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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—Rivers in Taiwan are short and steep and torrential during storm periods. Hydrographs are characterized by high peaks and short duration, with heavy silt content. Streamflom in winter and spring seasons is much lower than that in summer period. Shortage of water usually occurs in Feburary through May.

6. Hydrology—About 78% of annual precipitation falls from May through October, with major portion provided by storms or typhoons. In 1986, there were 120 hydrometric stations and 965 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 1979, the average annual rainfall for the entire Taiwan area is 2510 millimeters in depth or 90 billion cubic meters in volume. The average annual runoff is about 67.5 billion cubic meters However, only 22% of it, about 14.9 billion cubic meters, is available for use during the dry season. The annual pumped groundwater is estimated at 3.8 billion cubic meters, (Fig. 3, Fig. 4, Fig. 6 and Table 3).

8. Summary of Hydrological Events in 1982

(1).Climate

The climate of 1986 In Taiwan is quite normal. The monsoon season ended on June 11/ and was soon followed by typhoon season. There are 19 typhoons occurred in the Pacific Ocean this year. Only four of them influenced Taiwan area areatly (See Fig. 10).

I .Medium Typhoon " Nancy " landed between Hualien and Hsinkong on the early morning of June 24, and then went up north along Suhua highway. As Nancy washeading north, she brought a rainstorm that caused great damage to the traffic of Kaoping and eastern Taiwan areas.

II .Severe Typhoon " Peggy " Passed southeast region and southern area on the night of July 9. The flow around in circles brought strong wind, Pouring rain and torrential flood. The intruding sea water flooded the coastal region of KaoPing area, and destroyed the traffic systems of the eastern coast. Even the levees were washed away.

III.Medium Typhoon " waYne " passed Penflhu at about 3:30 a.m. on August 22. then landed Choshui river mouth at 6:40 a.m. and left Taiwan from Hualien at 1:00 P.m. He lingered at sea near Shihyuan island. Two days later, he moved towards southwest, and landed Hengchun Peninsula. Then it became a tropical low pressure and moved south towards Tungsa island. On August 28, this tropical low Pressure developed to a tYPhoo again and lingered at sea near Taitung. This was his third threat to Taiwan, and on September 1, he finally wentwest. This was a unique Typhoon at climate history In Taiwan. It had lasted for 16 days and it was the first typhoon which threatened Taiwan area for three times. Penahu, Southern and Central Taiwan areas were seriously damaged. Thousands of houses were collapsed of traffic and Power systems were destroyed, levees Mere ruptured. The fishin9 boats were toppled or capsized. The total loss is more than twenty billion New Taiwan dollars.

IV .Medium Typhoon " AbbY " landed between Taitun9 and Hsinkana at 8:00 a.m. on September 19 and left Taiwan from Wuchi at 11:00 P.m. The 9usts were 9reater than 10-scale because Typhoon Abby occurred during the duration of flood tide on lunar August 15 and 16. As aresult, Yunlin and Chianan areas were completely under intruding sea water. The levees along the coast were washed away. The total loss was estimated as 5 billion New Taiwan dollars.

(2).Rainfal L

The average annual rainfall of Taiwan in 1986 was 2,585

mm/ which was 3 % more than the long-term ( 1949-1986 )

average annual rainfall of 2,504 mm.

Re 田 on | Average annual rainfall in mm For the years indicated | Percentage
1986 | 1949-1986
North Central South East | 3502 2042 2126 3011 | 2911 2075 2513 2684 | 120 98 85 112
Total | 2585 | 2504 | 103

(3 ). Streamf Low

In 1986/ the annual stream-flow of northern Taiwan was more than the longterm (1949-1986) average annual streamflow, while the annual streamflow of other regions was less than the long-term's (1949-1986). The total stream runoff in 1986 was 67.1 billion cubic meters/ which was 1 X less than the long-term (1949-1986) average annual straamflow of 66.7 billion cubic meters. The streamflow of major river basins is shown in Table 2.

(4 ).Groundwater

There were 337 observation wells In Taiwan area in 1986. The investigation of groundwater level is the duty of Taiwan Provincial water Conservancy Bureau (a total of 326 wells) and Bureau of Public works of Taipei Municipal Government (11 wells totally). The average annual recharge of groundwater in Taiwan was estimated at 4 billion cubic meters. However/ 4.1 billion cubic meters were withdrawn in 1986. The amounts of groundwater drawing from Hsin-Miao coastal area, Choshui River alluvial fan area, Chia-Nan Plain area and Pingtung Plain area far exceeded the safe yield. This caused the land subsidence in some local areas. (See Fi9.-11/ Fi9. 12)

(5).Water consumption in 1986

The total amount of the annual consumptive water In 1986 was calculated at 18.4 billion cubic meters, of which 14.9 billion cubic meters ( 81% ) were used for agriculture 1.9 billion cubic meters ( 10% ) for domestic water supply, 1.6 billion cubic meters (9% ) for industrial use .

Unit：Billion (109)m3

Region | Agriculture | Domestic | Industrial | Total
North | 2.3 | 0.9 | 0.6 | 3.6
Central | 5.7 | 0.4 | 0.4 | 6.5
South | 5.3 | 0.4 | 0.6 | 6.3
East | 1.7 | 0.04 | 0.04 | 1.8
Total Taiwan Area | 15.0 | 1.8 | 1.6 | 18.4

(6) The Operation of major reservoirs

There were 32 reservoirs in 1986. The total storage and effective capacity of existing reservoirs are 1.96 and 1.48 billion cubic meters respectively. The effective capacity of 10 major reservoirs is 1.77 billion cubic meters, which is 95% of that of all reservoirs.

The analysis of water utilization and sedimentation for major reservoirs in 1986 is shown in Table 7.