中文摘要：

长江中下游的河湖水交换关系典型且复杂，为描述河湖水量相互交换过程，提出了河湖水量交换系数的概念，即某一时段内由支流汇入湖泊的径流量与湖泊泄入干流径流量的比值，表示河湖水量交换的激烈程度。根据水量平衡原理推导出河湖水量交换系数计算的经验公式，并把河湖水量交换过程分为3种状态：“湖分洪”、“稳定”和“湖补河”。近60多年来河湖水交换系数年际变化趋势表明：洞庭湖与长江干流的水交换状态从“湖分洪”到“稳定”，再到“湖补河”状态发展；鄱阳湖与长江干流的水交换系数在稳定状态附近波动，河湖水交换状态无明显趋势性变化，河湖系统演化稳定。河湖水交换系数与长江干流径流量相关性良好，而与湖泊支流径流量相关性较差，表明长江干流径流量的大小是河湖水量交换过程的主控因素。

英文摘要：

A new concept of river-lake water exchange coefficient is introduced to investigate the river-lake water ex- change in the middle and lower reaches of Changjiang River. The concept is constituted of the ratio of the amount of runoff from tributaries to a lake and the amount of runoff from the lake to the main stream of a river during a certain period of time. The equilibrium theory is used in the derivation of the river-lake water exchange coefficient. Based on the latter, the exchange process in river-lake water can be classified into three types： Diverting flood waters to lake, water balancing, and draining lake water out to river. Results show that the water exchange between the Dongting Lake and the main stream of Changjiang River has evolved from diverting flood waters to lake, to water balancing, and to draining lake water out to river during the past 60 years. No significant changes can be observed in water exchange types between the Poyang Lake and the main stream of Changjiang River, as revealed by fluctuations of the river-lake water exchange coefficient indicating mostly the water balancing type. The river-lake water exchange coefficient can be strongly correlated to the amount of runoff from the main stream of Changjiang River. In contrast, a week correlation is found in lake＇s tributaries. Thus, the river-lake water exchange is dominantly controlled by the water flow in the main stream of Changjiang River.