish a sustainable dam operation to enhance the resilience to hydrological extremes