Measurements of carbon dioxide and heat fluxes during monsoon-2011 season over rural site of India by eddy covariance technique

An increase in carbon dioxide (CO₂) concentrations in the atmosphere due to anthropogenic activities is responsible for global warming and hence in recent years, CO₂ measurement network has expanded globally. In the monsoon season (July–September) of year 2011, we carried out measurements of CO₂ and water vapour (H₂O) concentrations along with wind and air temperature over a tropical site in south-east India having rural topography. To collect these observations, the instrumentations used were the sonic anemometer for wind and temperature, and the open path H₂O/CO₂ infrared gas analyzer for CO₂ and H₂O concentrations. Using these observations, we explored the diurnal variability of CO₂ flux along with sensible and latent heat. The CO₂ flux was positive during night-time and negative during daytime and in phase with convective instability. The CO₂ flux relationships with the meteorological parameters such as wind speed, temperature and heat fluxes have been analysed. The seasonal (monsoon) half hour mean of CO₂ flux which was ?3.55 μmol m^???2 s^???1 indicated the experimental site as a CO₂ sink region (net seasonal uptake). An increase in CO₂ concentrations during weekends was not observed due to unavailability of heavy vehicular traffic.     

Carbon dioxide fluxes over a temperate meadow in eastern Inner Mongolia,China

Understanding the carbon dynamics in grassland is essential to precisely estimate global atmospheric carbon budget in response to climatic change. Eddy flux measurements were carried out during 2011 and 2012 to characterize seasonal and annual variability of carbon exchanges above a temperate meadow in eastern Inner Mongolia, China. The CO₂ flux showed obvious diurnal variations and the monthly mean amplitudes of diurnal course followed June/July > August > May > September. The daily maximum NEE reached up to ?8.0 and ?7.7 g C m^?2 for 2011 and 2012, respectively. CO₂ uptake was mainly from May to August, with seasonal peaks of ?16.0 g C m^?2 day^?1 in both two years. Gross primary production (GPP) and ecosystem respiration (Re) were ?1,084.5, 987.1 g C m^?2 year^?1 in 2011, and ?1,123.3, 1,040.2 g C m^?2 year^?1 in 2012, respectively. The meadow acted as a stable carbon sink, with integrated net ecosystem exchange (NEE) of ?97.4 and ?83.1 g C m^?2 year^?1 for 2011 and 2012, respectively. Compared with 2011, the ecosystem assimilated more carbon and meanwhile respired even more, leading to a less carbon sequestration in 2012. PAR and leaf area index (LAI) dominated the seasonal variations in NEE, with PAR explaining 61–69 % of the variance in NEE as LAI maintaining the plateau during June to July. Harvest significantly decreased ecosystem carbon uptake. The interannual variability in GPP and Re resulted primarily from the variations in temperature and its effect on biomass growth.     

Carbon dioxide,water vapour and energy fluxes over a semi-evergreen forest in Assam,Northeast India

The eddy covariance method is a powerful technique for quantification of \(\hbox {CO}_{2},\) \(\hbox {H}_{2}\)O and energy fluxes in natural ecosystems. Leaf area index (LAI) and its changes are significant drivers of \(\hbox {CO}_{2}\) and \(\hbox {H}_{2}\)O exchange in a forest ecosystem due to their role in photosynthesis. The present study reports the seasonal variation of \(\hbox {CO}_{2}\) and energy fluxes and their relationship with other meteorological parameters of a semi-evergreen primary forest of Kaziranga National Park, Assam, India during February 2016–January 2017. The diurnal pattern of half hourly average \(\hbox {CO}_{2 }\) fluxes over the forest was found to be mostly dominated by the incident photosynthetically active radiation. During the period of study, diurnal variations of \(\hbox {CO}_{2}\) flux showed a maximum value of \(-9.97\,\upmu \)mol \(\hbox {m}^{-2}\hbox {s}^{-1}\) in the month of June during summer which is also the beginning of the monsoon season. The monthly averaged diurnal \(\hbox {CO}_{2}\) flux and variation in LAI of the forest canopy closely followed each other. The annual net ecosystem exchange of the forest estimated from the \(\hbox {CO}_{2}\) flux data above the canopy is 84.21 g C \(\hbox {m}^{-2}\,\hbox {yr}^{-1}\). Further studies are in progress to confirm these findings. The estimated average annual evapotranspiration of the semi-evergreen forest is 2.8 ± 0.19 mm \(\hbox {day}^{-1}\). The study of partitioning of energy fluxes showed the dominance of latent heat fluxes over sensible heat fluxes. The energy balance closure was found to increase with an increase in instability and the highest closure of around 83% was noted under neutral conditions.     

