Ecological water level and its safeguard measures in Lake Dongting

Abstract

湖泊生态水位是维持湖泊生态系统健康的重要因素.基于洞庭湖城陵矶、杨柳潭、南咀3个水文站1959—2016年日平均水位序列进行分析，采用Mann-Kendall法、累积距平法和滑动T检验法综合确定洞庭湖水位变异时间节点，结合生态水位年内展布法以及IHA-RVA法，计算分析湖泊最小和适宜生态水位，并且采用Tennant法进行合理验证，在此基础上对水文变异前、后湖泊生态水位保障度进行研究.研究结果表明：（1）洞庭湖城陵矶和杨柳潭水文站年均水位呈上升趋势，而且城陵矶站水位上升趋势显著，南咀站年均水位呈显著下降趋势.（2）洞庭湖3个典型水文站水位年际变化突变年份为2003年，突变年份基本上与三峡工程蓄水时间相符.（3）城陵矶、南咀和杨柳潭年均最小生态水位分别为21.41、28.95和27.84 m，分别占多年平均水位的86.3%、95.9%和95.7%，城陵矶、南咀和杨柳潭年均适宜生态水位分别为23.29、29.51和28.36 m，分别占多年平均水位的93.9%、97.8%和97.5%，生态水位计算结果考虑了天然湖泊水位年内丰枯变化，满足了湖泊生态目标需求.（4）洞庭湖最低生态水位保障程度较高，基本能达到80%以上，但适宜生态水位保障程度相对较低，其中2003年以后洞庭湖10月和11月生态水位保障程度显著下降，与上游水利工程蓄水有关，建议在此期间采取调度措施适当增加洞庭湖水量，以保障湖泊生态系统的健康与生物多样性.;Lake ecological water level (EWL) is an important factor to maintain the health of lake ecosystem. Based on the analysis of the daily average water level series of Chenglingji, Yangliutan and Nanzui hydrological stations in Lake Dongting from 1959 to 2016, the hydrological variation characters of Lake Dongting were determined by Mann-Kendall method, cumulative anomaly method and sliding T test method. Based on the method of annual EWL distribution and the improved method of IHA-RVA, the minimum and optimal EWLs were calculated, and the Tennant method was used for the reasonability verification. Finally, the guaranteed rates before and after the abrupt point were analyzed. The results show that: (1) The average annual water level of Chenglingji and Yangliutan hydrology stations were both on the rise, with much significant trend for the former one. While the average annual water level of Nanzui station exhibited significant decline trend. (2) The time of abrupt changes in water levels of the three stations was 2003, which was basically consistent with the beginning operation time of the Three Gorges Dam. (3) The annual minimum EWLs of Chenglingji, Nanzui and Yangliutan stations were 21.41, 28.95 and 27.84 m, respectively, accounting for 86.3%, 95.9% and 95.7% of the multi-year average level. The optimal EWLs were 23.29, 29.51 and 28.36 m, respectively, accounting for 93.9%, 97.8% and 97.5% of the multi-year average level. Such defined EWLs could meet the requirements of lake ecosystem goals with the consideration on the annual changes in natural lakes. (4) the safeguarding on the minimum ecological water level in Lake Dongting is relatively high, basically reaching more than 80%, but the guarantee degree of the appropriate EWLs is relatively low. After 2003, the guarantee degree of Lake Dongting decreased significantly in October and November, which is related to water storage and conservancy projects in the upstream. The EWL guarantee measures in Lake Dongting were suggested to maintain the health and biodiversity of the lake ecosystem based on the low guarantee degree and the difference of annual EWL distribution.