洞庭湖近年干旱与三峡蓄水影响分析

季节性水文干旱是洞庭湖近年突出的水情问题，并因三峡影响而倍受关注。基于洞庭湖水文干旱的关注焦点（湿地生态影响）及其与湖泊水情的对应关系，建立了湖泊滩地出露与持续时间为依据的干旱度量指标；并利用水文分析方法，揭示了洞庭湖水文干旱发生的时空特点和水情机制；最后，基于BP神经网络模型获取的三峡水库蓄水对洞庭湖的水位影响量化了三峡水库秋季蓄水对湖区干旱的贡献分量。结果认为：（1）2000年后，洞庭湖的干旱频次明显增多、旱情加重，干旱程度以西洞庭最剧，东洞庭次之，南洞庭最轻；（2）洞庭湖不同湖区干旱成因存在一定差异，其中全湖的春旱基本由洞庭湖流域来水偏少引起，而东洞庭湖秋旱主要由长江来水减少引起，西、南洞庭湖秋旱则由长江和洞庭湖流域来水共同减少形成；（3）三峡水库蓄水对东洞庭湖秋旱起到一定的加重作用，但并非洞庭湖近年干旱的主要因素

RECENT HYDROLOGICAL DROUGHTS IN DONGTING LAKE AND ITS ASSOCIATION WITH THE OPERATION OF THREE GORGES RESERVOIR

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