Changsha belongs to subtropical monsoon climate and has four distinctive seasons. It is rainy at the end of the spring and in the beginning of the summer, but arid at the end of summer and during the autumn. In spring, the weather is humid and volatile. In summer and autumn, it is always sunny. However, the severe winter is short-lived but the hot summer weather lasts for a long time. Changsha City's frost-free period totals 275 days all year round, with mean annual temperature hitting 16.8~17.2℃. Records show that its ultra high temperature was 40.6℃, and ultra low temperature,-12℃. With mean annual precipitation of 1,422.4mm, Changsha City takes surface water as its major water resource. Thanks to its sufficient water sources, Changsha City's mean annual surface runoff hits 80.8 billion cubic meters. Aside from Xiangjiang River, Changsha City pools 15 tributaries including Liuyang River, Laodao River, Jingjiang River, and Weishui River. The largest reservoir in Changsha is Huangcai Reservoir in Ningxiang County and Zhushuqiao Reservoir in Liuyang City.