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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 T&% of annual precipitation falls from May through October, with major portion provided by storms or typhoons. In 1981, there were 122 hydrometric stations and 1170 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

In 1982, the climate of Taiwan was normal during January through April, the monsoon season begins since May. The rainfall was not much in the eastern region. After July, as influenced by the tropical cyclone, the weather was unstable and thunder-storm prevails. There were 26 Typhoons occurred in the northern Pacific Ocean in the year, however, only three of them influenced area of Taiwan. The severe typhoon Andy (July 28-30) and medium typhoon Dot (August 9-13) landed over Taitung and the area between Taitung and Hengchun respectively.

As a result, tremendous rainfall happened in Alishan and Taitung area, while a continuous strong winds occurred in Taipei area. Large damage for railroad and highway happened in the eastern region. At the same time the rice crops was damaged severely, near Taitung and Hengchun area.

Typhoon Cecil occurred on August 6-7, it did not hit the island directly yet. However, due to the circulation of air mass, it brought heavy rainfall and severe damage caused by flood and landslide in Wuku and Taishan area of northern region. In central region, flood over the bridge and damaged on Tachia Chi, and suspended the communication of the main north-south railway for short period.

(2) Rainfall

The average annual rainfall in 1982 over the whole Taiwan area was 2,298 mm, which was 8 percents less than the long-term (1949-1979) average annual rainfall of 2,510 mm; and was 22 percents lower than the average annual rainfall of 2,961 mm in 1981. The average annual rainfall of major river basins in 1982 refer to Table 2, Fig. 7 and 8.

Re 田 on | Average annual rainfall in mm For the years indicated | Average annual rainfall for 1982 as % of that for
1982 | 1981 | 1949-1978 | 1949-1979 | 1981
North Central South East | 2493 2051 2474 2215 | 3174 2679 3157 2893 | 2851 2075 2578 2679 | 87 99 96 83 | 79 77 78 77
Total | 2298 | 2961 | 2510 | 92 | 78

(3)Streamflow

In 1982 the annual streamflow of most river basins was lower than that of longterm (1949-1979) average, except Tachia and Wu basins in the central region as well as Erhjen and Kaoping Basins in the south region. The total streamflow of river systems in 1982 was calculated at 60.8 billion cubic meters which was 10 percents less than that of the longterm (1949-1979) average of 67.5 billion cubic meters. The stream-flow of major river basins are shown in Table 2.

(4) Groundwater

There were 323 observation wells in Taiwan area in 1982. The investigation of groundwater level was implemented by Taiwan Provincial Water Conservancy Bureau (Total 312 Wells) and Bureau of Public works of Taipei Municipal Government (Total 11 Wells). The average annual recharge of groundwater in Taiwan was estimated at 4 billion cubic meters. However, 3.8 billion cubic meters were withdrawn in 1982. Excessive groundwater withdrawn from Chiayun Coastal Area and Pingtung Coastal Area were recorded. It induced the land subsidence in both areas.

(5) Water consumpotion in 1982

The total amount of the annual consumptive water use in 1982 was calculated at 17.1 billion cubic meters, of which 14.1 billion cubic meters (82$^) were used for irrigation; 1.2 billion cubic meters (7 %') for domestic water supply; and 1.8 billion cubic meters (11^) for industrial use. The following table shows the water use in various regions in 1982.

Region | Domestic | Irrigation | Industrial | Total
North | 716 | 680 | 2368 | 3771
Central | 201 | 467 | 5883 | 6551
South | 298 | 674 | 3828 | 4773
East | 27 | 48 | 1969 | 2044
Total Taiwan Area | 1242 | 1849 | 14048 | 11739
% Of Total | 7 | 11 | 82 | 100

The analysis of water resources supply and demand in 1982 is shown in Table 5. Of the total water demand in 1982 22 percents (3.8 billion cubic meters) were drawn from groundwater, 78 percents (13.3 billion cubic meters) were from surface water. 24 percents of the surface water (3.2 billion cubic meters) were from reservoir regulation and To% (10.1 billion cubic meters) from streamflow. The utilization factrrs of surface and groundwater are 22 percents and 95 percents respectively.

(6) Operation Analysis of major Reservoirs

The total storage and effective capacities of existing reservoirs are 2.0 and 1.5 billion cubic meters respectively. The effective capacity of nine major reservoirs is 1.4 billion cubic meters, which is 91 percents of that of all reservoirs and ponds.

The analysis of water utilization and sedimentation for major reservoirs in 1982 is shown in Table 6.

Consumptive water released from these reservoirs in 1982 is approximately equal to the long-term average from 1965 to 1982 and larger than 1981 by 427 million cubic meters. Wusheh-Sun Moon Lake and Tsengwen-Wushantou Reservoirs are serial linked reservoirs; the high release ratios show that they have good effici ency of supply water. The release ratio of Peiho and Akungtien reservoirs were lower than others, only 1.81 and 1.41 respectively.

Sedimentation ratio of Akungtien Reservoir is 41.68 percents which is the highest among all reservoirs. Techi and Tsengwen reservoirs were completed after 1973; therefore, their ratio of sedimentation is 3.94 and 5.60 percents respectively. The amount of sedimentation of Shimen Reservoir is always less than the designed sediment discharge 800 thousand cubic meters per year. (890 thousand cubic meters in 1982) Since 1971, it is resulted by the accomplishment of soil and water conservation in the upstream of the watershed.