The correlation between the storage capacity and runoff, runoff + precipitation, runoff + precipitation-evaporation was analysed at the basis of monthly water balance calculation in the period of 1963 and 1980 according to the real hydrological processes of the research site. At the same time, the intermediate-term changes in the mean value of water level, runoff, precipitation and evaporation were detected with the cumulative sum technique in order to find the reason of water level dropping after 2000. Meanwhile, the distribution and mutual relations of water balance terms in a year was analyzed. Results show that:climate change exhibited a warmer and dryer trend in recent years that might be the major cause for water resources deficit after 2000. The crucial influence on water level comes from runoff, followed by precipitation on the water surface. In April and May of each year, accumulated melt water infiltrate the soil and then recharge the lake, while in the other time the groundwater recharged by the lake.