| Precipitation as a vital section of water cycle process,the study of its stable isotope is helpful to understand the water cycle process.The evaporation effect of precipitation from the bottom of the cloud to near-surface is known as the below-cloud secondary evaporation of raindrops.The process of raindrops falling from the bottom of the cloud to near-surface,due to the influence of the below-cloud secondary evaporation,the isotope value will be different.Therefore,this paper selected four sampling points in the Urumqi River Basin to detect the isotopes of precipitation.Based on the precipitation samples collected from May2012 to September 2013,the stable isotopes of precipitation in the Urumqi River Basin were analyzed systematically.And combining the meteorological data of the16 automatic weather stations in summer of 2013 and the meteorological data of the Urumqi and Caijiahu stations from 1980 to 2015,the temporal and spatial patterns of the whole basin and the simulation of the below-cloud secondary evaporation were further analyzed.At the same time,the applicability of the parameters was evaluated.The results were given as follows:?1?The?18O of the four sampling sites in the downstream of Urumqi River Basin was higher than the upstream and the spatial distributions of?D and?18O were similar.The?D and?18O in the precipitation of Urumqi and Caijiahu showed an obvious seasonal variation,and the?D and?18O in summer?April to October?was significantly lower than that in the winter?November to March the following year?.The stable isotope and temperature of the Urumqi River Basin showed a good correlation.The correlation coefficients of?18OT in the downstream were all greater than 0.9 and in the upstream the correlation coefficients were low.In addition,there were different correlations between precipitation,relative humidity and the?18O at the four sampling stations.And the correlation of?18ORH was weaker than that of?18OT.?2?The slope of the local meteoric water line at each sampling point in the Urumqi River Basin was lower than the global meteoric waterline.Among them,the slope of the atmospheric water line in Urumqi was 7.84,which was the most close to that of the global atmospheric waterline.The LWML slope of Houxia was6.63,which was the biggest difference with the global meteoric waterline slope.And the slope and intercept of the atmospheric waterline showed a decreasing trend with the increase of temperature,and showed an increasing trend with the increase of relative humidity.?3?The diameter of the raindrops in the successive precipitation events was mainly distributed in the range of 1.21.8 mm.Compared with the previous results measured in Tianshan area,the main distribution range of raindrop diameter was slightly larger in this paper.The diameter of the raindrops obtained from the automatic weather stations in the summer 2013 fluctuated between 0.72.7 mm,and the diameter in the range of 0.81.6 mm accounted for 54.74%.The velocity of raindrops fluctuated between 3.17.2 m/s in all samples of the whole basin,the velocity of raindrops above 4.5 m/s in most precipitation events,and the range larger than 3.54.5 m/s in Tianshan area.?4?The evaporative rate of raindrops in the Urumqi River Basin showed an obviously seasonal difference.In space,the evaporative rate in the downstream was greater than that in the upstream.Similarly,the temporal and spatial differences of f and?d were similar to the evaporative rate.In the time series,the changing tendency of the evaporative rate of raindrops in Urumqi and Caijiahu sites was similar.The f of Urumqi was larger than that of Caijiahu.The?d of Urumqi in most years was higher than that of Caijiahu.?5?The linear relationship between the evaporation rate and the change of d in the Urumqi River Basin was the?d decreasing 1.012‰when the evaporation increasing 1%.The relationship between the f and the?d measured by Meteorology sites was the?d decreasing 1.198‰when the evaporation increasing 1%in summer 2013.In the time series,the?d decreased 1.240‰when the evaporation increasing 1%in Urumqi.And the?d decreased 1.216‰when the evaporation increasing 1%in Caijiahu.Their relationship was close to 1.1‰/%while the evaporative rate was more than 90%.?6?The?d in the precipitation showed an increasing trend with the increase of precipitation.When the air temperature gradually increasing,the?d showed a decreasing trend.While the relative humidity increasing,the?d was significantly improved.The diameter of the raindrop was decreased from 0.2 mm to an increase of the range of 5 mm,the mean value of?d in sampling points precipitation increased from-7.9‰to-0.1‰,and the mean value of?d in weather stations precipitation increased from-42.2‰to-0.7‰.When the raindrop diameter was smaller,the?d was likely to be less than-30‰. |
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