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SUMMARY OF HYDROLOGICAL FEATURES

IN TAIWAN

1. Geography－The area of Taiwan, which is 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. Northeast monsoon prevails from October through March next year. Typhoons and thunderstorms bring in abundant rainfall in May through September. The crop growing season in the plains ex-tends throughout the whole year.

4. Geology－The mountains in Taiwan are dominated by sedimentary rocks. It is 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 period (Fig. 2, Tables I and 2). Hydrographs are characterized by high peak and short duration, with heavy silt content. Streamflow in winter and spring seasons is much lower than that in summer period. Shortage of water usually occurs in February through May.

6. Hydrology－About 78％ of annual precipitation falls from May through October with the major portion caused by storm rainfall or typhoons. In 1979, there are 131 hydrometric stations and 1,094 rainfall stations in operation. Due to the unfavorable natural conditions, many difficulties have been encountered for the measurements of streamflow and sediment.

7. Assessment of water resources – Based on island wide records from 1949 to 1979, the average annual rainfall for the entire area of Taiwan is 2,510 milli-meters in depth (Fig. 3) or 90 billion cubic meters in volume. Average annual ninoff is about 67.5 billion cubic meters. However, only 22 of the average annual runoff or 14.9 billion cubic meters are available for use during the dry season (Fig. 4 and Table 3). The annual pumped groundwater is estimated as 3.6 billion cubic meters (Fig. 6).

8. Summary of Hydrological Conditions in 1979

(1) Rainfall

According to the island wide records, the average annual rainfall for Taiwan was 2,333 millimeters in 1979 (Fig. 7), which is 7 lower than the long term average rainfall, 2,510 millimeters, and 4 lower than 2,416 millimeters rainfall in 1978. For the areal distribution of annual rainfall, most area were lower than that of long-term average, except the upstream part of a few river basins, (Table 2, Fig. 8).

(2) Streamflow

The streamflow for Taiwan was estimated as 92％ of longterm average in 1979. The streamflow for rivers in north, east regions and southern portion of south region were lower than the longterm average, except Touchien Chi, while the other rivers in central and south regions were above the long-term average, except Tachia Chi, Wu Chi and Peikang Chi.

(3) Floods

There were fewer damage made by typhoon in 1979, although twenty three typhoons formed in Western Pacific Ocean and parts of them developed into violent typhoons in this year. Owing to the systematic position of atmosphere did not match, all of them changed their direction and disappeared quickly and none of them landed into this island. In July, Gordon and Hope passed through the southern part of Luzon strait, which caused the disaster of salt water intrusion in coastal regions of southwestern part of the island and brought plenty of rainfall. In August, lrving passed through tile off sea in northeast Taiwan, which brought lots of rainfall for the north region. As the result destroyed the Chin Tan dam that influenced the water supply of Taipei metropolitan.

(4) Observation of Groundwater Level

In Taiwan, observation of groundwater level is implemented by the Provin-cial Water Conservancy Bureau and the Bureau of Public Works of Taipei Municipal Government. There were 274 existing observation wells on the island in 1979, 268 of which were operated by PWCB and other 6 wells by BOPW. I he data collected from these wells is used for development of groundwater (Fig. .5). The development of industry and commerce in Taipei municipal area have greatly evolved in the recent years. It causes the consi-imption of water increased rapidly, therefore, many users drilled deep wells themself and pumped a great amount of groundwater every year. It results in the sharply draw down of groundwater level and the subsidence within Taipei Basin. For collaboration of groundwater management of Taipei Area, the commission .has undertaken the investigation of ground-water level periodically since 1968,.and has schemed the eontourlines of groundwater level after each investigation. According to the observed data in 1979, for most parts of Taipei Area, 204 km2 or 85％ of the whole area of basin, its groundwater level was higher than that in 1978. The rest 15％ of the area in the marginal parts, the groundwater level has decreased. Analysis of Water Resources Supply and Demand

(5) Analysis of Water Resources Supply and Demand

The analysis of water resources supply and demand for 1979 is shown in Table 5. Total annual consumptive water was 16.7 billion cubic meters, 78％ of which was surface water. However only one quarter of the surface water was regulated by reservoirs. The utilization rate (the ratio of the consumed water to the annual runoff) of surface water was 21％, while the utilization rate of the groundwater was 91％.

(6) Analysis of Water Utilization and Sedimentation for Major Reservoirs

Water utilization and sedimentation of nine major reservoirs in 1979 are shown in Table 6. Total capacity of these reservoirs is 1,390 million cubic meters or 91％ of total capacity of all reservoirs and ponds. Consumptive water released from these reservoirs in 1979 is equal to longterm average from 1965 to 1979 and lower than 1978 by 481 million cubic meters. Wusheh - Sun Moon Lake and Tsengwen Wushantou Reservoirs are serial linked reservoirs, the high release ratios show they have good efficiency of supply water. The release ratio of Peiho and Akingtien reservoirs were lower than others, only 1.47 and 1.42, respectively. Sedimentation ratio of Akungtien Reservoir is 45.7％ which is the highest one among all the major reservoirs. Techi and Tsengwen reservoirs were completed recently, therefore, their ratio of sedimentation is as low as 2.5 and 3.6. respectively. The amount of sedimentation of Shimen Reservoir is always below the designed sediment by 800 thousand cubic meters per year, since 1977. In 1979, there were only 180 thousand cubic meters of sedimentation, it is resulted by the accomplishment of soil and water conservation in the up-stream of the watershed.

(7) Promotion of Hydrological Activities

In order to strengthen the investigation of Science and Technology Development for Taiwan water resource, the Commission has positively promoted the systematization of hydrological data. A pilot study of hydrologic data processing and its multipurpose applications was accomplished by the Council for Agricultural Planning and: Development. The streamflow data bank" is being compiled by the- Provincial water Conservancy Bureau and will be provided to the agencies concerned to use.