Thirteenth Congress

Rabat, 1987

A RENOVATION SCHEME FOR THE OLD IRRIGATION SYSTEMS ON BOTH THE BANKS OF THE CHOSHUI RIVER*

ABSTRACT

The old irrigation systems on both banks of the Choshui River have been in existance for more than three hundred years, Since 1664, the begin- ning of the late Ching Dynasty, when a large number of people immigrated from Mainland China to Taiwan. The existing diversion works along the river using bamboo cages have been somewhat improved, through many are still inconveniently utilized at the moment. The flow of water diverted by using those primitive diversion facilities may be interrupted immediately after a flood has passed through, consequently, the irrigation water supply can not be ensured.

A renovat ion scheme has been conceived and formulated including a fixed diversion weir across the river, two common intakes on both banks of river, and 90 km of connection canal along both banks. It i s proposed to construct a11 o f these to improve the function of the old systems. In addition to the description of the improvement of the irrigation facilities herein, the renovation of the water utilization pattern and the consideration of the artificial recharge of the ground water will also be described in this paper to present the major effects that could be improved by the rehabilita- tion scheme of the 300-year old irrigation systems.

RESUME

Depuis plus de trois cents ans, i I existe de vieux systemes d'irrigaticn aux deux bords de la rivitre Choshui. C'ettait depuis 1664, au debut de la Dynastie C hing, qu'une large popuIation a immigre en Chine Continentale

* Projet de rhovation de vieux systemes d'irrigation ;tux d e w bords de la riviere Choshui. 'Chief, PlaMing Team, Taiwan Provincial Water Conservancy Bureau. 'Junior Engineer, Planning Team, Taiwan Provincial Water Conservancy Bureau.

a Taiwan. Les ouvrages de dbrivation actuels le long de la rivitre qui utili- sent des cages de ba-mbou, ont connu une certaine amC1ioration quoique la plupart d'entre eux ne soient pas utilisb d'une rnaniere convenable a pr6sent. L'tcoulement d'eau dtri,Ce en utilisant ces facilitCs primitives de dbrivation, peuvent Etre i nterrornpu immidiatement apres une crue. Par consbquent, I'apport d'eau d'irrigation ne peut pas Ctre assurte.

Un projet de r6ncwation a i t 6 concu et formulk, y compris un dCver- soir fixe sur la rivibre, deux prise d'eau communes aux bords de Ia rivikre, et 90 km de canal de raccordement le long de deux bords. I1 est propose de construire touts ces ouvrages pour amdiorer le fonctionnement des. systlmes vie i 11 ies. Outre la description sur I'amdiorat ion des facilitCs d'irri- gation, le rapport decrit la renovation du modbIe de I'ut ilisation d'eau et examine la possibilite de recharge artificielle de l'eau souterraine et kvoque les principaux effets qui peuvent dtre arn6liorbs par la rbhabilitation des systemes d'irrigation qu.i datent de 300 ans.

1. INTRODUCTION'

The Choshui River is the major source of irrigation water for cultivated area of some 97.500 ha on the western coastal plain of Taiwan. This area, which occupied a large part of the natural delta formed by the Choshui River, is fertile and aIready intensively farmed. However, in the southern sector (Yunlin country) of the delta, the present-inadequacy of the water supply results in excessive removal o f ground water that serious land subsidence in the southwestern corner of this area.

The scheme of the Chichi Common Diversion (Figure 1) has been the consideration of an important premise of the comprehensive development of the Choshui River Basin since the early 1940's. As a proposed project, it was continuously studied by Water Resources Planning Commission (WRPC), the Ministry of Economic Affairs from 1958 through 1962. Since then, it was reviewed intensively by a Canadian Consultant Firm; Acres International Limited ~onsult ing ~ n ~ i n e e r s , under the sponsorship of the United Nations in 1964. Nevertheless, the scheme, is still in origin which has not been implemented yet, therefore, farms on both banks of the Choshui River are not guaranteed to get a regular water supply during the cropping period.

Interruption of irrigation water due to fiood damage oftemporary diversion facilities, such as bamboo cages, has frequently occurred, there- fore, the scheme of the Chichi Common Diversion is suggested again as a renovation project which has been carefully studied and hopefully formula- ~ e d to be a feasible plan. The Chichi Common Diversion Plan was presented to the Taiwan Provincial Government for approval by the Taiwan Provin- cial Water Conservancy Bureau (TPWCB) in 1984. The major reasons for the re-establishment of this plan are enumerated as follows :

(1) On both banks of the main stream of the Choshui River, there are 14 existing intakes connecting 12 canal networks which are irrigating 80,000 ha of direct irrigation area and 17,500 ha of

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irrigation area giving at total o f 97,500 ha wh.ich is the most excellent crop plantation in Taiwan. The old irrigation systems have been built up over a long period. Their bamboo-built temporary diversions have also been used to divert water for the past three centuries. These facilitie s regulate water and sediment diverting them into the canals. I11 addition to the interruption o f irrigation water due to the flood damage of the diversions, the high cost of system maintenance is aggravated by sediment deposition along the canals. In order to ensure the water supply and to reduce the operation difficulties, the modernization of these old irrigation systems is urgently needed.

