Climate background, relative rate, and runoff effect of multiphase water transformation in Qilian Mountains, the third pole region

Modeling and assessing the impacts of climate change on groundwater recharge in endorheic basins of Northwest China

Climate change imposing additional stressors on groundwater resources globally, thereby predicting groundwater recharge (GR) changes is crucial to sustainably managing water resources, especially in the arid endorheic basins. Groundwater in the Endorheic Basins of Northwest China (NWEB) is potentially impacting regional socio-economic output and ecosystem stability due to the imbalance between supply and extraction exacerbated by climate change. Hence, recognizing the impacts of climate change on past and future GR is imperative for groundwater supply and sustainable groundwater management in the NWEB. Here, the impact of historical (1971-2020) and projected (2021-2100) climate changes on GR across the entire NWEB and three distinctive landscape regions (i.e., mountainous, oasis, and desert) were assessed. A coupled distributed hydrologic model (CWatM-HBV model), which integrates the Community Water Model (CWatM) and the HBV model, was run with three shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, and SSP3-7.0) forcing from 10 general circulation models (GCMs) to simulate and analyze the interannual and seasonal variations of GR, along with their driving factors. Over the past 50 years, both precipitation and runoff have undergone significant increases, and leading to a dramatic rise in GR (0.09 mm yr-1). The future annual growth rate of GR is projected to range from 0.01 to 0.09 mm yr-1 from SSP1-2.6 to SSP3-7.0 across the entire NWEB, with the majority of the increase expected during the spring and summer seasons, driven by enhanced precipitation. GR from the mountainous region is the primary source (accounting for approximately 56-59 %) throughout the NWEB with the greatest increase anticipated. Precipitation and runoff have significant influences on GR in mountainous areas, and the impact of precipitation on GR is expected to increase over time. Changes in GR in oasis and desert areas are mainly limited by precipitation variation and increase in the SSP2-4.5 and SSP3-7.0 scenario. Additionally, the processes of glacial retreat and permafrost degradation will complicate the GR dynamics although the process is largely interfered with by anthropogenic environmental changes, especially in oasis-desert systems. The average annual recharge in the NWEB was 8.9 mm in the historical period and 13.6 ± 4.1 mm in the future. Despite an increase in GR due to climate change, groundwater storage is likely to continue to decline due to complex water demands in the NWEB. This study highlights the significance of future precipitation changes for GR and contributes to the understanding of the influence of climate change on groundwater systems and advances the sustainable management of water resources.

Spatiotemporal variability and controlling factors of groundwater depletion in endorheic basins of Northwest China

Recent global groundwater overpumping is threatening ecosystem stability and food security, particularly in arid basins. A solid investigation regarding the drivers of groundwater depletion is vital for groundwater restoration, hitherto, yet it remains largely unquantified. Here, a framework to quantify the contribution of natural forcing (NF) and anthropogenic perturbations (AP) to groundwater storage anomalies (GWSA) variability by separating the GWSA estimated by the Gravity Recovery and Climate Experiment (GRACE) satellite into natural- and human-induced GWSA was proposed in the northwest endorheic basin (NWEB) of China. Further, a multiple linear regression model was established for GWSA change prediction. Our results showed that, during the period 2003-2020, the GWSA depleted at a rate of 0.25 cm yr-1 in the entire NWEB. In addition, GWSA was found to decrease significantly (exceeding 1 cm yr-1) in the west of NWEB where there are heavily irrigated areas, and has become one of the regions with the most serious groundwater depletion in China. Whereas a significantly increasing trend (greater than 0.5 cm yr-1) was observed in the Qaidam basin and south part of the Tarim River basin, becoming a groundwater enrichment reservoir in NWEB. The negative contribution of AP to groundwater depletion has increased from 3% to 95% in the last decade, as determined by separating the effects of NF and AP on GWSA. The rapid expansion of the cropland area and the increase in water use due to population growth are investigated to be the main reasons for GWSA depletion, particularly in the North Tianshan Rivers, Turpan-Hami, and Tarim River basins. Therefore, we conclude that AP are dominating and accelerating groundwater depletion in the NWEB. The increase of GWSA in the Qaidam basin has been attributed to the increase in solid water melt and regional precipitation. The western route project of China's south-north water diversion and water-saving irrigation are important ways to solve the problem of groundwater depletion in NWEB. Our results emphasize that a more feasible framework capable of reliably identifying the driving factors of groundwater storage change is a necessary tool for promoting the sustainable management of groundwater resources under both NF and AP in arid endorheic basins.

