◢Page Up

中　　文

Page Down◣

SUMMARY OF HYDROLOGICAL FEATURES

IN TAIWAN

1. Geography－Taiwan, an island province of the Republic of China, is 35,981 square kilometers( 13,892 square miles ) in area. It is located in the subtropical region with the Tropic of Cancer traverses the island through its southern half.

2. Topography－Steep mountain terrain above 1,000 meters in elevation covers about 32％ of the island; hills and terrace land between 100 and 1,000 meters make up 31％; and alluvial plains below 100 meters, where communities and farming activities and industries are concentrated, comprise the remaining 37％ ( Fig.1 )

3. Climate－The climate is subtropical. Northeast monsoon prevails from October through March. During the summer and autumn, typhoons and thunderstorms bring in abundant rainfall. The growing season in the plains extends throughout the year.

4. Geology－The mountains in Taiwan are dominated by sedimentary rocks, It is weak, fractured, and deeply weathered. Heavy erosion occurs because of intense rainfall and high velocity of water flow and is further aggravated by earthquakes which damage the stability of the hill slopes.

5. Rivers－Rivers are short and steep, and torrential during storms. ( Fig.2 and Table 1 and 2 ). Hydrographs usually characterized by high peak and short duration, with heavy silt content. Flow in winter and spring seasons is much lower than in summer period, Shortage of water usually occurred during February to May.

6. Hydrology－About 76％ of annual precipitation occurs from May through October and major portion is caused from storm rainfall. In 1977, there are 121 hydrometric stations and 1100 rainfall station in operation. Due to the unfavorable natural conditions many difficulties have been encountered for the measurement of streamflow and sediment.

7. Evaluation of Water Resources－Based on long term island wide rainfall records, the average annual rainfall of the total area is calculated at 2,430 millimeters ( Figure 3 ) which equivalent to a total volume of 87.4 billion cubic meters. Average annual runoff of the total area is calculated at 2,430 millimeters ( Figure 3 ) which equivalent to a total volume of 87.4 billion cubic meters. Average annual runoff is about 67.1 billion cubic meters. However, only 22％ or 15 billion cubic meters are available for use during dry season ( Figure 4 and Table 3 ). The annual withdrawal of groundwater is estimated at 3.2 billion cubic meters ( Figure 5 ).

8. Summary of Hydrological Conditions in 1977

(1) Rainfall

According to the island wide records, the average annual rainfall for Taiwan in 1977 is 2,852 millimeters, which is 17％ higher than long-term average, 2,430 millimeters ( Fig. 7 ), and 25％higher than 2,280 millimeters in 1976. For the regional distribution, the rainfall in 1977 is higher than that of long-term average except in a few areas, such as the eastern and northeastern parts of the island, and the upstreams of Touchien and Taan River Basins.

The annual rainfall in the southern part of the island in 1977 is even higher than that of the long-term average by 49％. However the monthly distribution of rainfall in 1977 was uneven. The total rainfall in February through early May in 1977 was only 40％ of the long-term average rainfall, it resulted in a distressed drougnt ( Table 2 Fig 8 ).

(2) Streamflow

Accordance to the streamflow records, the streamflow of the eastern rivers and the northern rivers is lower than the long term average. The recorded streamflow of the rivers in the central and south regions is greater than the long term average ( Figure 8 and Table 2 ).

(3) Floods

There were 4 Typhoons in 1977, Thelam, Vera, Amy, and Dinah. Which attacked the island. Thelma and Vera which landed on the South and the north Taiwan, respectively,

In Taiwan, most floods are caused by typhoon storms. In this year, only one typhoon, named Billie hit the island on August 9. Typhoon Billie caused severe flood damage in central and northern part of the island.

(4) Drought

Due to the East-Asia continental high pressure was erratic westward, most parts of this island had a great deficiency in precipitation during February through early May 1977. In the whole spring, the rainfall in western part of the island was 40％ below 20 mm. ( Fig. 10 Fig. 11 ). Water supply and agricultural irrigation were seriously affected by the drought. The streamflow was less than the normal for the whole island. In normal streamflow ( Fig 12 ) except Chishui and Tsengwen chi, due to the regulation by reservoirs or pools. In this period, the total inflow of reservoirs in this island was only about 55％ of the long-term average, but the water supply still maintained at the 90％ of the long-term average. In south region the water released from Tsengwen, Wusantou, and Paiho reservoirs in this period was as much as long-term average, therefore, the drought damages were reduced to the minimum extent ( Table 5 ).

(5) Observation of Groundwater Level

In Taiwan, observation of groundwater level is implemented by Provincial Water Conservancy Bureau and Bureau of Public Works of Taipei Municipal Government. There were 281 existing observation Wells in the island in 1977, of which 274 wells were implemented by PWCB and 7 wells by BOPW. The data collected from these wells is used for the development of groundwater ( Fig. 5 ).

(6) Supply and Demand Analysis of Water Resources

The supply and demand analysis of water resources in 1977 is shown in Table 6. The total annual consumptive water was 168 hundread million cubic meters, 79％of which was surface water in which more than one quarter was regulated by reservoirs. The utilization rate ( the ratio of the developed and diverted water to the average runoff ) of surface water was 19.8％, while the utilization rate of the groundwater was 87.3％.

9. ERTS Photomap of Taiwan

The NASA Earth Resources Technology Satellite-1 ( ERTS-1 ) launched July 23, 1972 is the first satellite dedicated wholly to the sask of obtaining imagery of the earth’s surface for earth survey purposes. In order to introduce the applications of remote sensing technology to hydrology and water resources, four ERTS Photomaps of Taiwan are attached ( Figures 9).