(2) The variation of release what from the power plant of Sun Moon Lake is too tremendous to use the tail water efficiently. Also, the water released from the reservoir is unevently distributed both in the wet season and in the dry season. In order to enhance the efficiency of tail water ultilization, a regulating basin i s necessary as soon as possible.

(3) Since the water resources of the Choshui River are not solely provided for the use o f crop land, a great amount of ground water is withdrawn for rice and fish farming. Consequently, the ground water is overdrawn and the saltwater intrusion and land subsi- dence have seriously increased, resuIting in the difficulties of shore- line protection and storm water drainage. The excessive removal of ground water should be prohibited. Thus, not only a regula- tion is required but also a definite, plentiful source of surface water for crop and fish use is needed.

(4) The Choshui River alluvial plain is the major agriculture area in Taiwan. The decline of agricultural revenue caused by the shor- tage of water resources supply also influences the population reduction in this area. The implementation of the Chichi Common Diversion Project could be the major stabilizing fkctor in the economic development of this country.

For the reasons mentioned above, there is a most urgent need to draw up an acceptable solution to renovate the old existing irrigation systems in order to improve the productivity of the area concerned. The major facilities and their functions are enumerated as follows:

(a) Building of a permanent diversion facility, including a diversion weir, two intakes and 90 km of canals to connect 12 existing canals, on both banks o f the Choshui River, could solve the diversion problems of protecting the production of 97,500 ha crop fields. On the other hand, the large amount of money used for the rebuilding of temporary diversions after every flood could be saved. Consequently, the water resources of the Choshui River could be regulated and utilized efficiently.

(b) A settling basin will be erected in conjunction with the diversion .facility to reduce the sediment deposition in the canals for improvement of the water conveyance capacity, an.d the money

Company. In the past, a small agricultural benefit, has no doubt been obtained incider~tally from the operation of the reseivoits. An increased benefit for irrigation purposes in the future would be achieved by the develop- ment of this project. The major concerns of this prqiect are described as foflows :

( I ) BASIC POTENTJAI,

According to the records and analysis of precipitation and evaporation, it is estimated that the Chichi upstream watershed area is only 43 per cent of the whole basin, though 65 pcr cent of water resources of the basin are collected. Approximately, there i s 420,000 ha-m of average annual flow, which i s induced from estimated precipitation of 608.200 ha-m a year.

(2) GROUNDWATER POTENTIAL

The annual natural recharge of groundwater in the project area of about 2,063 km2 covering most of Yunlin and Changhwa countries, is esti- hated at 95,WO ha-m which is accumulated from rainfal!, stteamflow and irrigation water in canals and rice paddies. Although the plentiful natural recharge and porous strata in the Choshui River delta area cause an abundant groundwater potential, however, removal of groundwster has reached, or

to the safety limit. Tlie current removal of groundwater in this area is estimated at 89,300 ha-m, very close to the annual natural recharge amount of 95,000 ha-m. Only 5,700 ha-rn could be developed in the part i- cular area near the apsx of the delta. The groundwater in the bottom of this delta, i s over removed for fish farming which causes the strata to pack more closely together, result jog in a gradual settling of land, land subsidence, at the southwestern corner of Yunlin country. The rate of land subsidence is estimated at 10 cm a year recently and reached a total of 40 ern by 1984. The water resources, which are developed by the Chichi Common Diversion Project, will be used for artificial recharge o f the groundwater and provided for fish Fdrdrming in the coastal area to prevent the land subsidence and salt- water intrusion from becoming tnore serious

(3) RESERVOIR POTENTiA I.

The arc three reservoirs and dams in the upper waters of the Choshui River and its tributaries, all of them solely f o r generating hydropower, irriga- tion requirements have been taken into account in a compromise agreement between the Taiwan Power Company and t he focal irrigation association. which has been cordinated by the Water Resources Planning Commision since 1962. At present, Wusheh and Sun Moon Lake reservoirs are functioning as multiple purpose reservoirs. I t is estimated that the increased water amount by two-reservoir regulatio~l I S 13,800 ha-m annually. Mean- while, 29 reservoirs and dam sites have been surveyed. From these, four reser- voirs, Zuifeng, Hushan, Hunan and Hwangte. have been. investigated and studied with the conclusion that they are the most suitable of the 29 sites. All of the four reservoirs arc arranged to be off-stream reservoirs to divert and operate the stream water of the C h i n g h u i River which is a relatively large tributary of the Choshui River. '[t i s anticipated that the potential of daily water supply will be 40 x 10L rnR/day from the Zuife~lgreservoir and 20 x I O4 myday from the Hushan, .-Hunan and Hwangte reservoirs syste,m(j).

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