Simulation of future land use/cover change (LUCC) in typical watersheds of arid regions under multiple scenarios

The rapid development of the social economy has promoted a continuous increase in the intensity and scale of land use by humans, which has seriously affected the sustainable development of the region. It is important to understand the land use/cover change (LUCC) in the arid region and its future development trends and to make reasonable planning recommendations for the sustainable development of the ecological environment. This study validates the patch-generating land use simulation (PLUS) model in a typical arid region, the Shiyang River Basin (SRB), and analyzes the applicability of the model in arid regions. On this basis, the PLUS model is combined with the scenario analysis method to design four scenarios including no policy intervention, farmland protection, ecological protection and sustainable development to analyze the dynamic changes in past and future land use in the SRB and to make corresponding planning recommendations for the development of each type of land use in the arid region. The results showed that the PLUS model had a better simulation effect in the SRB (its overall accuracy reached 0.97). Coupled models obtain better simulation results than quantitative and spatial models by comparing the mainstream models, with PLUS model that combines CA model and patch generation strategy showing better simulation results in the same category. From 1987 to 2017, the spatial centroid of each LUCC in the SRB moved to varying degrees due to a continuous increase in human activities. The spatial centroid of water bodies had the most obvious change, with a moving speed of 1.49 km/a, while the moving speed of built-up land increased year by year. The spatial centroid of farmland, built-up land and unused land all shifted toward the middle and lower plains, which is a further indication of increased human activity. Due to different government policies, the development trend of land use was also different under different scenarios. However, the four scenarios all showed that the area of built-up land will be increasing exponentially from 2017 to 2037, which would seriously affect the surrounding ecological land and have a negative impact on the local agro-ecological environment. Therefore, we proposed the following planning recommendations: (1) Land leveling work should be carried out on scattered farmland located at high altitudes and with slopes over 25°. Additionally, the land use of low-altitude areas should strictly adhere to basic farmland, increase the diversification of cropping patterns and improve the efficiency of agricultural water. (2) The relationship between ecology, farmland and cities should be reasonably coordinated and the existing idle built-up land should be efficiently used. (3) Forestland and grassland resources should be strictly protected and the ecological redline should be strictly observed. This study can provide new ideas for LUCC modeling and prediction in other parts of the world and provide a strong basis for ecological management and sustainable development in arid areas.

Spatio-Temporal Development of Vegetation Carbon Sinks and Sources in the Arid Region of Northwest China

