High-frequency storm event isotope sampling reveals High-frequency storm event isotope sampling reveals of diurnal cycles

摘要

High-frequency sampling of stable water isotopes in precipitation and stream waternduring winter and summer storm events was carried out in a 2·3 km2 lowlandnagricultural catchment. During peak flows of monitored events, the responses ofnd2H and d18O were comparable and inferred the dominance (ca 70%) of ‘old’ preeventnwater. Transit Time Distribution (TTD) inferred by a gamma function werenfitted (Nash–Sutcliffe = 0·8) and were also similar for d2H and d18O. However,nthe shape (a) and scaling (b) parameters were markedly different for summernand winter events. Consequently, when antecedent wetness was high, mean transitntimes were in the order of days; when drier, they increased to months. Moreover,nwhile the responses of d2H and d18O exhibited similar gradual recovery to preeventnconditions during winter hydrograph recessions, they differed dramaticallynon summer recessions. Time series analysis showed that d2H isotope content wasncorrelated with the diurnal cycle of air temperature, suggesting an evaporativenfractionation pattern which could be reproduced by a temperature-based firstordernautoregressive model. The heavier d18O isotope showed no evidence fornsuch diurnal variability. The study highlights the utility of high-frequency stablenisotope sampling to explore the time-variant nature of TTDs. Furthermore, it showsnthat the time of sampling in a diurnal cycle may have crucial significance forninterpreting stream isotope signatures, particularly d2H. Copyright  2011 JohnnWiley & Sons, Ltd.