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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 1996

(1) In general the hydrological condition of 1996 was more than the long term average situation. During this year the water demand requested from different specific targets was satisfied with the suitable water allocation. In the last ten days of July, 1996, the tropical low air pressure in the West Pacific developed into typhoon Herb. It moved to the direction to the North and attacked Taiwan on July 31 and August 1. The storm range of Herb reached 700 kilometers (7th degree storm with a radius of 350 kilometers), the max. wind velocity in the center reached 191 kilometers .(16th degree wind with 35 meters/sec.). In comparison with previous typhoons, Herb was not the strongest one, but the rich rainfall and strong wind it brought caused serious damage in Taiwan.

This violent typhoon brought strong wind and downpour that made considerable raining intensity and accumulated rainfall, especially Mt. Ali Station in central Taiwan where the rainfall accumulated to be 899.5 millimeters in 9 hours, 1157.2 millimeters in 12 hours, 1575 millimeters in 18 hours, and 1748.5 millimeters in 24 hours. All these records exceeded the richest rainfall record in Taiwan and created the new record of the richest rainfall in 9, 12, 18, and 24 hours continuously (the original record of 862 millimeters in 12 hours was created in Dahgwang at Hualian stream, 1427 and 1672 millimeters in 18 and 24 hours separately was created in Shinliau at Dongshan river). The richest rainfall in 24 hours brought by Herb was very close to the extreme value of the world record of 1870 millimeters. One-day rainfall of 1094.5 millimeters and two-day rainfall of 1986.5 millimeters were about va (27%) and Vz (49%) separately of the average annual precipitation of 4065 millimeters.

(2) rainfall

The average annual rainfall of Taiwan in 1996 was 2,860 mm which was 14% more 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
1996 | 1949-1990
North | 3432 | 2934 | 117
Central | 2402 | 2081 | 115
South | 2507 | 2501 | 100
East | 3375 | 2715 | 124
Total | 2860 | 2515 | 114

(3).Streamflow

The annual streamflow of river in Taiwan in 1996 was more than the long term average. The total stream runoff in this year was 73.3 billion cubic meters which was 10% more 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
1996 | 1949-1990
North | 17.9 | 16.1 | 111
Central | 18.9 | 17.0 | 111
South | 17.0 | 17.2 | 100
East | 19.5 | 16.5 | 118
Total | 73.3 | 66.8 | 110