Drylands, which account for 41% of Earth's land surface and are home to more than two billion people, play an important role in the global carbon balance. This study analyzes the spatio-temporal patterns of vegetation carbon sinks and sources in the arid region of northwest China (NWC), using the net ecosystem production (NEP) through the Carnegie-Ames-Stanford approach (CASA). It quantitatively evaluates regional ecological security over a 20-year period (2000-2020) via a remote sensing ecological index (RSEI) and other ecological indexes, such as the Normalized Difference Vegetation Index (NDVI), fraction of vegetation cover (FVC), net primary productivity (NPP), and land use. The results show that the annual average carbon capacity of vegetation in NWC changed from carbon sources to carbon sinks, and the vegetation NEP increased at a rate of 1.98 gC m-2 yr-1 from 2000 to 2020. Spatially, the annual NEP in northern Xinjiang (NXJ), southern Xinjiang (SXJ) and Hexi Corridor (HX) increased at even faster rates of 2.11, 2.22, and 1.98 gC m-2 yr-1, respectively. Obvious geographically heterogeneous distributions and changes occurred in vegetation carbon sinks and carbon sources. Some 65.78% of the vegetation areas in NWC were carbon sources during 2000-2020, which were concentrated in the plains, and SXJ, the majority carbon sink areas are located in the mountains. The vegetation NEP in the plains exhibited a positive trend (1.21 gC m-2 yr-1) during 2000-2020, but this speed has slowed since 2010. The vegetation NEP in the mountain exhibited only intermittent changes (2.55 gC m-2 yr-1) during 2000-2020; it exhibited a negative trend during 2000-2010, but this trend has reversed strongly since 2010. The entire ecological security of NWC was enhanced during the study period. Specifically, the RSEI increased from 0.34 to 0.49, the NDVI increased by 0.03 (17.65%), the FVC expanded by 19.56%, and the NPP increased by 27.44%. Recent positive trends in NDVI, FVC and NPP have enhanced the capacity of vegetation carbon sinks, and improved the eco-environment of NWC. The scientific outcomes of this study are of great importance for maintaining ecological stability and sustainable economic development along China's Silk Road Economic Belt.

The role of land use change in affecting ecosystem services and the ecological security pattern of the Hexi Regions, Northwest China

The land use and land cover change (LUCC) associated with climate change and human activities is supposed to exert a significant effect on ecosystem functions in arid inland regions. However, the role of LUCC in shaping the spatio-temporal patterns of ecosystem services and ecological security remain unclear, especially under different future LUCC scenarios. Here, we evaluated dynamic changes of ecosystem services and ecological security pattern (ESP) in the Hexi Regions based on LUCC and other environment variables by integrating morphological spatial pattern analysis (MSPA), entropy weight method and circuit theory. Our result showed that the LUCC was generally stable from 1980 to 2050. Compare to 2020, the land conversion under natural growth (NG), ecological protection (EP) and urban development (UD) scenarios in 2050 has changed by 10.30 %, 10.10 %, and 10.31 %, respectively. The forest, medium-cover grassland and water increased in the EP scenario, and construction land and cropland greatly expanded in the other two scenarios. Ecosystem services grew larger in the EP scenario by 2050 in comparison with the NG and UD scenarios. The ESP in the Hexi Regions has obvious spatial differences during 1980-2050. The larger ecological sources and less resistance corridors were mainly distributed in the central and eastern of the Hexi Regions with high ecosystem services. Conversely, fragmented ecological sources and larger resistance corridors were mostly located in the western regions blocked by sandy land, bare land or mountains. Compared to 2020, the area of ecological sources and pinch points under the EP scenario in 2050 increased by 4.10 × 10³ km² and 0.31 × 10³ km², respectively. The number of ecological corridors reduced while the length and resistance increased apart from the EP scenario. Our results highlighted the importance of ecological protection in shaping the LUCC, which further enhances the integrity of ecosystem and ecological security.

Environmental significance of atmospheric nitrogen deposition in the transition zone between the Tibetan Plateau and arid region

The ecological effect of atmospheric N deposition has become a hot research point along with intensive human activities and global climatic change. As the transition zone between the Tibetan Plateau and the arid region, the Qilian Mountains are important ecological barriers and source regions of inland rivers in northwest China. However, the quantification of N deposition in the transition zone between the Tibetan Plateau (TP) and the arid region remains unclear, primarily due to the lack of in situ measurements. Hence, an observational study was conducted on the Qilian Mountains, and precipitation data were collected. Approximately 1382 samples were collected and analyzed regarding their characteristics and environmental effects of the atmospheric N wet deposition. The annual wet deposition of atmospheric N was 10.05 kg/hm², and showed a decreasing trend from the south to the north of the Qilian Mountains. NO₃^--N deposition was the main form of wet deposition of atmospheric N on the Qilian Mountains, accounting for 73.83% of the DIN deposition. Additionally, altitude, meteorological factors, and ecosystem types were found to influence the wet deposition of atmospheric N. The contribution of NO₃^--N to the wet deposition of atmospheric N in meadows, forests, grasslands, farmlands, and towns was 48.38%, 71.55%, 77.54%, 69.61%, and 82.84%, respectively. This study provides a scientific basis for the effective management and sustainable development of environmental protection in the transition zone between the TP and the arid region. The results showed that relevant policies, as well as scientific and governmental measures, could contribute to reducing N deposition effectively. However, the further mitigation measures should be proposed and strictly enforced.