Catchment scale spatial variability of soil salt content in agricultural oasis,Northwest China

Soil salinization is a serious environmental problem in the world, especially in arid and semi-arid regions. Therefore, estimating spatial variability of soil salinity plays an important role in environmental sciences. Aiming at the problem of soil salinization inside an oasis, a case study was carried out at the Sangong River catchment in Xinjiang province, northwest China. Methods of classical statistics, geostatistics, remote sensing (RS) and geographic information system (GIS) were applied to estimate the spatial variability of soil salt content in the topsoil (0–20 cm) and its relationship with landscape structure at catchment scale. The objective of this study was to provide a scientific basis to understand the heterogeneous of spatial distribution of soil salt content at a large scale. The results revealed that (1) elevation of landform was a key factor for soil salt content’s spatial variability, and soil salt content had a strong spatial autocorrelation, which was mainly induced by structural factors. (2) Mapping of soil salt content by Kriging and comparing it with landscape maps showed that area of soil salinization in old oasis was smaller than that in new oasis, and degree of soil salinization in old oasis was also lower than that in the new one. Among all landscapes, cropland was mostly affected by salinity, with 38.8% of the cropland in new oasis moderately affected by soil salinity, and 8.54% in old oasis.     

Water requirements and stability of oasis ecosystem in arid region,China

This research was conducted to determine the water requirements of oasis ecosystem with crop evapotranspiration method, and to analyse the water balance between the supply and requirement using water balance model, and then assess the stability of oasis ecosystem in the middle of Heihe River basin, China. The results indicated that the summations of the water supply and requirement approximated to 82.54 and 110.13 Mm³ years⁻¹ in 2007, and the water deficit was 27.59 Mm³ years⁻¹. The farmland was the largest water consumer with a consumption of 57.07 Mm³ years⁻¹ and accounted for 51.82% of the total water requirements. It was followed by the water area 38.83 Mm³ years⁻¹, forestland 12.13 Mm³ years⁻¹ and domestic and industrial 2.10 Mm³ years⁻¹, and accounted for 35.26, 11.01 and 1.92%, respectively. The stability index was 0.74, which implies that the oasis ecosystem have already started degenerating in sub-stability state. However, the water requirement of unit area was 1243.70 mm years⁻¹ and larger than other oases in arid region of China, which implies that the water resource scarcity do not exist in the middle basin where the excessive waste of the flood irrigation method has broken the balance between the water supply and requirement in the basin scale.     

Evaluation of spatial interpolation methods for groundwater level in an arid inland oasis,northwest China

The Wuwei oasis, situated in the upper reaches of the Shiyang River basin in the arid inland of northwest China, is intensively cultivated using both groundwater and irrigation water originating from the Qilian Mountains. Groundwater levels are declining due to overuse of irrigation water. To estimate the decline over the entire Wuwei oasis, eight different interpolation methods were used for interpolating groundwater levels over 3 years, i.e. starting in 1983, followed by 1988 and ending with 1992. Cross-validation and orthogonal-validation were applied to evaluate the accuracy of the different methods. Root mean squared error and the correlation coefficient (R ²) were calculated for each of the interpolation methods and years. Three kriging methods (simply, ordinary, and universal) gave the best fit. Modified ordinary kriging was found better than simple and universal kriging methods with a smaller number of points having large differences (>50 m) between estimated and predicted values. Based on the groundwater surfaces determined by the ordinary kriging as modified by Yamamoto, the groundwater decline was found from 1983 to 1992 to be a modest 2.1 m in average.     

Responses of phreatophyte transpiration to falling water table in hyper-arid and arid regions,Northwest China

Quantitative assessment of the impact of groundwater depletion on phreatophytes in (hyper-) arid regions is key to sustainable groundwater management. However, a parsimonious model for predicting the response of phreatophytes to a decrease of the water table is lacking. A variable saturated flow model, HYDRUS-1D, was used to numerically assess the influences of depth to the water table (DWT) and mean annual precipitation (MAP) on transpiration of groundwater-dependent vegetation in (hyper-) arid regions of northwest China. An exponential relationship is found for the normalized transpiration (a ratio of transpiration at a certain DWT to transpiration at 1 m depth, T_a*) with increasing DWT, while a positive linear relationship is identified between T_a* and annual precipitation. Sensitivity analysis shows that the model is insensitive to parameters, such as saturated soil hydraulic conductivity and water stress parameters, indicated by an insignificant variation (less than 20% in most cases) under ± 50% changes of these parameters. Based on these two relationships, a universal model has been developed to predict the response of phreatophyte transpiration to groundwater drawdown for (hyper-) arid regions using MAP only. The estimated T_a* from the model is reasonable by comparing with published measured values.© 2021 China Geology Editorial Office.     

