Chapter 6 : Hydrology and Soil

Hydrological Cycle (Water Cycle)

        When water evaporates from land or sea, it enters the atmosphere as vapor. When the moisture in the atmosphere cools, it condenses into cloud. Finally, it becomes rain or snow and falls down to the earth¡¦s surface again. Part of it seeps underground and becomes groundwater; the rest of it trickles down into rivers and lakes and flows into the ocean. The earth¡¦s water is continuously cycled from the atmosphere to the earth, from the earth to the ocean, and from the ocean back to the atmosphere. This cycle is called the ¡§Hydrological Cycle.¡¨

        When rain falls, it follows the terrain of the earth and forms small rivulets. These rivulets combine to form tiny creeks. As more and more tiny creeks merge, they gradually form rivers. If we look down from the sky or check a map, we will find that rivers often have many tributaries. Together, the main stream and its tributaries make up a river system. River systems are divided by watersheds, which are usually mountain ranges. Each river system has a drainage area; the precipitation that falls within this area either evaporates or flows into the river. So we can also say that watersheds separate drainage areas.

 

River Systems in Taiwan

 

        Figure 6-1 shows the distributions of rivers in Taiwan. There are 129 rivers of different sizes. The river density of Taiwan is very high. The rivers are distributed throughout the island, so that if you look at a map of Taiwan¡¦s rivers, you basically see an outline of Taiwan. Taiwan is a long and narrow mountainous island, so its rivers are relatively short. There are only six rivers longer than 100 kilometers.    Taiwan¡¦s shape and terrain result in riverbeds with very steep gradients. In other words, Taiwan¡¦s rivers flow downhill very quickly and there are a lot of rapids. They also have considerable washout power. Seasonal rainfall variations result in tremendous differences in river volume. During summer, torrential rains cause rivers to swell and surge furiously. Such a river is a spectacular sight but also very dangerous. Winter is the dry season. The rivers in southern Taiwan shrink to tiny creeks, or dry up leaving nothing but dust and gravel.

Figure 6-1 The River Systems of Taiwan                 ²H¤ôªe¡GDanshuei River                 ¤j¥Ò·Ë¡GDajia River                 ¤j¨{·Ë¡GDadu River (Wu River)                 ¿B¤ô·Ë¡GJhuoshuei River                 ´¿¤å·Ë¡GZengwun River                 °ª«Ì·Ë¡GGaoping River                 ¤½¨½¡Gkm

 

Water Resources

        The average annual rainfall of Taiwan is around 2,500 mm, almost 3.5 times of the average global rainfall. If we subtract evaporation from precipitation, the remainder is the water we can tap as a resource. Although the rainfall of Taiwan is abundant, the population density is also high. In addition, terrain and weather reduce the amount water that is actually available. The result is that Taiwan has a water problem: per capita Taiwanese people are relatively water deprived. Taiwan¡¦s per capita water supply is only half the world average. Water conservancy facilities such as reservoirs, ponds and pools are used to store water and maintain the water supply.

        Taiwan¡¦s water resources include surface and groundwater. Water is consumed by households, traditional farms for irrigation, fish farms, and industry.

        In recent years, improper land use and outright exploitation have polluted Taiwan¡¦s water. In some areas, excessive pumping of groundwater has caused serious environmental problems like stratum subsidence. There is an urgent need for effective governmental regulation.

 

Soil

        The rocks on the surface of the earth will become soil after long-term weathering. They will form different kinds of soil depending on terrain, climate, time, and what organisms live in the soil. Different soils vary in color, viscosity, and particle size. These features determine what we can do with a certain kind of soil. The three main soil types of in Taiwan include laterite and loess, both of which are formed by weathering, and alluvial soil, which is deposited by rivers. Human beings put different kinds of soil to different uses. When deciding land usage, we also take terrain into account.

        The formation of soil is a slow process. Even a thin layer of soil takes at least three to five hundred years to form. With some types of soil in certain conditions, a thin layer may even take thousands of years to form. However, it does not take long for it to wash away in a torrential rain. When willful human exploitation is unrestrained by effective water and soil conservation policy, washout erosion becomes a serious problem. Once washed away, soil can never be reclaimed.

        Soil is also susceptible to pollution. The main sources of pollution are homes and factories that discharge enormous amounts of untreated waste water contaminated with heavy metals. Farmers who use excessive pesticides and fertilizers add to the pollution problem. These noxious substances contaminate the soil, threatening people¡¦s health through bioaccumulation. Therefore, protecting the soil from erosion and pollution is important for everyone.

 

Water and Soil Conservation and Wildlife

        Taiwan is a beautiful, densely-wooded island. The ecological environment allows wildlife to thrive. Many species of wild animals once inhabited the entire length of the island. Place names such as Lugang, literally Deer Harbor, give us an indication of the richness of animal life in the past. As the human population of Taiwan grew, habitat for wildlife was reduced. The number of wild animals and even plants declined. This is still happening today: many species of the wildlife are endangered or already extinct. Deforestation makes it even harder to maintain water and soil balance.

        Plants need to assimilate nutrients and water from the soil in order to grow. Plants also protect the soil from washout. Taiwan¡¦s abundant rainfall and steep terrain mean that washout is still a serious problem. Without proper water and soil conservation, the ecological environment may suffer irrevocable damage. Instead of merely seeking short-term benefits, we should put more effort into water and soil conservation, especially on slopeland. This is the only way to ensure a sustainable ecological environment.