Assessment of Effectiveness for the Watershed Best Management Practices Using AGNPS model

Agricultural non-point source pollution (AGNPS) caused by large-scale agriculture is hard to detect and prevent. The best management practices are methods of improvement and prevention for AGNPS, that might include expertise, facility, measures of management and project. The AGNPS model was used to simulate the Leeyutan Reservoir watershed conditions. The best management practices were also used to simulate the effects on different measures. The Leeyutan Reservoir is one of the most important reservoirs in Central Taiwan. Orchard is the main landuse pattern for the watershed. This study give the Leyutan Reservoir watershed three BMPs. Project 1 is the farmland retrieve for reforestation at steep area, project 2 is establishing the riparian buffer zones, and project 3 is the vegetation restoration at landsides. Watershed conditions and three BMPs were simulated by AGNPS and GIS to a scale of sub-watershed. The results show that the improvement efficiency per unit area of Project 1, decreasing 0.05％ in peak flow at the outlet of No.2 sub-watershed, decreasing 0.39％ in sediment yield at the outlet of No.5 sub-watershed and decreasing 0.33％ and 0.47% in total nitrogen and total phosphorus at the outlet of No.7 sub-watershed. In project 2, no difference between sub-watershsds, decreasing 0.63％ in sediment yield at the outlet of No.4 sub-watershed, worst to collect 0.06％ in sediment yield at the outlet of No.7 sub-watershed and decreasing 0.61％ and 1.15% in total nitrogen and total phosphorus at the outlet of No.4 sub-watershed. In project 3, low efficiency at all sub-watersheds, decreasing 0.88％ in sediment yield at the outlet of No.3 sub-watershed and decreasing 1.26％ in total nitrogen at the outlet of No.7 sub-watershed. Whole result shows that each project received low efficiency improvement, but project 1 and project 2 have certain effects on sediment yield control and nutrition transportation, Project 3 shows no significant in improvement result. It might result from the small-scale landslides, the model sensitivity is not enough to estimate the effenciency. It could be more methods to increase reservoir life and to adjust water resource by finding more measurements such as non-structural BMPs.大面積的農業活動在缺乏管理下所造成的非點源污染不易察覺且防制困難，因此所謂的「最佳管理作業」被提出。「最佳管理作業」係指用來改善防治農業非點源污染的一切方法，內容可能為任何技術、設施、管理措施或是規劃原則。本研究選取鯉魚潭水庫集水區利用AGNPS模擬最佳管理作業配置前後差異，評估最佳管理作業施作效率。 鯉魚潭水庫為中部地區一重要水庫，果園為集水區內主要農業土地利用形態，約佔總面積的34.71%。本研究對鯉魚潭水庫集水區提出三種不同的最佳管理作業，分別為方案1：陡峭區位農地回收；方案2：水系周邊配置緩衝帶；方案3：崩塌地植生復育。將集水區現況及三種方案配置後以上游子集水區為單元，配合地理資訊系統及AGNPS模式推估輸出結果。 結果顯示方案1所得單位面積改善效率，其中逕流洪峰以2號集水區最佳，達0.05%；泥砂產量以5號集水區的0.39%最好；總氮與總磷以7號集水區的0.33%與0.47%最高。方案2中逕流洪峰各集水區相差不大；泥砂產量以4號集水區最佳，達0.63%，7號集水區效益最差，僅0.06%；營養鹽部分，4號集水區的總氮和總磷分別以0.61%和1.15%最高。方案3中逕流洪峰均偏低，泥砂產量則以3號集水區0.88%最高；總氮以7號集水區1.26%為首。整體結果各方案對逕流洪峰流量改善效率不大，但方案1及方案2對泥砂產量及營養鹽輸出均有一定效果，方案3改善效果不明顯，推測原因為本區崩塌地面積不大，模式敏感度不足以推估其改善效果。 除本文所提三種最佳管理作業之外若能針對水庫集水區提出更多方案進行評估，如添加非結構型最佳管理作業，對集水區內農業做出各項管理措施，應可找出更適合、更有效的方法來延長水庫壽命，調節水資源。摘要 I ABSTRACT III 目錄 V 表目錄 VI 圖目錄 VII 壹、前言 1 貳、前人研究 2 一、農業非點源污染模式 2 二、最佳管理作業 11 參、研究材料與方法 18 一、樣區概況 18 二、研究流程 27 三、研究方法 28 肆、結果與討論 39 一、最佳管理作業配置區位 39 二、降雨量計算結果 44 三、最佳管理作業配置效益 48 伍、結論與建議 59 陸、參考文獻 6