從供水成本探討柯子湖溪導水管工程之效益

Abstract

台灣地區降雨量相對世界平均而言已算豐沛，但由於時間較集中於雨季數月，且以空間上而言降雨分佈亦極不均勻，再加上河川陡峻，豐枯季流量相差懸殊，故為克服此困境，皆以興建水庫蓄水來調節用水量，以達到蓄豐濟枯之功效。這種現象在新竹地區亦不例外，但大新竹地區供自來水使用之水庫較少，僅有寶山水庫535萬立方公尺之蓄水，對於新竹地區枯旱季長達三個月以上之每日48萬立方公尺之需求實無法滿足，故長期以往均需永和山水庫、石門水庫、鯉魚潭水庫等之淨水場聯合支援，以補頭前溪取水之不足。但至枯季河川流量極小，且適逢農民春耕用水時，自來水公司於頭前溪取水量幾乎為零，各淨水場支援已達極限仍時有不足，故自經濟部90年「農業用水調度使用協調作業要點」頒佈作為休耕依據後，更輔以休耕辦法，移用農業用水以應民生及工業之需。 故當寶二水庫於民國九十五年六月完工蓄水後，在一般普遍認為缺水問題自此告終時，卻潛藏了一項隱憂，就是當乾旱來臨時，可能空有水庫之水而無足夠之淨水場來處理原水，而遠在用水中心之新竹二場卻徒有淨水場而無原水。柯子湖溪導水管工程就在這樣的情形下，因應而生。 本研究先就本研究區域予以介紹，以了解本區各水庫與淨水場現況及其供水能力，再針對工程相關資料蒐集、彙整，將其所產生之效益區分成可量化及不可量化(或量化困難)之項目，並對可量化效益中之兩大項目休耕費用之節省、淨水調度之動力費損失予以計算；成本部分則以連通管線之全部相關工程費用予以統計。再以平均報酬率法、投資回收期法、淨現值法、等值年金法、內在報酬率法、益本比法等六種計畫評估方法予以評估。 評估結果，不論以各種評估法則而言，均為可行。且以益本比法則觀之，其益本比在降雨量為最可能之情形下達9.78，故在該柯子湖溪流量較小時有潛在污染之虞顧慮下，最後建議研議及評估將導水管向上游延伸，避開污染河段以確保用水安全。

The annual rainfall in Taiwan is above the average in the world. However, most rainfalls occur only in those months of the rainy seasons and the rainfall distribution in terms of geography is extremely uneven. Such rainfall tendencies coupled with the bold terrains lead to huge variations in the river flow from wet to dry seasons. Reservoirs are thus widely constructed to regulate the daily water use, with no exception in HsinChu. However, there are only a few reservoirs in HsinChu. The PoShan Reservoir with storage of 5.35 million cubic meters cannot meet the daily water demand of 0.48 million cubic meters in this area during the dry seasons up to three months or more. Therefore, a combined supply from the water purification plants of Yunghoshan Reservoir, Shihmen Reservoir, Liyutan Reservoir is required to replenish the water supply shortage from the Tochien River. However, during the dry season when the runoff is small or the seeding season when large amount of water is demanded in agriculture, the water intake from Tochien River is almost zero. Moreover, the load of the supporting water purification plants sometimes reaches the maximum during those seasons so additional water supplies are unavailable. The Ministry of Economic Affairs announced the “Agriculture Water Supply Act” and the “Fallow Act” in 2001 in order to solve such water shortage problems by shifting the water use in agriculture to the industry and the public. The Poshan No.2 Reservoir was constructed in June, 2006. By that time, it was widely acknowledged that the water shortage problems were solved with the exception that when the seeding season comes, there were not enough purification plants to deal with raw water while the Hsinchu No.2 Water Purification Plant far from the area of water demand was lack of raw water. The Kotzehu River Aqueducts were thus later constructed. This study firstly introduces the background information of the focused region, including the current status and capacities of water supply of the reservoirs and the purification plants. Second, this study summarizes the gathered data and classifies those as quantifiable and non-quantifiable (difficult to be quantified) items on the basis of their producing benefits. For quantifiable items, this study calculates and analyzes the savings from the fallow and the costs of the water purification transfer, including the total cost of the viaducts and the related constructions, using the method of Average rate of return,Payback period,Net Present Value,Uniform annual series,Internal rate of return and Benefit/cost ratio. Results show that each method mentioned previously is applicable. In Benefit/cost ratio, the most likely rainfall could reach 9.78. So, as far as the potential water contamination in the small runoff of the Kotzehu River is concerned, extensions of the viaducts to the upstream are suggested in order to circumvent the contaminated water reach and ensure the safety of water use.