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SUMMARY OF HYDROLOGICAL FEATURES

IN TAIWAN

1. Geography—The area of Taiwan, an island and also a province 'of the Republic of China, is 36,002 square kilometers (13,900 square miles). It is located in the subtropical region with the Tropic of Cancer traversing its southern part.

2. Topography—The area of steep mountain terrain above 1,000 meters high covers about 32% of that of the island; hills and terraces between 100 and 1,000 meters high make up 31%; while alluvial plains below 100 meters high, where most communities, farming activities and industries are concentrated, cover the remaining 37% (Fig. 1).

3. Climate—The Climate is subtropical. The Northeast monsoon prevails from October through March of the next year. Typhoons and thunderstorms bring in abundant rainfall from May through September. The crop-growing season in the plains extends throughout the year.

4. Geology—The mountains in Taiwan are dominated by sedimentary rocks. They are 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—There are 21 Princpal Rivers, 29 secondary rivers and 79 ordinary rivers in Taiwan. Rivers in Taiwan are so iods. Hydrographs characterized by high peaks and short duration, with heavy silt content. Strreamflow in winter and spring seasons is much lower than that of summer period. Shortage of water usually occurs in Febuary through May.

6. Hydrology—About 78% of annual precipitation falls from May through October, with major portion provided by storms or typhoons. In 1991 there were 134 hydrometric stations and 1000 rainfall stations in operation. Due .to the unfavorable natural conditions, many difficulties were encountered for the measurements of streamflow and sediment, (Fig. 2, Table 1 and Table 2).

7. Assessment of water resources—Based on island wide records from 1949 to 1990, the average annual rainfall for the entire Taiwan area is 2,515 millimeters in depth or 90.5 billion cubic meters in volume. The average annual runoff is about 66.8 billion cubic meters However, only 23% of it, about 15.4 billion cubic meters, is available for use during the dry season. The annual pumped groundwater is estimated at 7.1 billion cubic meters, (Fig. 3, Fig. 4, Fig. 6 and Table 3).

8. Hydrological features in 1993

(1) During the first half of this year, the precipitation appeared normal. Insummer 1993, there were 29 typhoons originated in the West Pacific Ocean, but none of them reached Taiwan. Consequently, no sufficient rainfalloccurred and Taiwan area, thus, suffered a severe drought.

(2) 1993 was a dry year. The average annual rainfall was 1,645 mm which was 35% less than the long term ( 1949-1990 ) average of 2,515 mm. The total stream nmoffwas 38.1 billion cubic meters which was 43% less than the long term ( 1949-1990 ) average of 66.8 billion cubic meters.

(3) According to the entire rainfall record, the driest year was 1980, and its average annual rainfall was 1,605 mm. This year, the average annual rainfalll was 1,645 mm, and it became the second driest year in record. Even more worse was that the drought period was in the wet season. In 1980, Typhoon Nooris brought in abundant rainfall in August, and the drought condition was relieved considerably, even with a lowest annual total rainfall. However, no single typhoon reached Taiwan area this summer, and drought condition became much severe.

During the drought period of this year,all the agencies concerned devotedgood effects and coordination to overcome this cruel period. All availablewater resources were effectively used.