咸阳地热田钻孔温度测量及水动力系统

地热田温度场分析, 不仅为地热田类型划分和热源机理研究提供科学根据, 而且可以为确定地热田有利开采区域和深度提供直接依据。本文报道了咸阳地热田13口钻孔的系统(准)稳态测温数据, 对研究区温度的垂向分布特征做了初步分析, 并据此划分了地热田水动力系统。结果表明, 咸阳地热田属于以传导为主的沉积盆地型地热田, 地温梯度为26.2~40.1 ℃/km, 平均为32.4 ℃/km。然而, 与典型的传导型地热田相比, 咸阳地热田的地温场特征又存在特殊性, 表现为钻孔温度-深度曲线分段性明显: 浅部受地表水流动对温度场的影响, 地温曲线呈现出锯齿形波动; 钻孔中上部受地表水和深部水热活动影响较小, 温度曲线为传导性地热特征; 井孔中下部测温曲线明显"下凹", 揭示了地下水沿渭河断裂侧向补给的同时使地层温度降低; 井孔下部温度随深度异常增大, 表明存在异常压力流体封存箱。测温资料揭示了咸阳地热田水动力系统在垂向上存在多层结构: 浅部为垂向重力驱动型, 中上部为正常压实型, 中下部为侧向重力驱动型, 下部为封闭型。基于咸阳地热田水动力系统的多层结构, 建议将各系统赋存的地热资源分别进行规划和开发。     

过去千年中国西北干旱区温度变化特征及成因分析

本研究基于过去千年多成员集合模拟试验(CESM-LME)数据, 在与同化资料(LMR)、参与CMIP6两个模式(MRI-ESM2和MIROC-ES2L)过去千年模拟结果以及观测/再分析资料(CN05.1)进行对比验证模式模拟结果可靠性的基础上, 分析了中国西北干旱区过去千年地表温度变化特征和影响因子。研究表明, CESM-LME全强迫试验能够较好地模拟出中世纪气候异常期、小冰期和现代暖期温度异常特征, 模式模拟的中国西北干旱区中世纪气候异常期温度变化幅度与现代暖期的相当。通过计算不同尺度上各单因子敏感性试验与全强迫试验中地表温度序列相关系数发现, 不仅在年尺度上, 年代际、多年代际、百年尺度上火山活动强迫出的地表温度序列与全强迫试验中的均呈现显著的相关关系, 但二者的关系在过去千年并不稳定; 另外, 1850年后温室气体排放对西北干旱区地表温度升高起到了主导作用。根据火山喷发纬度位置和气溶胶注入量两个标准将过去千年火山活动分为3种类型, 即北半球火山(NHV)、赤道火山(TRV)和南半球火山(SHV)。NHV爆发对西北干旱区降温的影响最强, TRV次之, 但是二者差别不大, 均在爆发10~12个月之内降温至-2.0 ℃以下。与以往研究不同的是, SHV爆发后中国西北干旱区地表温度在10个月后降温幅度能达到-1.7 ℃, 且该冷异常可持续到22个月之后。

     

Geostatistical analysis of temporal and spatial variations in groundwater levels and quality in the Minqin oasis, Northwest China

