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The average annual rainfall of Taiwan is about 2,500 mm.It is about 2.6 times the global average. However, the amount of water available for each person per year is about 4,000 cubic meters. This volume is only one-fifth of the global average, and Taiwan still ranks 19th on the world list of water scarce countries. The major reason is the steep terrain in Taiwan. Rivers are short and flow velocity is fast. When precipitation falls into rivers, most flows directly into the ocean. In particular, the rainfall distribution is uneven in time and space in the southern region of Taiwan.About 90% of annual precipitation concentrates in the period from May to October, and most occurs in rainy season and typhoon events. During this period, river water is not easy to use because of its high turbidity level. This makes water resources management more difficult. The watershed area of Gaoping River is large. Rain falls and turns into a river called Gaoping River, which is with the largest watershed in Taiwan. The watershed is about 3,257 square kilometers. Part of rain water infiltrates into soil to store in the ground, and becomes abundant groundwater resources.

Hyporheic Flow

Now, here is an explanation for what hyporheic flow is. The material beneath the earth’s surface is not just soil.It is like a sponge cake and has different levels.There are not only aquifers, which are composed of coarse particles (sand and gravel) and full of water between soil particles,but also aquitards, which are composed of fine particles (clay) and restricts water flow from one aquifer to another.And hyporheic flow is the water flowing or stored in the sand and gravel layers below river bed.