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SUMMARY OF HYDROLOGICAL FEATURES

IN TAIWAN

1. Geography－Taiwan, an island province of the Republic of China, is 35,990 square kilometers( 13,896 square miles ) in area. It is located in the subtropical region with the Tropic of Cancer traverses the island through its southern half.

2. Topography－Steep mountain terrain above 1,000 meters in elevation covers about 32％ of the island; hills and terrace land between 100 and 1,000 meters make up 31％; and alluvial plains below 100 meters, where communities and farming activities and industries are concentrated, comprise the remaining 37％ ( Fig.1 )

3. Climate－The climate is subtropical. Northeast monsoon prevails from October through March. During the summer and autumn, typhoons and thunderstorms bring in abundant rainfall. The growing season in the plains extends throughout the 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 storms. ( Fig.2 and Table 1 and 2 ). Hydrographs usually characterized by high peak and short duration, with heavy silt content. Flow in winter and spring seasons is much lower than in summer period, Shortage of water usually occurred during February to May.

6. Hydrology－About 78％ of annual precipitation occurs from May through October and major portion is caused from storm rainfall. In 1978, there are 126 hydrometric stations and 1100 rainfall station in operation. Due to the unfavorable natural conditions many difficulties have been encountered for the measurement of streamflow and sediment.

7. Assessment of water resources – Based on island wide long term rainfall records, the average annual rainfall for the entire area of Taiwan is 2,559 millimeters in depth or 92.1 billion cubic meters in volume. Average annual runoff is about 67.7 billion cubic meters. However, only 22％ of the average annual runoff of 15 billion cubic meters are available for use during the dry season. The annual pumped groundwater is estimated as 3.6 billion cubic meters.

8. Summary of Hydrological Conditions in 1978

(1) Rainfall

According to the island wide records, the average annual rainfall for Taiwan was 2,416 millimeters in 1978, which is 6％lower than the long-term average rainfall, 2,559 millimeters, and 18％lower than 2,852 millimeters rainfall in 1977. For the various regional distribution, the annual rainfall recorded in the coastal plains of central, south and east regions in 1978 is lower than that of long-term average rainfall by 10％~40％, however, the other parts of the island is higher than that of long-term average.

(2) Streamflow

According to the observed records in 1978, streamflow for the rivers in east and central regions were below the normal, except Hsiukuluan Chi, Houlung Chi, and Peikang Chi, while the other rivers in north and south regions were above the long-tern average, except Tanshui River, Lanyang Chi, Tsengwen Chi, Erhjen Chi, and Linpien Chi.

(3) Floods

There were 4 typhoons, Olive, Rose, Della and Ora, attacked the island 1978. Rose and Della landed at Hsinkang and at south to Ilan respectively and then weakened quickly. The other two typhoons, Olive and Ora passed Luzon Strait and Pacific Ocean near east Taiwan, respectively. Only Ora caused flood in the suburban area of Taipei metropolitan and land slide in lowland. The other typhoons didn’t cause any damage fortunately, on the contrary, brought lots of rainfall which solved drought.

(4) Observation of Groundwater Level

In Taiwan, observation of groundwater level is implemented by the Provincial Water Conservancy Bureau and the 280 existing observation wells on the island in 1978, 273 of which were operated by PWCB and other 7 wells by BOPW. The data collected from these wells is used for the development of groundwater. The development of industry and commerce in Taipei municipal area have greatly evolved in the recent years, It causes the consumption of water increased rapidly. Therefore many users drill deep wells themselves and pumped a great deal 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 groundwater level periodically since 1968, and has schemed the contour lines of groundwater level after each investigation. According to the observed data in 1978, however, it is shown that the groundwater level for 204 KM2 or 85％ of the area of whole basin, has raised significantly, and for 70 KM2 or 29％ of the area of the basin has raised two meters high due to the effect of groundwater control carried out in the past a few years.

(5) Analysis of Water Resources Supply and Demand

The analysis of water resources supply and demand for 1978 is shown in Table 5. Total annual consumptive water was 17.2 billion cubic meters, 79 of which were surface water. However only one third 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 20, 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 1978 are shown in Table 6. Total capacity of these reservoirs is 1.404 billion cubic meters, or 91％ of total capcity of all reservoirs and pools. Consumptive water released from these reservoirs in 1978 is higher than the average amount by 351 million cubic meters. Sedimentation ratio of Akungtien Reservoir is 46％, which is the highest one among these major reservoirs. Techi and Tsengwen reservoirs were completed recently; therefore, their ratio of sedimentation is as low as 1.1％ and 3.2％ respectively. The total sedimentation of Shihmen Reservoir during the period from November 1977 to November 1978 was 42.46 million cubic meters, which is equivalent to 13 of the reservoir capacity. Most reservoirs in Taiwan have been encountered serious sedimentation problem. For lengthening reservoir life and beneficial use, soil conservation in the catchment area of reservoirs should be enforced.

(7) Promotion of Hydrological Activities

Owing to the rapid growth of economics in the recent years, demands of water have increased rapidly. Therefore, the basic hydrological data collection and process to agree with the expeditiously increased water resources development is in urgently need. The 11th Hydrological Conference was held at the Commission on May 25, 1979 for discussing hydrological activities of various agencies, exchanging experience and promoting the academic and pratical studies of hydrology. The conference concludes that the systematization of hydrological and hydro meteorological data for the establishment of a modern and complete hydrological database should be undertaken by the Com-mission. It has been positively carried out by the Commission.