Recognition of the temporal and spatial variations in groundwater levels and quality has become a prerequisite of formulating strategies for the sustainable development and utilization of water resources. In this study, data were obtained from 51 observation wells of depth to groundwater from 1999 to 2008 and 30 sampling wells of hydrochemical characteristics of groundwater in the Minqin oasis. The Kolomogorov–Semirnov test revealed that all data followed normal or log-normal distribution. A set of well-structured semivariograms also confirmed that the data had moderate (only for Mg²⁺ and K⁺) or strong spatial dependence. Based on spatial distribution, maps drawn using the ordinary Kriging interpolation method in different periods, the declining trend of the groundwater table was found to have been relieved since 2007, with the mean water table dropping 4.65 m from 1999 to 2008. Spatial comparison results further showed that the variations in groundwater levels in Baqu and Quanshan were more evident than those in Huqu, with mean decline rates of 0.64, 0.93, and 0.41 m/year, respectively. The mean value of total dissolved solids (TDS) was 3.34 g/L, and the groundwater in 76.2 % of the study area was brackish (TDS ≥ 1.0 g/L). From south to north, the groundwater types can be classified into three obvious zones based on the major ion distributions: SO₄ ^2?–HCO₃ ^?–Na⁺–Ca²⁺ type in Baqu, SO₄ ^2?–Na⁺–Ca²⁺ type in Quanshan, and SO₄ ^2?–C1^?–Na⁺ type in Huqu. TDS increased from the 1970 to 2005 and gradually decreased thereafter.     

Variations in groundwater levels and quality due to agricultural over-exploitation in an arid environment: the phreatic aquifer of the Souf oasis (Algerian Sahara)

Groundwater is a major source of supply for domestic and agricultural purposes, especially in arid and semi-arid regions. In this study, we followed the variations in water levels in the Souf oasis in the Algerian Sahara by measuring depths to groundwater across 65 points during the period from 2010 to 2015. Additionally, electrical conductivity (EC) was measured for assessing variations in groundwater salinity in the same groundwater monitoring network over the same time interval. The results from these investigations indicated that there are significant and continuous declines in the groundwater level across all study areas throughout the period of investigation. This is especially the case in the northern part of the study area where the water table declined by up to 18.2 m in Ghamra in 2015. Additionally, this study has indicated that the rate of decline of groundwater levels has increased from 0.29 m/year as an average in 2011 to 2.37 m/year in 2015, where the situation has become alarming. As a consequence of this, the depth to groundwater now exceeds 2 m over more than 77% of the study area, and only about 17% of the study area now has a water table depth that lies within the optimal depth interval for extractive uses (between 1 and 2 m). This decline in groundwater levels has been accompanied by a significant increase in the electrical conductivity values (salinity) of this water, and there is a strong correlation between these variables (R > 0.99). This alarming situation has been caused by the continuous over-exploitation and unsustainable management of this limited resource, especially by the agricultural sector. For a long time, this critical situation led to the demise of the agricultural world heritage cultivation system (Ghout) due to the increasing salinity of groundwater. Two solutions are proposed to manage the effects of groundwater depletion in the area: firstly, rationalizing groundwater use through effective groundwater allocation management measures, and secondly by implementing the reuse of treated wastewater as an alternative water source for agricultural use. This latter measure could be in two ways: either by direct use in irrigation to relieve pressure on the phreatic aquifer, or by artificial recharge of the phreatic aquifer.     

Quantitative assessment of the impact of an inter-basin surface-water transfer project on the groundwater flow and groundwater-dependent eco-environment in an oasis in arid northwestern China

Inter-basin water transfer projects (IBWTPs) can involve basins as water donors and water receivers. In contrast to most studies on IBWTPs, which mainly impact the surface-water eco-environment, this study focuses on the impacts of an IBWTP on groundwater and its eco-environment in a water donor basin in an arid area, where surface water and groundwater are exchanged. Surface water is assumed to recharge groundwater and a groundwater numerical simulation model was constructed using MODFLOW. The model was used to quantitatively evaluate the impact of an IBWTP located in the upstream portion of Nalenggele River (the biggest river in the Qaidam basin, Northwest China). The impact involved decrease in spring flow, drawdown of groundwater, reduction in oasis area, and an increase in species replacement of oasis vegetation in the midstream and downstream of the river. Results show that the emergence sites of springs at the front of the oasis will move 2–5 km downstream, and the outflow of springs will decrease by 42 million m³/a. The maximum drawdown of groundwater level at the front of the oasis will be 3.6 m and the area across which groundwater drawdown exceeds 2.0 m will be about 59.02 km², accounting for 2.71% of the total area of the oasis. Under such conditions, reeds will gradually be replaced by Tamarix, shrubs, and other alternative plant species. These findings have important implications for the optimization of water resource allocation and protection of the eco-environment in arid regions.     

