| 摘要: | 因大氣中二氧化碳增加造成全球暖化的問題，碳循環近年在學術研究上受到重視。碳元素在地球系統中可簡單分為有機、無機兩類。其中陸域上有機碳(Organic Carbon, OC)可分成顆粒態有機碳(POC)與溶解態有機碳(DOC) 。然而，溶解性有機碳在亞熱帶地區中的實測研究卻非常缺乏。本研究透過2002-2005年間在淡水河流域之密集採樣及分析的結果，及水利署之觀測流量進行溶解性有機碳輸出之研究；因觀測流量資料上有些許遺漏；因此使用TOPMODEL進行流量模擬，同時為了減少流量上的不確定性，本研究選定的子集水區皆位於中上游流域或是未受水庫影響之支流。淡水河流域主要由大漢溪、新店溪及基隆河三條支流所組成，其中石門水庫跟翡翠水庫分別位於大漢溪及新店溪中上游地區。 通量計算的結果在淡水河中上游地區為12-45kg-C/km2/yr；不同計算方式間奇差異約為±10％，除福山及逮魚崛兩個集水區不同計算方式間差異可達±30％，而到了下游地區的支流其通量為50-115kg-C/km2/yr不同計算方式上差異為±5％，除寶橋及五堵集水區兩個集水區不同計算方式間差異可達±25％，溼季通量約為全年的75％季節間輸出量的差異大致上是3倍。研究結果指出淡水呵流域溶解性有機碳輸出和溫度人口密度有顯著正關係。本研究中發現淡水河流域中溶解性有機碳輸出與幾類土地利用類型有明顯之相關，分別為水稻田、建地和竹林呈正相關，和天然林為明顯的負相關。。溶解性有機碳輸出與天然林為明顯的負相關，但和人工林有微弱之正相關，推測應是造林撫育作業使得林地出現部分裸露及大量的枯落物所致。此外竹林與溶解性有機碳之關連有待進一步的研究及更多的證據才可證實。 Global warming due to enhanced greenhouse effect that makes the global carbon cycle became a hot issue in the past few decades. River export of carbon is an important part of global carbon cycle, but most current studies presumed that the carbon export by rivers remains unchanged despite all the other changes. The organic carbon exported by river can be distinguished into two types: particular organic carbon (POC) and dissolved organic carbon (DOC). Past data showed that the major form of the river exported organic carbon was in dissolved form with DOC accounting for 60% of total organic carbon (TOC). In Taiwan the river export of carbon is dominated by the particular form, but most of it is fossil carbon. TO the biogenic organic carbon in river loads, both the POC and DOC are equally important. For this study, we used DOC data obtained from 2002-2005, and the discharge data of Danshuei River Basin from the Water Resource Agency to investigate the export of dissolved organic carbon. Since several parts of the discharge data were missing, we used the TOPMODEL simulate the discharge in order to make up for the missing data. To avoid the reservoir effect, we choose the middle-upper sections of the main channel and, the tributaries that were not affected by the reservoir. The Danshuei River System comprises three major tributaries, namely, the Da-Han Creek, the Shin-Dien Creek and the Keelung River. There are two reservoirs in the Danshuei River System: the Shimen Reservoir on the Da-Han Creek and the Fei-tsui reservoir on the Shin-Dien Creek. The yields of DOC in different watersheds of the middle-upper section of the Danshuei River System was 12-45kg-C/km2/yr, which were averaged from results calculated by four different methods, which resulted in variations of 10-30% between methods. By contrast, watersheds in the downstream area gave yields of 50-115kg-C/km2/yr with a similar between methods variation of 5-25%. The seasonal variation of DOC export showed a range of about three folds, with nearly 75% exported in the wet season. The DOC yields in the Danshuei River Basin were positively correlated with temperature and population density and also correlated with some types of landuse/cover. They exhibited positive correlations with percentages of paddy, building and bamboo, and negative correlation with the percentage of natural forest, whereas they showed no significant correlation with that of artificial forest. The lack of correlation with the artificial forest might cause by weeding, which may bare some area and cause accumulation of litter on the ground. Such conditions may result in leakage of DOC out of the system and diminish the carbon retaining effects of the forest. It is noted that the apparent positive correlation with the percentage of bamboo in the watershed rested upon only one high point. More data are needed to confirm such a trend. |
