Isotopic composition of atmospheric precipitation and its tracing significance in the Laohequ Basin, Loess plateau, China

Highlights: • The variability of δ¹⁸O and δD in precipitation was generated by a combination factors including temperature and precipitation amount. • The precipitation amount effect plays key role in rainy season, however, the temperature results in isotopic enrichment in drought conditions. • The precipitation isotopic variations at micro and synoptic scale is useful in tracing local hydrological process. Based on the δ¹⁸O and δD values in precipitation and related meteorological parameters in the Chinese Loess Plateau (CLP) from June 2014 to April 2016, the effects of temperature and precipitation amount effect on stable isotopic compositions were analyzed, as well as the tracing significance of deuterium excess. The results show that the Local Meteoric Water Line (LMWL) was defined as δD = 7.08δ¹⁸O + 4.73(R² = 0.99), with a slope of r = 0.113, n = 150, p = 0.168), which shows a weak positive correlation between temperature and isotopic composition. Meanwhile, the precipitation amount effect was defined as δD = −0.7883P–34.152 (r = −0.223, n = 150, p = 0.011), which shows a weak negative correlation between temperature and isotope composition. The slopes in cross plotting δD - δ¹⁸O records were 8.3, 7.7, 7.4, and 6.3, displaying a decreasing trend in the temperature gradient of T ≤ 0 °C, 0 < T ≤ 10 °C, 10 < T ≤ 20 °C and T > 20 °C, respectively. The amount effect is significant (δD = −7.0946P + 48.686, r = −0.548, n = 20, p = 0.012) in the precipitation range from 10 to 20 mm. The average deuterium excess values in winter were 24‰ and 25.3‰ for 2015 and 2016, respectively; however, the low values of 3.1‰ and 5.5‰ occurred in the summers of 2014 and 2015, respectively. These results suggested that intense sub-cloud evaporation decreased the deuterium excess values in the Chinese Loess Plateau under the climate conditions of the rainy season with high temperatures. However, deuterium excess values of snow in wintertime (LT-1, LT-2) could be reflective of the reliable characteristics of atmospheric vapor for weak sub-cloud evaporation and surface evaporation.