中国西北内陆干旱盆地地下水资源评估与开发

以多年的地下水勘查经验、成果为基础，总结西北内陆干旱盆地地下水资源评估与开发中的主要问题，分析了产生这些问题的客观和主观原因；认为地下水动态监测和水运行测量统计是实现逐步逼近地下水资源量的最有效方法；提出了开发地下水库，实施地表水、地下水联合调度，稳定供水，是西北内陆干旱盆地水资源开发的最佳模式。     

DRAV model and its application in assessing groundwater vulnerability in arid area: a case study of pore phreatic water in Tarim Basin,Xinjiang, Northwest China

According to the characteristics of groundwater in arid area, this paper proposes DRAV model for groundwater vulnerability assessment, where D is groundwater depth, R is the net recharge of aquifer, A is the aquifer characteristics, and V is the lithology of vadose zone. As a case study, the paper assesses the vulnerability of pore phreatic water in Tarim Basin of Xinjiang, China by using the DRAV model. The results indicate that the areas of phreatic water with vulnerability index ranges of 2–4, 4–6, 6–8 and >8 accounting for 10.1, 80.4, 9.2 and 0.2% of the total plain area of the Tarim Basin respectively, and the areas with the latter two vulnerability ranges (6–8 and >8) are mainly located in the irrigation districts with thin soil layer (20–30 cm thick surface soil of vadose zone, mainly with underlying sandy gravel) and with silty and fine sand layer. Such vadose zone generally lacks sandy loam and clayey soil and has larger recharge by infiltration of irrigation water.     

柴达木盆地地温场分布特征及控制因素

含油气盆地的地温场在油气的生成、运聚及盆地演化过程中起着十分重要的作用。柴达木盆地是中国西部重要的含油气盆地,位于喜马拉雅构造域的东北部,盆地现今地温场特征研究不仅为柴达木盆地及周缘陆内或板内大陆动力学及盆地动力学研究提供了科学依据,同时也是油气成烃、成藏及资源评价等工作的研究基础。柴达木盆地现今地温场研究的前期工作主要集中在盆地西部,盆地的北缘、东部开展的工作很少,所用研究数据多取自20世纪之前,盆地现今地温场特征的系统研究尚比较缺乏,亟须开展相关研究工作。文中采用先进的钻孔温度连续采集系统,实现了深井稳态测温工作的大规模化、高精度化,使用光学扫描法测试岩石热导率,获得了批量的、高精度的岩石热导率数据,新增了17口钻井的测温剖面。研究表明：柴达木盆地现今地温梯度平均为(28.6±4.6) ℃/km,地温梯度分布具有西部高,中、东部低的特点。柴达木盆地现今大地热流值平均(55.1±7.9) mW/m2,盆地不同构造单元大地热流分布存在差异。大地热流分布特征表明：柴达木盆地总体属于温盆,热流值低于我国大陆地区大地热流平均值,但高于西部的塔里木、准噶尔盆地。柴达木盆地现今地温场分布特征受地壳深部结构、岩石热导率性质及盆地构造等因素的控制。     

Evaluation of landscape changes and ecological degradation by GIS in arid regions: a case study of the terminal oasis of the Shiyang River,northwest China

The development and utilization of water and land resources in the Shiyang River basin are the highest among the inland rivers in northwestern China. Using GIS and the landscape structure analysis program FRAGSTATS, landscape metrics of a study area in the lake-district in Minqin Oasis in the lower reaches of Shiyang River basin were calculated and analyzed based on TM images from 1987 to 2001 at class level and landscape level, respectively. Transformation matrix of the landscape mosaics was also examined. The driving forces of landscape changes were analyzed, including the dynamics of runoff and groundwater resources and their exploitation, and influences of policies and market economy. The results showed that: (1) Patch number of farmland and desert decreased, but their mean patch area increased, indicating that reclamation and desertification was very serious in the study area. Contagion index for oasis increased and edge density and landscape diversity were reduced. (2) Farmland and desert areas have enlarged by 1,520.46 and 4,941.27 hm², respectively. This increase was at the cost of woodland and grassland, which shrank by 1,520.46 and 4,941.27 hm², respectively. (3) The net utilization of surface water decreased by 50% in the last 10 years, and dropped to 22.3 × 10⁶ m³ in the later 1990s. Exploitation of groundwater increased remarkably, up to 80 × 10⁶ m³ per year in the last 5 years. (4) The capability of the economic structure to withstand pressures of the market economy has depressed distinctly because of the simplified planting pattern, which is dominated by salinity-enduring cash crops, such as cotton. Based on the above analysis, strategies of ecological reconstruction in the lake-district were suggested.