Effects of land use and land cover change on soil organic carbon storage in the Hexi regions, Northwest China

Soil organic carbon (SOC) storage in arid inland regions is significantly affected by land use and land cover change (LUCC) associated with climate change and agricultural activities. A systematic evaluation to the LUCC effects on SOC storage could enable us to better manage soil carbon pools in arid inland regions. Here, we evaluated the effects of LUCC on SOC storage in the Hexi Regions based on high-resolution SOC and LUCC maps derived from Landsat imagery and digital soil mapping using machine learning algorithm and environmental covariates. The results showed that SOC generally increased from northwest to southeast over the Hexi Regions with an average stock of 7.15 kg C m^-2 at a soil depth of 100 cm and a total storage of 2783.05 Tg C. The SOC stock and storage in the Qilian Mountains (mountains) was about 3.90 and 4.55 times higher than that in the Hexi Corridor (plains), respectively. It was estimated that LUCC over the past four decades caused a net increase of 23.41 and 18.19 Tg C in total SOC storage for the Qilian Mountains and Hexi Corridor, respectively. Specifically, the development in grasslands quality as well as the land-use category conversion from the bare land to grassland mainly contributed to the increase in SOC storage of the Qilian Mountains, where the LUCC was mainly driven by climate change. By contrast, the SOC storage change in the Hexi Corridor was mainly associated with the conversion from sandy land and low-cover grassland to cropland as well as sandy land to grassland, being mainly affected by intense cropland expansion and desertification control. Our results highlighted the importance of climate change and cropland expansion in enhancing SOC storage of the Qilian Mountains and Hexi corridor, respectively.

Quantitative Assessment for the Spatiotemporal Changes of Ecosystem Services, Tradeoff–Synergy Relationships and Drivers in the Semi-Arid Regions of China

Ecosystem services in arid inland regions are significantly affected by climate change and land use/land cover change associated with agricultural activity. However, the dynamics and relationships of ecosystem services affected by natural and anthropogenic drivers in inland regions are still less understood. In this study, the spatiotemporal patterns of ecosystem services in the Hexi Region were quantified based on multiple high-resolution datasets, the InVEST model and the Revised Wind Erosion Equation (RWEQ) model. In addition, the trade-offs and synergistic relationships among multiple ecosystem services were also explored by Pearson correlation analysis and bivariate spatial autocorrelation, and redundancy analysis (RDA) was also employed to determine the environmental drivers of these services and interactions. The results showed that most ecosystem services had a similar spatial distribution pattern with an increasing trend from northwest to southeast. Over the past 40 years, ecosystem services in the Hexi Region have improved significantly, with the water retention and soil retention increasing by 87.17 × 108 m3 and 287.84 × 108 t, respectively, and the sand fixation decreasing by 369.17 × 104 t. Among these ecosystem services, strong synergistic relationships were detected, while the trade-offs were found to be weak, and showed significant spatial heterogeneity in the Hexi Region. The spatial synergies and trade-offs in the Qilian Mountains were 1.02 and 1.37 times higher than those in the Hexi Corridor, respectively. Human activities were found to exacerbate the trade-offs between ecosystem services by increasing water consumption in the Hexi Corridor, with the exception of carbon storage. In particular, there were significant tradeoffs between food production and water retention, and between soil retention and habitat quality in the oases of the Hexi Corridor, which is affected by rapid population growth and cropland expansion. Additionally, precipitation, temperature and vegetation cover in the Qilian Mountains have increased significantly over the past four decades, and these increases significantly contributed to the enhancements in water retention, carbon storage, habitat quality, soil retention and food production. Nevertheless, the amount of sand fixation significantly decreased, and this was probably associated with the reduction in wind speed over the past four decades. Our results highlighted the importance of climate wetting and water resource management in the enhancement of ecosystem services and the mitigation of food production trade-offs for arid inland regions.

