Determination of the optimal ecological water conveyance volume for vegetation restoration in an arid inland river basin, northwestern China

Do adaptive policy adjustments deliver ecosystem-agriculture-economy co-benefits in land degradation neutrality efforts? Evidence from southeast coast of China

Ecosystem restoration projects (ERPs) facilitate land degradation neutrality (LDN). However, the response dynamics and interactions of sectors within ecosystem-agriculture-economy nexus (EAEN) have not been sufficiently explored, which constrains the coordinated efficacy of LDN efforts. To bridge the knowledge gaps, the present study selected a land restoration hotspot in southeastern China as a case to investigate the simultaneous responses of the EAEN sectors to ERPs from a novel social-ecological system (SES)-based LDN perspective. Various biophysical models and Manne-Kendall trend test as well as multi-source spatially explicit data and socioeconomic statistics were applied to quantify the co-evolution of natural and socioeconomic indicators. ERPs converting cropland to woodland and grassland promoted vegetation restoration, reduced soil erosion, and enhanced carbon sequestration. However, cropland loss initially resulted in a decline in grain productivity. Policy adjustments and improvements in ecosystem restoration efforts and agricultural production conditions improved food security and increased agricultural production capacity. Effective policymaking and favorable resident engagement accelerated the transformation from a grain-production-based agriculture to diversified industries and, by extension, economic output, income, and population. The success of socioeconomic development under the SES framework for LDN demonstrated that this strategy could achieve the desired environmental, agricultural, and economic targets. EAEN under the SES conceptual framework provides an inclusive, comprehensive LDN perspective and improves ERP efficacy. The findings of the present work might be applicable to other land restoration areas challenged by the complex interactions among multidimensional factors. Comparably successful implementation of these ERPs could be realized if individual environmental and socioeconomic conditions are thoroughly considered during the formulation of coordinated development policies.

Improved statistical models for the relationship between riparian vegetation and river flow in arid environments: Implications for flow management

Riparian vegetation (RV) provides critical ecosystem services but has been degraded worldwide due to river flow change. Quantitative relationships between RV and river flow are essential for understanding RV developments and managing flow to conserve RV. Based on the improved statistical model framework that incorporates previous RV conditions into explanatory variables to estimate later RV conditions, this study quantified the RV-flow relationships on the annual scale in the arid Ejina Delta through regression analysis coupled with the normalized difference vegetation index (NDVI) and hydrological data during 2002-2020. The median of NDVIs over the April-October growing season (SMN) was used to indicate annual vegetation conditions, and annual RV cover was derived using a dynamic SMN threshold (0.077-0.084) based on its better vegetation conditions than surrounding deserts. The water year was determined as September-August based on the defoliation time and lag time of the groundwater response to river flow. The results showed that (1) the RV cover approximately expanded from 1619 to 2914 km^2, and the total SMN of RV cover increased from 3711 to 7880; (2) the spatial pattern of SMN declining away from rivers was well described by an exponential function with two physically meaningful parameters (R² = 0.99); (3) the water-year runoff ranged from 4.0 × 10⁸ to 10.6 × 10⁸ m³ with an increasing trend; and (4) the annual RV condition, including both the total SMN and the spatial pattern of SMN, was well estimated by the multiple linear models incorporating a previous RV condition with a coefficient <1 and the subsequent water-year runoffs (R² = 0.98). The results suggest that previous RV conditions are necessary to improve the rationality and performance of RV-flow relationship models, and in arid environments, annual RV conditions depend on the RV's degradation characteristics under zero flow conditions and the ecological benefit by river flow.

Achieving balance between socioeconomic development and ecosystem conservation via policy adjustments in Guangdong Province of southeastern China

Rapid urbanization improves socioeconomic development but challenges ecosystem sustainability. Meanwhile, the gradient responses of ecosystem services (ESs) to landscape structures and associated regime shifts of the agriculture-ecosystem-economy nexus (AEEN) have not been sufficiently addressed, preventing an effective balance between socioeconomic prosperity and ecosystem conservation. To bridge this knowledge gap, this study selected the Guangdong Province of southeastern China to explore landscape dynamics from 1985 to 2020 and their spatially heterogeneous impacts on ESs and the AEEN, based on Integrated Valuation of Ecosystem Services and Trade-offs approach and other biophysical models as well as statistical records about socioeconomic factors. AEEN elements, including ESs, responded directly to policy adjustments in terms of ecosystem restoration and landscape management and presented remarkable regime shifts (i.e., phase changes) and spatial heterogeneity. Aggressive agricultural reclamation before 1999 increased crop productivity but caused vegetation degradation and biomass decline. Accelerated urban expansion and ecosystem restoration efforts have improved economic and ecological benefits but have substantially reduced crop productivity and threatened food security. However, timely policy adjustments since 2009 reversed the declining trend and maintained the grain supply. Landscape composition presented patterns of gradual decline along the urban-rural gradient, which in turn determined ES gradient patterns. For instance, water yield and nitrogen export positively correlated with each other (p < 0.0001) but negatively correlated with other ESs. Our study enriches the understandings of social-ecological systems' response to man-made interventions from AEEN perspective allowing for spatial variabilities and regime shifts, which support policy formulation for coordinating ecological and economic benefits.

