Fenli Chen

Factors controlling stable isotope composition of precipitation in arid conditions: an observation network in the Tianshan Mountains, central Asia

Approximately one-third of the Earths arid areas are distributed across central Asia. The stable isotope composition of precipitation in this region is affected by its aridity, therefore subject to high evaporation and low precipitation amount. To investigate the factors controlling stable water isotopes in precipitation in arid central Asia, an observation network was established around the Tianshan Mountains in 2012. Based on the 1052 event-based precipitation samples collected at 23 stations during 2012–2013, the spatial distribution and seasonal variation of δD and δ18O in precipitation were investigated. The values of δD and δ18O are relatively more enriched in the rainfall dominant summer months (from April to October) and depleted in the drier winter months (from November to March) with low D-excess due to subcloud evaporation observed at many of the driest low elevation stations. The local meteoric water line (LMWL) was calculated to be δD=7.36δ18O – 0.50 (r2=0.97, p<0.01) based on the event-based samples, and δD=7.60δ18O+2.66 (r2=0.98, p<0.01) based on the monthly precipitation-weighted values. In winter, the data indicate an isotopic rain shadow effect whereby rainout leads to depletion of precipitation in the most arid region to the south of the Tianshan Mountains. The values of δ18O significantly correlate with air temperature for each station, and the best-fit equation is established as δ18O=0.78T – 16.01 (r2=0.73, p<0.01). Using daily air temperature and precipitation derived from a 0.5° (latitude)×0.5° (longitude) gridded data set, an isoscape of δ18O in precipitation was produced based on this observed temperature effect.

Contribution of recycled moisture to precipitation in oases of arid central Asia: A stable isotope approach

Terrestrial moisture contributed by surface evaporation and transpiration, also known as recycled moisture, plays an important role in hydrological processes especially across arid central Asia. The stable hydrogen and oxygen isotopes can be used for water budget analysis to calculate the contribution of recycled moisture to precipitation between two locations along the moisture flow. Based on a three-component isotopic mixing model, the moisture recycling in oasis stations of arid central Asia during summer months is assessed. At large oases of Urumqi, the proportional contribution of recycled moisture to local precipitation is approximately 16.2%, and the mean proportions of surface evaporation and transpiration are 5.9% ± 1.5% and 10.3% ± 2.2%, respectively. At small oases like Shihezi and Caijiahu the contribution of recycled moisture is less than 5%, and the proportion of surface evaporation is much less than that of transpiration. The vegetative cover in arid central Asia is generally sparse, but the evapotranspiration contribution to precipitation cannot be ignored at the widely distributed oases. The oasis effect shows great variability depending on locations and water availability for evapotranspiration.

Comparison of GCM-simulated isotopic compositions of precipitation in arid central Asia

The isotope-equipped GCMs (general circulation models) are useful tools to investigate the isoscape in precipitation and water vapor, especially for the regions without enough in-situ observations. To study the spatial distribution and seasonal variation of precipitation isotopic composition in arid central Asia, several isotope-equipped GCMs are applied, and the long-term observations in two stations are used to verify the GCM-simulated results. Generally, seasonality of isotopic composition can be well simulated in each GCM, and the values of δ18O in precipitation are larger in summer months (from April to October) and lower in winter months (from November to March). Higher latitude usually shows lower values of δ18O in precipitation, and lower latitude has higher values. The values of δ18O are relatively low in the eastern section, and higher in the western section. Among these simulations, ECHAM is good at describing the isotopic composition in the study region, which can be seen from the mean absolute error (MAE) and root-mean-square error (RMSE). The ECHAM-derived values of δ18O in precipitation positively correlate with the observed series, and the correlation coefficient based on ECHAM is the largest among these GCMs.

Ammonium nitrogen concentration in the Weihe River, central China during 2005–2015

Ammonium nitrogen is a widely used indicator to assess water quality of lakes, rivers and other water bodies. Based on an observation network of five sampling sections on the trunk stream and four on the tributaries along the Weihe River (the largest tributary of the Yellow River), the interannual and seasonal variations in ammonium nitrogen concentration of river water during 2005–2015 are analyzed. The annual mean concentrations generally increase downstream, and the concentration after 2011 is much lower than that before 2011. Seasonality of ammonium nitrogen concentration is observed along the Weihe River, especially on the trunk stream. The interannual and seasonal patterns of ammonium nitrogen concentration are jointly influenced by the anthropogenic and natural factors. The decreasing concentration is related to the strict policy on water pollution control in the recent years, and the seasonal variation in concentration is influenced by the high flow in summer months and low flow in winter months.

Environmental controls on stable isotopes of precipitation in Lanzhou, China: An enhanced network at city scale

Stable hydrogen and oxygen isotopes in precipitation are very sensitive to environmental changes, and can record evolution of water cycle. The Lanzhou city in northwestern China is jointly influenced by the monsoon and westerlies, which is considered as a vital platform to investigate the moisture regime for this region. Since 2011, an observation network of stable isotopes in precipitation was established across the city, and four stations were included in the network. In 2013, six more sampling stations were added, and the enhanced network might provide more meaningful information on spatial incoherence and synoptic process. This study focused on the variations of stable isotopes (δ18O and δD) in precipitation and the environmental controls based on the 1432 samples in this enhanced network from April 2011 to October 2014. The results showed that the precipitation isotopes had great spatial diversity, and the neighboring stations may present large difference in δD and δ18O. Based on the observation at ten sampling sites, an isoscape in precipitation was calculated, and the method is useful to produce isoscape for small domains. The temperature effect and amount effect was reconsidered based on the dataset. Taking meteorological parameters (temperature, precipitation amount, relative humidity, water vapor pressure and dew point temperature) as variables in a multi-linear regression, the result of coefficients for these meteorological parameters were calculated. Some cases were also involved in this study, and the isotopic characteristics during one event or continuous days were used to understand the environmental controls on precipitation isotopes.