||Feitsui Reservoir provides water supply in Taipei City as well as parts of Taipei County in northern Taiwan. The variations of hydrogen, oxygen, and carbon isotope compositions of waters record the sources and mixtures of waters; they are ideal and important tools to study hydrology. This study discussed the relationships among the waters in Feitsui Reservoir, rain waters and river in its watershed in order to provide basic isotope information in hydrology for the appropriate water resource management. From April 2003 to April 2004, rain waters of respective rainy days were collected at Feitsui Reservoir; waters of the reservoir and major branches of its watershed were also sampled monthly. These water samples were analyzed for hydrogen, oxygen, and carbon isotope compositions. In addition, information of meteorology and hydrology was also compiled and integrated with isotope data to study the hydrology cycle of Feitsui Reservoir.|
Analytical results showed that the hydrogen and oxygen isotope compositions of rain waters in Feitsui Reservoir exhibit a negative correlation with air temperature; an opposite feature with those observed in the continent. Apparently, the variations of hydrogen and oxygen isotope compositions of the rain waters in Feitsui Reservoir’s watershed have been primarily affected by the air masses of different seasons rather than the air temperature; because the sources and the paths of air masses change with seasons and cause distinct isotope fractionations accordingly. Two intervals (southwest and northeast monsoons) were differentiated to discuss the meteoric water lines in this study. Southwest monsoon prevails from May to September, and the ranges of δ18O and δD were -0.7‰ ~ -11.7‰ and1.1‰ ~ -87.4‰, respectively; the least-squares regression line was: ? ?Northeast monsoon prevails from October to March, and the ranges of δ)7.04.7()1.08.7(18±+±=ODδδ18O and δD were 0.6‰ ~ -6.2‰ and 20.5‰ ~ -38.4‰, respectively; the least-squares regression line was: .Over this study period, the least-squares regression line of all precipitation data was: . )4.19.13()5.02.7(18±+±=ODδδ)8.01.13()2.02.8(18±+±=ODδδ
The hydrogen and oxygen isotope compositions of the river waters in Feitsui Reservoir’s watershed showed significant and systematic discrepancy with those of rain waters. The isotope data of rain samples were processed as five-month moving average and weighted according to the precipitation amount. The resulting trends of processed rain waters showed similar seasonal pattern with those of river waters probably due to the hysteresis effect of rain infiltration. The isotopic ranges of the river waters were relatively smaller and depleted than those of the precipitation. The possible causes are: (1) Rain waters had a larger precipitation amount effect in summer with relatively depleted isotope signatures and higher amount of infiltration;(2) The isotope compositions of the rain waters have an altitude effect that provides relatively depleted isotope values at higher locations; (3) The isotope compositions of the rain waters for previous years might carry relatively depleted isotope values.
The main source of the Reservoir derives from upstream branch waters, and base flows of those branches are groundwaters after rain infiltration. The observed seasonal isotope fluctuations of base flows were small, δ18O ranging from -4.7‰ to -8.3‰ with a mean value of -5.8‰. On the other hand, the overland flow displayed a large impact on the isotope compositions of the Reservoir during the typhoon periods. For example, the lowest δ18O (-8.3‰) value was detected in the middle water layer of the Feitsui Reservoir after the typhoon of September 2003.
The dissolved δ13C values of water samples ranged from -11.7‰ to -16.0 ‰, with relative enriched compositions in June and depleted values in March. The main cause of this season discrepancy is due to the higher growth of algae in June, which absorbed the lighter carbon, leaving the water carbon isotope relatively enriched. The chlorophyll reduced significantly after July with carbon dioxide continually introduced from respiration, resulting lighterδ13C values in the reservoir.
In short, results of this work demonstrated that hydrogen, oxygen, and carbon isotope compositions are good tracers in studying hydrology. These isotope tracers not only can effectively explain the sources and mixtures from various sources in Feitsui Reservoir but also enhance our understanding of the hydrology cycle in its watershed and the close relationship between the biology and geochemistry. The isotope data collected in this study are also valuable in the management of the Feitsui Reservoir waters in terms of quality and quantity.
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