Abstract
Characterization of spatiotemporal variation of the stable isotopes δ18O and δD in surface water is essential to trace the water cycle, indicate moisture sources, and reconstruct paleoaltimetry. In this study, river water, rainwater, and groundwater samples were collected in the Yarlung Tsangpo River (YTR) Basin before (BM) and after the monsoon precipitation (AM) to investigate the δ18O and δD spatiotemporal variation of natural water. Most of the river waters are distributed along GMWL and the line of d-excess = 10‰, indicating that they are mainly originated from precipitation. Temporally, the δ18O and δD of river water are higher in BM series (SWL: δD = 10.26δ18O+43.01, R2 = 0.98) than AM series (SWL: δD = 9.10δ18O + 26.73, R2 = 0.82). Spatially, the isotopic compositions of tributaries increase gradually from west to east (BM: δ18O = 0.65Lon (°)-73.89, R2 = 0.79; AM: δ18O = 0.45Lon (°)-57.81, R2 = 0.70) and from high altitude to low (BM: δ18O = −0.0025Alt(m)-73.89, R2 = 0.66; AM: δ18O = −0.0018Alt(m)-10.57, R2 = 0.58), which conforms to the “continent effect” and “altitude effect” of precipitation. In the lower reaches of the mainstream, rainwater is the main source, so the variations of δ18O and δD are normally elevated with the flow direction. Anomalously, in the middle reaches, the δ18Omainstream and δDmainstream values firstly increase and then decrease. From the Saga to Lhaze section, the higher positive values of δ18Omainstream are mainly caused by groundwater afflux, which has high δ18O and low d-excess values. The δ18Omainstream decrease from the Lhaze to Qushui section is attributed to the combined action of the import of depleted 18O and D groundwater and tributaries. Therefore, because of the recharge of groundwater with markedly different δ18O and δD values, the mainstream no longer simply inherits the isotopic composition from precipitation. These results suggest that in the YTR Basin, if the δ18O value of surface water is used to trace moisture sources or reconstruct the paleoaltimetry, it is necessary to rule out the influence from groundwater.
Highlights
The stable isotopes, δ18O and δD, in precipitation are widely used as a fingerprint for the hydrological processes and atmospheric circulation (Fette et al, 2005; Zhu et al, 2007; Singh et al, 2013; Guo et al, 2017; Wu et al, 2019)
In the Yarlung Tsangpo River (YTR) Basin, most of the river waters originate in precipitation and inherit the δD and δ18O characteristics of precipitation
Under the predominance of precipitation, the isotopic composition of river water is high before the monsoon precipitation and low after the monsoon precipitation
Summary
The stable isotopes, δ18O and δD, in precipitation are widely used as a fingerprint for the hydrological processes and atmospheric circulation (Fette et al, 2005; Zhu et al, 2007; Singh et al, 2013; Guo et al, 2017; Wu et al, 2019). Because of the so-called “continent effect” and “altitude effect” of isotope composition in precipitation (i.e., the negative relationship between Δδ18O or ΔδD in precipitation and transport distance and elevation) producing heavier monsoonal rainfall first with transported distance, and with orographic lifting, stable isotopes have been used to trace moisture sources and to reconstruct paleoelevation of the TP (Garzione et al, 2000; Poage and Chamberlain, 2001; Rowley et al, 2001; Hren et al, 2009; Li and Garzione, 2017). Significant negative isotope–altitude relationships have been reported: for example, the lapse rate of δ18O in river water is about −0.21‰ per 100 m in the northeastern TP (Bershaw et al, 2012), −0.24‰ per 100 m in the southern TP (Ding et al, 2009), −0.28‰ per 100 m in the high Himalayas of Nepal and −0.31‰ per 100 m in the Niyang River watershed (Florea et al, 2017), while it is as low as −0.36‰ per 100 m in the southern Himalaya (Wen et al, 2012) and about −0.19‰ per 100 m in the Hengduan Mountains (Hoke et al, 2014)
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