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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 extends 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 nigh 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. 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 1980, there are 119 hydrokinetics stations and 1,090 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 1980

(1) Climate

From May until August the sub-tropical high pressure from Pacific Ocean dominated the climate over Taiwan area. The air temperature. Was relatively high and there was very few rainfall had recorded during the rainy season. As a result, unprecedingly a severed drought was suffered on Taiwan area. On August 27, the drought was finally released by the landing of typhoon Norris that brought plenty of rainfall over the area. On September 18, typhoon Percy passed the southern tip of Taiwan and also brought abundant rainfall.

(2) Rainfall

The rainfall was no less then the normal for the period from January through April, but unusual deficiency of rainfall was experienced for the period from May through July. During this period, rainfall for the coastal area of Central and Sough regions was less than 300 mm, which is only about 20 of the average value of the same period (Table 5).

The average annual rainfall from January through December of1980 over Taiwan area was 1605 mm (Fig. 7) which is 64 as compared with the long-term average annual rainfall of 2510 mm (Period 1949 to 1979). The same comparison of the annual rainfall to long-term average for the four regions respectively of Taiwan area was as follows: North region 84, Central region 67, South region 45, and East region 64. The average annual rainfall of major river basins in 1980 refers to Table 2, Fig. 7 and 8.

(3) Streamflow

In North and Central regions, the streamflow of June and July was 30 and 20 respectively to the average figure; while on the Kaoping basin in the South region, it was only 4 to 6 to the average (Table 6).

The total stream runoff of Taiwan in 1980 was calculated at 37,2 billion cubic meters, which is only 55 of 67.5 billion cubic meters of the longterm average (Period 1949 to 1979). The streamflow of all the river basins refer to Table 2.

(4) Rainfall

There were 28 Typhoons occured in the Western Pacific Ocean, only two of them landed Taiwan. Typhoon Norris landed llan area on 27 August, resulted slight damage but brought abundant rainfall over northeastern area and the area to north of Chia 1. The rainfall recorded at Alisan station on August 27 and 28 was 595 mm.Typhoon Percy, which was classified as severe type, landed in South tip of Hengchun area on 18 September with maximum wind velocity of 14 grade. Many farm land and houses were damaged seriously in the peninsular of Hengchun. The torrential rainfall caused landslides and floods. A major bridge named Jenshou linking Hualien-Taitung highway was destroyed from torrential flood that suspended the traffic in the East region.

(5) Groundwater

The Taiwan Provincial Water Conservancy Bureau and Bureau of Public Works of Taipei Municipal Government implement investigation of groundwater level respectively. There were total 300 observation wells in 1980.

The average annual recharge of groundwater in Taiwan is estimated at 4 billion cubic meters. 3.8 billion cubic meters of groundwater had been withdrawn in 1980.

(6) Water Consumption in 1980

Total annual consumptive water was calculated at 16.2 billion cubic meters. Irrigation water used accounted for 83 or 13.3 billion cubic meters, domestic water supply 6 or I billion cubic meters, industrial water 11 or 1.7 billion cubic meters respectively. The water uses in various regions in 1980 are shown in the following table.

Region | Irrigation | Domestic | Industrial | Total
North | 628 | 645 | 2,560 | 3,833
Central | 156 | 432 | 5,630 | 6,220
South | 243 | 603 | 3,324 | 4,170
East | 22 | 46 | 1,882 | 1,950
Total Taiwan Area ％to total | 1,049 | 1,726 | 13,398 | 16,173

The analysis of water resources supply and demand for 1980 is shown in Table 8. Of the total water demand for 1980, 77 or 12.4 billion cubic meters from surface water and reservoir regulation, 23 or 3.8 billion cubic meters were drawn from groundwater. The utilization rates of surface and ground water are 33 and 95 respectively.

(7) Observation of Groundwater Level

i) Systematization of Hydrological Data Processing

Several meetings have been held among agencies concerned to promote the systematization of hydrological data processing since1979. Some achievements are obtained for unification of data format and promotion of the general use of hydrological data.

The research reports as "Study on Systematization of Taiwan Hydrological Data Management", "The Second Stage Study on Computerization of Meteorological Data Management", and “ The Computerized Operation System of Rainfall Data ” were published in this year by the Commission, Central Weather Bureau and Taiwan Power Company respectively.

A program was proposed by the Commission for establishing the water resources data center to set up the systems of storage; delivery and exchange of basic data.

ii) Reservoir Operation in Coordination with Downstream Irrigation Requirements

Choshui and Tachia are the two major river basins in Central region. The available dependable streamflow in dry seasons is quite limited. All reservoirs in the upstream mainly for the purpose of hydro power generation. Consequently, the rice production are always affected during the period from January to May due to deficiency of irrigation water. The Commission has coordinated with the agencies concerned for operation of the reservoirs for the both benefit of power generation and irrigation. There have been some outstanding achievements for man@ years. Irrigation water was fully supplemented from the pertinent reservoir operations during the period of the severe drought of this year.

iii) Water Study for Proposed Reservoirs

In collaboration with water resources planning of the Central and South regions, hydrological investigation and analysis for various proposed dam sites have been undertaken by the Commission and agencies concerned. The study will be continued in 1982.

iv) Operation Analysis of Major Reservoirs

The total storage and effective capacities of existing reservoirs are 20 and 15 billion cubic meters respectively. The effective capacity of the nine major reservoirs is 14 billion cubic meters or 91 of that of all reservoirs. Owing to the abnormal summer drought, the total consumptive water released of this year is 29 billion cubic meters which is 0.7 billion cubic meters less than that of last year.

Sun-Moon Lake and Wushantou reservoir are serial-linked with the upstream reservoir Wushien and Tsengwen respectively, therefore, their release ratios are the highest among all reservoirs. Although the sediment yield from reservoir watershed has tended to be reduced gradually by the soil conservation means during past years, but soil conservation works should be still strengthened to extend the reservoir life. The analysis of water utilization and sedimentation for major reservoirs in 1980 is shown in Table 9.

v ) Survey of Groundwater Level and Land Subsidence of Taipei Basin

In Taipei basin, overpumpage of groundwater has already induced severe land subsidence. In collaboration with the regulation to restrict use of ground water and control of land subsidence of Taipei basin, the Commission has undertaken the investigation of grovndwater level periodically to construct contour lines of ground-water level and the survey of benchmarks annually. Owing to the strict regulation on groundwater pump age in the basin groundwater levels are recovering and land subsidence decreasing in recent years. According to the investigation of this year, although the pumped water (152 million cubic meters) was similar to that of last year, but because of the severe drought, the natural recharge was decreased. Therefore, the groundwater level lowered again and subsidence of some portions were more than that of last year. The situation should be further controlled.