Interaction of climate change, potentially toxic elements (PTEs), and topography on plant diversity and ecosystem functions in a high-altitude mountainous region of the Tibetan Plateau

Potentially toxic elements (PTEs) generated from mining activities have affected ecological diversity and ecosystem functions around the world. Accurately assessing the long-term effects of PTEs is critical to classifying recoverable areas and proposing management strategies. Mining activities that shape geographical patterns of biodiversity in individual regions are increasingly understood, but the complex interactions on broad scales and in changing environments are still unclear. In this study, we developed a series of empirical models that simulate the changes in biodiversity and ecosystem functions in mine-affected regions along elevation gradients (1500-3600 m a.s.l) in the metal-rich Qilian Mountains (∼800 km) on the northeastern Tibetan Plateau (China). Our results confirmed the crucial role of PTEs dispersal, topography, and climatic heterogeneity in the diversification of plant community composition. On average, 54% of the changes in ecosystem functions were explained by the interactions among topography, climate, and PTEs. However, merely 30% of the changes were correlated with a single driver. The changes in species composition (explained variables = 94.8%) in the PTE-polluted habitats located in the warm and humid low-elevation deserts and grasslands were greater than those occurring in the dry alpine deserts and grasslands. The ecosystem functions (soil characteristics, nutrient migration, and plant biomass) experienced greater changes in the humid low-elevation grasslands and alpine deserts. Our results suggest that the processes driven by climate or other factors can result in high-altitude PTE-affected habitat facing greater threats.

Environmental effect and spatiotemporal pattern of stable isotopes in precipitation on the transition zone between the Tibetan Plateau and arid region

In the transition zone between the Tibetan Plateau and the arid region of northwestern China, the spatiotemporal patterns and environmental controls of stable isotopes in precipitation remain unclear. A network of 19 sampling stations was established across the Qilian Mountains to observe stable isotopes in precipitation, and 1310 precipitation event-scale samples were collected. The local meteoric water line (LMWL) was obtained and expressed as δD = 7.99δ¹⁸O + 14.57 (R² = 0.96). The spatiotemporal patterns of the stable isotopes were mainly dominated by the co-influence of the water vapor sources and the local environment. The westerly circulation, monsoon circulation, and Arctic circulation accounted for 79%, 13%, and 8% of all precipitation events in the study region, respectively. The rainout process also caused oxygen isotope depletion for continuous precipitation events. When the temperature increased by 1 °C, δ¹⁸O increased by 0.47‰, but this increase varied with the temperature range. The effect of precipitation amount was apparent in summer and was caused by sub-cloud evaporation. In addition, δ¹⁸O decreased by 0.13‰ for every 100 m increase in altitude in the Qilian Mountains. Future research should focus on quantifying the co-influence of sub-cloud evaporation, local moisture recycling, and water vapor sources on stable isotopes in precipitation.

Quantitative Analysis of the Sub-Cloud Evaporation of Atmospheric Precipitation and Its Controlling Factors Calculated By D-Excess in an Inland River Basin of China

