標的間用水移轉對區域乾旱缺水風險分佈之影響

Abstract

導致乾旱發生之原因通常是由於降雨不足所引起，而造成乾旱發生的頻率增加通常由於用水需求成長而導致供需失衡所致。台灣目前乾旱管理主要以暫時性移用農業用水支援民生工業為主要措施之一，故本研究的目的將以現況的乾旱管理措施，模擬各種標的間用水移轉之調配策略，評估各標的之區域性缺水風險從農業部門移轉至非農業部門的流動狀況。 本研究以災害風險分析的整體架構，包括潛勢分析、易損度評估與風險管理措施等，利用地理資訊系統(GIS)建立農業灌溉、民生及工業活動的空間資料庫，作為推估區域各標的用水量之基礎；並以系統最小缺水易損度為決策目標，利用混合整數線性規劃(Mixed Integer Linear Programming, MILP)建立區域性用水調配模型，並模擬各種缺水時期標的間用水調配措施，包括農業部門不允許支援民生與工業（情境一）、以固定水量的移轉方式（情境二）與考慮經濟效益的移轉方式（情境三），建立缺水風險的超越機率曲線及空間風險分佈。 研究結果發現在農業部門不支援民生與工業部門之情境下（情境一），嘉南地區農業灌區之平均缺水率約為14.8%，而民生與工業部門的平均缺水率約為8.6%。同時，農業部門缺水較嚴重的地方多出現於水稻之三年輪區或蔗作區；而至於民生與工業部門，嘉義沿海地區，如東石、朴子、六腳等鄉鎮市，以及靠山區的鄉鎮，如阿里山、大埔等地，均有較高的缺水風險。在以固定水量作為移轉方式的模擬中（情境二）發現嘉南地區透過增加農業用水移轉率可改善民生與工業之供水穩定度。當用水移轉率約20%，可舒緩台南沿海地區(例如，北門、將軍、七股等地)之民生與工業部門缺水風險。然而，農業用水移轉的最大調配潛能約40%，這表示移轉超過40%以上的農業用水對減緩嘉南地區之民生與工業缺水風險的助益已不顯著。若以用水經濟效率為調配準則時（情境三），民生與工業部門之平均缺水率減少至1.2%，其缺水易損度亦大幅度從原本4381.4萬元減少至703萬元，減少幅度約83.9%，而缺水風險從民生工業移轉至農業部門，農業缺水易損度從原本2364.4萬元增加至3004.8萬元，增加幅度約27.1%。 本研究比較不允許用水支援調度之情境（情境一）與以經濟效率為準則（情境三）的模擬結果發現，農業用水移轉將對於區域整體缺水易損度的減緩幅度約60%，故從公平原則上，因農業用水移轉所減緩的工業與民生缺水易損度應反映在移轉補償價格上，而非僅以農業休耕損失作為補償金額的計算基礎。因此，若提高對農業部門的補償金額使工業部門的邊際效益在每噸 $22-26間，對工業用水的缺水量並無顯著影響，但當工業部門的邊際用水效益從原本之每噸 $26降至$22以下時，將顯著降低工業部門向農業部門移轉用水的意願。本研究亦發現工業部門每噸$18的用水邊際效益應為工業部門願意向農業部門移轉用水的底線，而每噸 $6-8之農業用水移轉的補償價格可視為標的間水量移轉之合理範圍。

The droughts often result from inadequate precipitation for meeting the water demands. The increase of water demands makes drought a more frequent phenomenon. Due to the low economic efficiency of agriculture activities as well as political and national economy concerns, irrigation water supplies are usually regarded as temporary water reallocation sources available for supporting municipal and industrial (M&I) activities during severe droughts. The purpose of the study is to evaluate the impacts of different water transfer strategies among sectors on regional drought risk distribution. A comprehensive framework of risk analysis for natural hazards is used in this study, including potential hazard analysis, vulnerability assessment, and risk management instruments. Drought vulnerability is defined as the product of the vulnerability weight and water deficit. The weight reflects f the economic impacts of suffering droughts in spatial, temporal, and sectoral dimensions. A geographic information system (GIS) is used to capture spatial variations of human activities, including irrigation and M&I sectors in order to estimate regional water demands and establish spatial relationship between demand sites and supply infrastructures. The study area located in southern Taiwan incorporates 8 surface and 2 underground water supplies, 8 reservoirs and 629 demand sites, including 570 irrigated areas and 59 M&I areas in ChiaNan area. A mixed integer linear programming (MILP) model with minimizing regional drought vulnerability as the objective is established for inter-sectoral drought allocation. Exceeding probability curves and spatial mapping of drought risk are established under different water transfer scenarios, including inhibition of water transfer among sectors (scenario 1), and transfer criteria based on specific amount of agricultural water (scenario 2) and priority of economic-efficiency (scenario 3). The results show that average deficit rate is about 14.8% for irrigation, 8.6% for M&I under scenario 1. Under this scenario, higher drought risk in irrigated areas appears in the areas with rotated paddy crop and area with sugarcane. The M&I sectors suffer from higher risk more in the mountainous and coastal regions. In scenario 2, transferring about 20% of the irrigation water can reduce the drought risk for M&I sector in coastal areas. It is also shown from the simulation when the transfer rate is more than 40%, no further benefits are shown for retarding M&I drought risk. Comparing scenario 3 with scenario 1, it could be found that average deficit rate of M&I sectors reduced from 8.6 % to 1.2 %, and that of agricultural sector increased from 14.8% to 23%. M&I drought vulnerability reduced substantially from 43,814 thousands to 7,030 thousands NT dollars (83.9%), and the agricultural sector increased from 23,644 thousands to 30,048 thousands NT dollars (27.1%). The results of this study show that the water transfer strategy based on economic-efficiency consideration (scenario 3) could reduce about 60% M&I drought vulnerability in ChiaNan region. The compensation for paddy-farming fallow from the M&I sectors should reflect this contributions accordingly rather than merely the actual farming loses. However, the results also show that the M&I sectors tend to reduce water demands if their marginal benefits for water consumptions are less than $22/ton. And the M&I drought vulnerability will be even larger than the agricultural sector if marginal benefits for M&I sector decrease less than $18/ton. This study shows that the compensation of $6-8/ton for fallow of paddy-farming is reasonable for water transfer to M&I sector.