考量河川流態之環境流量評估－以大漢溪為例

Abstract

水為生物生存必需之資源，為滿足人類用水需求，常以建造水工結構物，來滿足用水需求。惟若過度的取水，易使河流自然環境受到衝擊，而保留河道內足夠的流量，為降低衝擊維持河川生態之關鍵。傳統上對此保留流量的考量，多僅對單一保護標的，或為單一最小流量，惟此對河川生態之整體性考量較不足。另外，河川流量的保留亦可能對人類用水產生影響，因此不同可能方案之影響分析，以作為在兩者之間協調平衡的基礎乃是必需。本研究目的為在維持河川生物生存、人類活動及自然景觀等基本原則下，考量多項量化管理標的，提出多種可能之河川流態替代方案，其中流態包括水量、水質、延時等，並分析各替代方案對於人類用水及生態環境之衝擊等，以為折衝平衡的基礎。 大漢溪水資源已呈現高度利用狀況，嚴重影響下游河川生態環境，故選擇大漢溪為研究區域。首先蒐集該流域的生態、水文、地文等資料，並依此決定環境流量管理標的。各管理標的再以不同評估方法而得其不同量化值。組合前述不同的標的量化值，提出共8組環境流量替代方案，並與大漢溪現況系統(Case0)予以比較。各種替代方案對於人類用水需求及生態環境之衝擊，則以缺水指數(SI)及整體水文改變度代表。 本研究將各河段劃分為如下：河段1為石門水庫至鳶山堰，河段2為鳶山堰至三峽河匯流口，河段3為三峽堰下游至三峽河匯流口，河段4為三峽河匯流口至大漢溪匯入淡水河口。就對於人類用水衝擊最低之方案(Case3)及對於生態環境衝擊最低之方案(Case4)而言；Case3中桃園地區之缺水指數較Case0增加了0.136，對於人類用水衝擊有造成輕微影響，而河段1、2、4之整體水文改變度較Case0分別下降了1.7%、1%及0.1%，僅有些微之改善。Case4中桃園地區之缺水指數較Case0增加了13.3，其原因在於此方案所供應之基流量較高，故對於人類用水有較大的衝擊，但河段1~4之整體水文改變度也較Case0分別下降10.3%、13.8%、10.5%及10.4%，各河段之改變度有了明顯的改善。整體而言，本研究之環境流量為以河川流態表示，並考量多種可能之量化管理標的，評估各種替代方案對人類及生態之衝擊，可作為後續政府機關及利害關係人等進行替代方案決策之依據。

Water is the necessary resource for living creatures. Human beings usually build hydraulic structures to satisfy human’s water needs. Excessive water usage impacts the natural environment of rivers. The key factor to protect the river ecology is the reserved discharge in the river. Traditionally, this reserved discharge is determined for single objective or is determined as a minimal discharge rate which did not consider the complexity of the river ecology. In addition to ecology, the reserved discharge also affects the water usage for human beings. Therefore, the influence analysis for different scenarios is necessary for balancing both side of water needs. The main purpose of this study is developing a systematic way to propose alternative scenarios for trade off analysis among river ecology, human activities, and natural environment. For these alternative scenarios, this study not only considers water quantity, water quality, and duration time, but also evaluates the corresponding impact related to human water usage and ecological environment. This study applies the proposed method to Dahan River. Current Dahan River is under the situation of excessive use. The ecological environment at the downstream river is seriously damaged. This study collects the ecology, hydrology, and geology data to assist the determination of the environmental discharge management targets. Each management target is quantified using different evaluation methods. Eight alternative scenarios are proposed based on the combination of these quantified indices. These alternative scenarios are then compared with the current system, as defined the based case or Case 0. The Shortage Index (SI) and the integrated hydrology changes are used to evaluate the impact of human water usage and ecological environment. The study found that the lowest impact scenarios for human water usage and ecological environment are Case 3 and Case 4 respectively. In Case 3, with comparison of Case 0 for Taoyuan area, the SI value increases 0.136 which indicates a slightly negative influence for human water usage. For river sections 1, 2, and 4, the integrated hydrology is reduced by 1.7%, 1%, and 0.1% respectively. This reduction refers slightly improvement for these river sections. In Case 4, with comparison of Case 0 for Taoyuan area, the SI value increases 13.3 which indicates a severely negative influence for human water usage due to high reserved discharge rate. On the other hand, the river ecology is improved according to the integrated hydrology is reduced by 10.3%, 13.8%, 10.5% and 10.4% for river sections 1 through 4 respectively. In this study, the environmental discharge is represented by river status. In addition, multiple quantitative management objectives are considered to evaluate the impacts for human beings and ecology using alternative scenarios. The results can be a decision reference for government and interested parties.