Atmospheric precipitation is an important part of the water circle in an inland basin. Based on the analytical results of 149 precipitation samples and corresponding surface meteorological data collected at four sampling sites (Lenglong, Ningchang, Huajian and Xiying) at different elevations in the Xiying river basin on the north slope of Qilian Mountains from May to September 2017, the sub-cloud evaporation in precipitation and its controlling factors are analyzed by the Stewart model. The results show that sub-cloud evaporation led to d-excess value in precipitation decrease and d-excess variation from cloud-base to near surface (Δd) increase with decreasing altitude. The remaining evaporation fraction of raindrop (f) decreases with decreasing altitude. The difference of underlying surface led to a difference change of f and Δd in the Xiying sampling site. For every 1% increase in raindrop evaporation, d-excess value in precipitation decreased by about 0.99‰. In an environment of high relative humidity and low temperature, the slope of the linear relationship between f and Δd is less than 0.99. In contrast, in the environment of low relative humidity and high temperature, the slope is higher than 0.99. In this study, set constant raindrop diameter may affect the calculation accuracy. The Stewart model could have different parameter requirements in different study areas. This research is helpful to understand water cycle and land–atmosphere interactions in Qilian Mountains.

Recent Changes in Water Discharge in Snow and Glacier Melt-Dominated Rivers in the Tienshan Mountains, Central Asia

Global warming has generally led to changes in river runoffs fed by snow and glacier meltwater in mountain ranges. The runoff of the Aksu River, which originates in the Southern Tienshan Mountains, exhibited a positive trend during 1979–2002, but this trend reversed during 2002–2015. Through a comprehensive analysis, this study aims to estimate potential reasons for changes in the runoff of its two contrasting headwaters: the Toxkan and Kumalak Rivers, based on climatic data, the altitude of the 0 °C isotherm, glacier mass balance (GMB), snow cover area (SCA), snow depth (SD) and the sensitivity model. For the Toxkan River, the decrease in spring runoff mainly resulted from reductions in precipitation, whereas the decrease in summer runoff was mainly caused by early snowmelt in spring and a much-reduced snow meltwater supply in summer. In addition, the obvious glacier area reduction in the catchment (decreased to less than 4%) also contributed to the reduced summer runoff. For the Kumalak River, a sharp decrease rate of 10.21 × 108 m3/decade in runoff was detected due to summertime cooling of both surface and upper air temperatures. Reduced summer temperatures with a positive trend in precipitation not only inhibited glacier melting but also dropped the 0 °C layer altitude, resulting in a significant increase in summertime SCA and SD, a slowing of the glacier negative mass balance, and a lowering of the snow-line altitude.

The sources of supra-permafrost water and its hydrological effect based on stable isotopes in the third pole region

The sources of supra-permafrost water and its hydrological effects were studied, based on the presence of stable isotopes in 562 samples collected in different ablation periods from the source regions of the Yangtze River. The δ¹⁸O (δD and d-excess) values for the initial ablation, ablation, and end ablation periods were -10.18‰ (-71.39‰ and 10.08‰), -12.14‰ (-85.58‰ and 11.51‰) and -11.50‰ (-78.75‰ and 13.23‰), respectively. The order of the slopes for the supra-permafrost water evaporation lines from the different ablation periods was initial ablation (IA) > ablation (A) > end ablation (EA). An anti-altitude effect is documented here, for a specific altitude range, in what is believed to be the first record of such an occurrence. Outside of that range, clear altitude effects were apparent. We have been able to show that supra-permafrost water was mainly recharged by atmospheric precipitation, ground ice, and glacier and snow meltwater, in the initial ablation and end ablation periods, and contributions from glacier and snow meltwater were mainly concentrated in higher altitude regions. In contrast, in the ablation period, supra-permafrost water was mainly recharged by atmospheric precipitation and ground ice. The contributions of precipitation to supra-permafrost water were 78.79%, 85.47%, and 82.99% in the initial ablation, ablation, and end ablation periods, respectively. The contributions of ground ice to the supra-permafrost water were 14.05%, 14.53%, and 11.94%, respectively, while contributions of glacier and snow meltwater were 7.15% and 5.07% in the initial and end ablation period. For the initial ablation, ablation, and end ablation periods, contributions from atmospheric precipitation to the supra-permafrost water were 85.47%, 86.86%, and 86.84%, while contributions from ground ice were 14.53%, 13.14% and 13.16%, respectively.