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Using stable isotopes to estimate young water fractions in a heavily regulated, tropical lowland river basin
Author(s) -
Trinh Anh Duc,
Do Thu Nga,
Panizzo Virginia N.,
McGowan Suzanne,
Leng Melanie J.
Publication year - 2020
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/hyp.13878
Subject(s) - drainage basin , streamflow , hydrology (agriculture) , baseflow , environmental science , precipitation , monsoon , rainwater harvesting , seasonality , catchment hydrology , discharge , climatology , geology , geography , ecology , cartography , geotechnical engineering , meteorology , biology
The young water fraction of streamflow ( Fyw ), an important hydrological variable, has been calculated for the first time, for a monsoon‐fed coastal catchment in northern Vietnam. Oxygen stable isotopes (δ 18 O) from six river sites in the Day River Basin (DRB) were analysed monthly, between January 2015 and December 2018. River δ 18 O signatures showed sine wave variability, reflecting the amount effect and tropical (dry‐rainy) seasonality of the region. The δ 18 O composition of precipitation ranged from −12.67 to +1.68‰, with a mean value of −5.14‰, and in‐streamflow signatures ranged from −11.63 to −1.37‰ with a mean of −5.02‰. Fractions of young water ( Fyw ) were calculated from the unweighted and flow‐weighted δ 18 O composition of samples. Unweighted Fyw ranged between 29 ± 8% and 82 ± 21% with a mean value of 51 ± 19%, and was not significantly different from flow‐weighted Fyw (range between 33 ± 25% and 92 ± 73%, mean 52 ± 36%). Both unweighted and flow‐weighted Fyw were highest in the middle of stream and lowest in downstream sites, capturing the impacts of landuse changes, hydrology and human activities in the catchment. Our calculations imply that more than a half of rainwater reaches the DRB river mainstream within the first 3 months. The Fyw is much higher than the global average (of one‐third) and insensitive to discharge due to the combination of a humid catchment with high rainfall, low storage capacity, flat landscape and an intensive drainage system in the DRB. Also the low discharge sensitivity of Fyw in the DRB implies that the regional hydrology is severely altered by humans.