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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 132 hydrometric stations and 915 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. Summary of Hydrogical Events in 1995

(1).In general the hydrological condition of 1995 was less than the long term average situation, During this year the water demand requested from different specific targets was satisfied with the suitable water allocation,No natural calamity resulted from the draught or Typhoon events.

a. From Jane 6 to 9,1995 in Taiwan area, Both the strong air current induced by the Typhoon DEANNA and the front made a double effect on shower and heavy rain occurred island-wide, Intensity rainfall caused flood disaster in south-center area.

b. A outer surround current induced by the Typhoon KENT from August 30 to August 31,1996,that made an effect on shower inland wise, heavy rainfall occurred over south island wise. heavy rainfall occurred over south-east area. In addition the Typhoon carried gale over the inland.

c. The outer surround current resulted from Typhoon RYAN dated September 20,1995 activated shower island wise, Duing the next two days, shower occurred over whole island and heavy rain appeared in south-east area, intensity rainfall located in norther mountain area and north-east area. that were affected by the Typhoon RYAN, Furthermore, an induced gale hit both the south-east area and southern area.

(2). Rainfall

The average annual rainfall of Taiwan in 1995 was 1,914 mm which was 24 % less than long term average annual rainfall of 2,515 mm (see table below):

Region | Average annual rainfall in mm for the year indicated | Average annual rainfall in 1998 as % of that in 1949-1990
1995 | 1949-1990
North | 2292 | 2934 | 78
Central | 1603 | 2081 | 77
South | 1637 | 2501 | 65
East | 2324 | 2715 | 86
Total | 1914 | 2515 | 76

(3).Streamflow

The annual streamflow of river in Taiwan in 1995 was less than the long term average.The total stream runoffin this year was 49.0 billion cubic meters which was 27 % less than the long term average 66.8 billion cubic mater.(see table below) :

Region | Average annual streamflow in billion cubic meters for the year indicated | Average annual rainfall in 1998 as % of that in 1949-1990
1995 | 1949-1990
North | 11.8 | 16.1 | 73
Central | 12.5 | 17.0 | 74
South | 11.0 | 17.2 | 64
East | 13.7 | 16.5 | 83
Total | 49.0 | 66.8 | 73