分类： 核科学技术 >> 核材料与工艺技术 提交时间： 2023-06-18 合作期刊: 《Nuclear Science and Techniques》
摘要： The existing 210Pbex mass balance models for the assessment of cultivated soil erosion are based on an assumption that 210Pbex is quite evenly mixed within the plough layer. However, the amount of 210Pbex distributed in the soils below the plough depth, like a downward tail in the lower part of the 210Pbex profile, has been largely ignored. In fact, after the initial cultivation of undisturbed soils, 210Pbex will diffuse downward from plough layer to the plough pan layer due to the concentration gradient. Assuming 210Pbex inventory is constant, the depth distribution in the two layers of the cultivated soils will achieve a steady state after continuous cultivation for 10.37 years, when 210Pbex is evenly distributed in the soils of the plough layer with an exponential concentration decline with depth in the soils of the plough pan layer, and the 210Pbex concentration at any depth will be invariable with time. The work reported in this paper attempts to explain the formation of the 210Pbex tail in the soil profile below the plough depth by theoretical derivation of the 210Pbex depth distribution process in the two layers of the cultivated soils, propose a 210Pbex mass balance model considering 210Pbex diffusion based on the existing model, and discuss the influence of the 210Pbex tail to the existing model.
分类： 地球科学 >> 地理学 提交时间： 2021-06-04 合作期刊: 《干旱区科学》
Nirmal M DAHAL
XIONG Donghong
Nilhari NEUPANE
Belayneh YIGEZ
ZHANG Baojun
YUAN Yong
Saroj KOIRALA
LIU Lin
FANG Yiping
摘要： Drought is an inevitable condition with negative impacts in the agricultural and climatic sectors, especially in developing countries. This study attempts to examine the spatial and temporal characteristics of drought and its trends in the Koshi River Basin (KRB) in Nepal, using the standardized precipitation evapotranspiration index (SPEI) over the period from 1987 to 2017. The Mann-Kendall test was used to explore the trends of the SPEI values. The study illustrated the increasing annual and seasonal drought trends in the KRB over the study period. Spatially, the hill region of the KRB showed substantial increasing drought trends at the annual and seasonal scales, especially in summer and winter. The mountain region also showed a significant increasing drought trend in winter. The drought characteristic analysis indicated that the maximum duration, intensity, and severity of drought events were observed in the KRB after 2000. The Terai region presented the highest drought frequency and intensity, while the hill region presented the longest maximum drought duration. Moreover, the spatial extent of drought showed a significant increasing trend in the hill region at the monthly (drought station proportion of 7.6%/10a in August), seasonal (drought station proportion of 7.2%/10a in summer), and annual (drought station proportion of 6.7%/10a) scales. The findings of this study can assist local governments, planners, and project implementers in understanding drought and developing appropriate mitigation strategies to cope with its impacts.
分类： 地球科学 >> 地球科学史 提交时间： 2018-10-29 合作期刊: 《干旱区科学》
XU Guobao
LIU Xiaohong
SUN Weizhen
CHEN Tuo
ZHANG Xuanwen
ZENG Xiaomin
WU Guoju
WANG Wenzhi
QIN Dahe
摘要： Stable isotopes in tree-ring cellulose provide important data in ecological, archaeological, and paleoenvironmental researches, thereby, the demand for stable isotope analyses is increasing rapidly. Simultaneous measurement of cellulose 13C and 18O values from tree rings would reduce the cost of isotopic commodities and improve the analytical efficiency compared with conventional separate measurement. In this study, we compared the 13C and 18O values of tree-ring -cellulose from Tianshan spruce (Picea schrenkiana) in an arid site in the drainage basin of the Urumqi River in Xinjiang of northwestern China based on separate and simultaneous measurements, using the combustion method (at 1050°C) and the high-temperature pyrolysis method (at 1350°C and 1400°C). We verified the results of simultaneous measurement using the outputs from separate measurement and found that both methods (separate and simultaneous) produced similar 13C values. The two-point calibrated method improved the results (range and variation) of 13C and 18O values. The mean values, standard deviations, and trends of the tree-ring 13C obtained by the combustion method were similar to those by the pyrolysis method followed by two-point calibration. The simultaneously measured 18O from the pyrolysis method at 1400°C had a nearly constant offset with that the pyrolysis method at 1350°C due to isotopic-dependence on the reaction temperature. However, they showed similar variations in the time series. The climate responses inferred from simultaneously and separately measured 13C and 18O did not differ between the two methods. The tree-ring 13C and 18O values were negatively correlated with standardized precipitation evapotranspiration index from May to August. In addition, the 18O was significantly correlated with temperature (positive), precipitation (negative), and relative humidity (negative) from May to August. The tree-ring 13C and 18O values determined simultaneously through the high-temperature pyrolysis method could produce acceptable and reliable stable isotope series. The simultaneous isotopic measurement can greatly reduce the cost and time requirement compared with the separate isotopic measurement. These results are consistent with the previous studies at humid sites, suggesting that the simultaneous determination of 13C and 18O in tree-ring -cellulose can be used in wide regions.
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