[Influence of below-cloud secondary evaporation on stable isotope composition in precipitation in Northwest China.]

The precipitation isotope data and meteorological data of eight stations provided by GNIP (Global Network for Isotopes in Precipitation) and two stations from the present study, combined with HYSPLIT model and water droplet evaporation model were used to examine the spatial and temporal distribution of precipitation δ¹⁸O and d values in Northwest China. The secondary evaporative effect of existence was evaluated and then quantitatively discussed, with the sensitive factors of secondary evaporative effect being considered. The results showed that during the summer monsoon, the δ¹⁸O and d values decreased from south to north in Xinjiang, while the δ¹⁸O value increased but d values decreased from south to north and from east to west of Shaanxi-Gansu-Ningxia region. During the winter monsoon, the δ¹⁸O value decreased from east to west in whole Northwest region, while the d value increased from south to north in Xinjiang, decreased from south to north and increased slightly from east to west in Shanxi-Gansu-Ningxia. The slope and intercept (6.80, -0.07) of the atmospheric precipitation line in the summer monsoon period was significantly lower than that of annual mean (7.27, 3.37) and winter monsoon period (7.46, 6.07), indicating that the secondary evaporation was stronger during the summer monsoon. The evaporation ratio in the summer monsoon was 4.49%, which was higher than 3.65% in the winter monsoon. However, the evaporation ratio of the winter monsoon was higher than the summer monsoon around of Loess Plateau, which might closely relate to the increasing drought of the Loess Plateau in recent years. Finally, the intensity of secondary evaporation decreased with increasing relative humidity, precipitation and vapor pressure but increased with increasing temperature (greater than 0 ℃). The influences of those factors (humidity, precipitation, temperature and vapor pressure) on the secondary evaporation were dependent on the differences of ranges.