| dc.description.abstract | 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.
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