According to Water Resources Agency statistics, Taiwan has annual rainfall of about 88,000 MCM. Over 25 % of this evaporates and some 4 % seeps into the ground, leaving around 70 % to enter the oceans. Because of the steep lay of the land, most river water flows very quickly. Only some 12,000 MCM is retained in reservoirs to be used for irrigation.
據水利署統計，台灣每年平均降下的雨量約為八百八十億噸，其中二成多蒸發至大氣中，4 % 入滲到地面下，約七成為河川逕流量，而流入河川中的雨水又因為坡陡流急，大部分奔流入海，因此被人們攔河築壩蓄水、引入灌溉溝渠利用的，只剩下一百二十多億噸了。