Abstract
In many areas of the Loess Plateau, groundwater is too deep to extract, making meteoric water (snow and rain) the only viable water resource. Here we traced the rainwater and water vapor sources using the δ2H and δ18O signature of precipitation in the northern mountainous region of Yuzhong on the Loess Plateau. The local meteoric water line in 2016 and 2017 was defined as δ2H = 6.8 (±0.3)∙δ18O + 4.4 (±2.0) and δ2H = 7.1 (±0.2)∙δ18O + 1.5 (±1.6), respectively. The temperature and precipitation amount are considered to be the main factor controlling the δ2H and δ18O variation of precipitation, and consequently, relationships were first explored between δ18O and local surface air temperature and precipitation amount by linear regression analysis. The temperature effect was significant in the wet seasons but was irrelevant in the dry seasons on daily and seasonal scales. The amount effect was significant in the wet seasons on a daily scale but irrelevant in the dry seasons. However, based on the data of the Global Network of Isotopes in Precipitation (GNIP) (1985–1987, 1996–1999) of Lanzhou weather station, the amount effects were absent at seasonal scales and were not useful to discriminate either wetter or drier seasons or even wetter or drier decades. Over the whole year, the resulting air mass trajectories were consistent with the main sources of water vapor were from the Atlantic Ocean via westerlies and from the Arctic region, with 46%, 64%, and 40% of water vapor coming from the westerlies, and 54%, 36%, and 60% water vapor from the north in spring, autumn and winter, respectively. In the summer, however, the southeast monsoon (21%) was also an important water vapor source in the Loess Plateau. Concluding, using the δ2H and δ18O signatures of precipitation water, we disentangled and quantified the seasonal wind directions that are important for the prediction of water resources for local and regional land use.
Highlights
Understanding the precipitation sources is important for meteorological, hydrological, and ecological studies that underpin well-informed water resource management [1]
Results were expressed as parts per thousand deviations from the Vienna Standard Mean Ocean Water (V-SMOW) in the following form: δ18 O = (Rs /Rstd − 1)
The δ2 H and δ18 O composition differed for individual precipitation events due to the complexity of vapor sources and the extreme natural climate in the cold semi-arid region
Summary
Understanding the precipitation sources is important for meteorological, hydrological, and ecological studies that underpin well-informed water resource management [1]. The δ18 O and δ2 H signatures of precipitation follow the Rayleigh-type fractionation model during the water cycle, influenced by meteorological factors, such as temperature, atmospheric humidity, precipitation amount, and residence time [8,9] These effects of environmental factors on stable isotope signatures vary with geographical location, atmospheric circulation, weather, and terrain [10]. Since the 1950s, the stable isotope composition of precipitation has been extensively used in studies of hydrologic cycle processes [18,19,20], including the moisture transport trajectory [21], climate models with isotope capability [22,23,24], river or lake water sources, [25,26], groundwater recharge [27], amount effects and paleoclimate reconstruction in certain regions [28,29,30,31], and the use of isotopic time series derived from speleothems to study the past monsoon intensity [32,33,34].
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