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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 1998 there were 133 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.Summary of weather and Hydrogical Events in 1998

In general the hydrological condition of 1998 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 , no natural calamity resulted from the draught or flood events.

(1). Meteorology

There were local severe showers in Taiwan between June 1st and 11th due to the southwestern current and stagnating frontal edge. Showers were considerably severe in the central and southern parts of Taiwan. Areas with precipitation exceeding 900 mm include Kaohsiung and Pingtung. It brought in 300 million cubic meters of rainwater to reservoirs. The rains also tied in serious disasters.

Typhoon Otto landed at Taitung around the noon time on August 4th and departed from Wuchi, Taichung at 6 P.M. on the same day. Although the typhoon radius was smaller and its duration was short, it still brought plenty of rain. Areas with rainfall exceeding 400 mm of rainwater include Chiayi and Alishan. Typhoon Yanni swept through the eastern ocean between September 27th and September 29th without touching down at Taiwan. However it came in with deluges at northern and northeastern Taiwan. In Shuangshi the precipitation hit 743 mm. Keelung and Kulu received more than 500 mm precipitation. All the reservoirs in northern Taiwan almost reached their full capacities.

Deluges appeared in northern, northeastern and central mountain parts of Taiwan between October 14th and 17th due to the southbound frontal edge and the peripheral whirlwinds of Typhoon Zeb. In addition to the above, heavy showers appeared at several other places. Chuchihu, Meishan and Pinglin received more than 800 mm precipitation of rain which caused serious flooding. Shichi was particularly damaged.

The recovery relies on the government efforts to improve earth/land, rivers and urban sewage conditions. There were showers appeared at northern and northeastern parts of Taiwan between October 24th and 28th due to the seasonal northeastern winds and the peripheral whirlwinds which came with Typhoon Babs. Shinlioa, Hanshi and Loutung in Ilan received precipitation of rain more than 1000 nun. Shichi and Keelung areas again were flooded.

(2).Rainfall

The average annual rainfall of Taiwan in 1998 was 3,322 mm which was 32% 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
1998 | 1949-1990
North | 4674 | 2934 | 159
Central | 2641 | 2081 | 127
South | 2948 | 2501 | 118
East | 3443 | 2715 | 127
Total | 3322 | 2515 | 132

(3).Streamflow

The annual streamflow of river in Taiwan in 1998 was more than the long-term average. The total stream runoff in this year was 83.4 billion cubic meters which was 25% 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
1998 | 1949-1990
North | 4674 | 2934 | 159
Central | 2641 | 2081 | 127
South | 2948 | 2501 | 118
East | 3443 | 2715 | 127
Total | 3322 | 2515 | 132