未管理廢耕農地對河川水質與河岸侵蝕之影響──以有勝溪為例

Abstract

施肥是導致水質惡化的主因，因此停止農業活動被認為是改善水質的最有效方法，但是過往鮮少有研究仔細探討農地廢耕時間與河川水質變化之關連性。本研究使用了1992年至2018年間的航空照片和衛星影像數化了有勝溪沿岸農地廢耕面積的時間序列，並與逐月水質檢測結果兩相比較。結果顯示，直到2018年有勝溪流域內的農地有65%停止了農業活動，其中絕大多數的農地廢耕集中在2004年至2008年（廢耕期），佔總廢耕面積的75%。利用變異範圍法分析水質量測數據後，發現有勝溪廢耕期的氨氮濃度相比於2000年至2003年的耕作期，其高濃度發生機會增加120%，廢耕後期（2009年之後）中濃度發生的機會增加188%。另外本研究自影像資料中發現2005至2018年，有勝溪沿岸共有11%的農地面積遭遇河岸侵蝕，且以廢耕地為主，疏於管理的廢耕農地似乎更容易受到侵蝕的威脅，同時廢耕後期的總磷發生高濃度的機會也增加132%。本研究結果顯示，未管理廢耕地除短期內可能因缺乏地表敷蓋而造成肥料流入河川外，長期將遭受河岸侵蝕，除應正視廢耕地的管理外，亦表示農業活動對於河川的長遠影響。Fertilization results in deterioration of the quality of water bodies. Therefore, stopping agricultural activities was regarded as the most effective method to improve the water quality. However, few studies have used the time-series data to explore the relations between river water quality and the abandonment of agricultural land. This study used aerial photos and satellite images from 1992 to 2018 to identify time series changes in the area of agricultural land abandonment along the Yousheng Creek, and compared to the monthly river water quality measurement. It was found that 65% of the agricultural land area had stopped agricultural activities by 2018, and 78% of it occurred in 2004 to 2008 (Abandonment Period). However, according to the result of the RVA alteration based on the water quality measurement data ,it was found that the chance of high-concentration of ammonia nitrogen in the Abandonment Period was greatly increased by 120% compared to that in the Pre-abandonment Period (2000-2003). Moreover, the chance of mid-concentration was increased by 188% compared to that in Post-abandonment Period (after 2009). In addition, a total of 11% of the agricultural land along the river was eroded by river water after 2005, and most of the eroded agricultural land was abandoned farmland. These poorly managed abandoned farmland seem more susceptible to flood, perhaps resulting in the occurrence of high-concentration total phosphorus has increased by 132% in the Post-abandonment Period. The results of this study indicate that the remaining fertilizer on abandoned and unmanaged farmland still poses threats to the river water quality via leaching in a short-term period and via bank erosion in a long-term period after the abandonment, implying the lingering effects of agricultural activities on river water qualities.