The potential for carbon sequestration by afforestation can be limited in dryland river basins under the pressure of high human activity

Dryland regions cover >40 % of the Earth's land surface. Both human activities and climate change have driven forest expansion in parts of dryland regions. Afforestation has been implemented widely to enhance carbon sequestration and benefit the ecological environment of many global drylands. However, the potential and available afforestation space in drylands is uncertain due to the conflicts between additional forest areas and available water. How afforestation will affect the potential for forest carbon stock is also unclear. This paper assessed the future spatial distribution of afforestation and potential forest carbon stock in a typical dryland region, the Yellow River Basin (YRB), which has experienced rapid afforestation and high human activity pressure over the past several decades. Combining the future land use change model (FLUS) and local important development planning, we simulated future afforestation distributions and estimated potential forest carbon stock under the ecological restoration, urban expansion, and cultivated land protection scenarios. The afforestation carbon stock was predicted by considering the dynamic change trends of the mature forest, the immature forest, and new afforestation. The results demonstrated that the potential afforestation area would be limited to 4000 km² in the YRB accounting for less than one-twentieth of the total forest area. Accordingly, the maximum potential forest carbon stock would increase only 59.5 × 10⁶ t. These findings implies that afforestation programs in drylands should further consider the optimum allocation of afforestation space and the balance between carbon and water in drylands, especially under a changing climate with increasing human activities.

Cumulative ecosystem response to Hydraulic Engineering Infrastructure Projects in an arid basin

Hydraulic Engineering Infrastructure Projects (HEIPs) typically show profound effects on hydrological systems and ecosystems. However, data restrictions have limited the exploration of the influences of compound HEIPs on ecosystems to a few studies. This study proposes a watershed-wide ecosystem assessment framework to investigate the impact of HEIPs in the Tarim River Headwaters-Hotan River Basin on the ecosystem of the arid zone. The framework includes a deep learning-meta cellular automata algorithm (DLMCAA) based on the spatiotemporal characteristics of HEIPs and hydro-meteorological and human activities. Moreover, the spatiotemporal relationships between compound HEIPs and ecosystem variances were quantified. The framework including DLMCAA showed a good performance in simulating landcover in 2020, with a Kappa coefficient of 0.89. Therefore, the DLMCAA could be used to simulate and predict ecosystem changes under the HEIPs, which suggested that the framework is effective and practical. An analysis of the spatiotemporal distribution of each ecosystem from 1980 to 2020 showed that the low shrub ecosystems changed most significantly (26.38 %) between 1980 and 2020. Also, the use of spatially driven hydrological project data from different ABC scenarios showed that ecosystems driven by HEIPs were more stable compared to those without HEIPs under future climate change. In particular, the DLMCAA indicated that compound HEIPs had a more positive impact on ecosystem oases in arid lands compared with that of single HEIPs. The results of this study can serve as a scientific reference for assessing the impact of HEIPs, as well as for understanding ecosystem changes and facilitating sustainable water resource management in the arid regions.

Coupling water cycle processes with water demand routes of vegetation using a cascade causal modeling approach in arid inland basins

Vegetation degradation is the key cause of land desertification in arid areas. Water stress is one of the most critical factors leading to vegetation degradation. The water needed for vegetation growth is inseparable from the water cycle processes. It is a new scope to reveal the vegetation water demand mechanisms from the water cycle processes. Water cycle processes in arid inland basins can be conceptually separated as RFA (runoff formation area) and RCA (runoff consumption area). In this study, both the water demand mechanisms of natural vegetation and farmland were discovered by creatively constructing the vegetation water demand route model. The TRB (Tarim River Basin), a typical arid inland basin system that RFA is separated from RCA, is considered as the study area. The tendency and relevance of water cycle factors and NDVI were detected. The dominant factors of vegetation growth were identified. According to the interaction causality of water cycle factors and vegetation, the PLS-SEM (partial least squares structural equation models) were constructed in RFA and RCA. Results displayed that SMroot (root-zone soil moisture), groundwater and precipitation were the dominant water sources for natural vegetation in RFA. The water demand for natural vegetation in RCA mainly came from SMroot and that for farmland mainly came from SMsurf (surface soil moisture). New findings showed that blue and green water circulations were more active in RFA than in RCA. Natural vegetation had better adaptability and resilience to water shortages compared with farmland. The higher effect of vegetation on AET (actual evapotranspiration) denoted the better growth status. It is contributed to the rational utilization of water resources in arid basins.

A novel multi-model fusion framework diagnoses the complex variation characteristics of ecological indicators and quantitatively reveals their driving mechanism

Systematic analysis of the change law and driving mechanism of ecological indicators (GPP, ET, WUE), as well as the study of maximum threshold of water resources benefit changing with ecological benefit, are important prerequisites for realizing the scientific allocation and efficient utilization of water resources in desert riparian forests. However, previous studies have defects in the detailed description of the change characteristics of ecological indicators. How to accurately diagnose the characteristics of a site, mutation year, pattern (linear, exponential, logarithmic, etc.), duration of change, future change trends of ecological indicators in a desert riparian environment, as well as quantitatively revealing their driving mechanisms, are major scientific problems that need to be solved urgently. In this regard, an ensemble function coupling a logistic function and an asymmetric Gaussian function was creatively adopted, a novel framework was created to integrate the time-series trajectory fitting method and the sensitivity analysis method, and the arid and ecologically fragile Tarim River Basin was taken as a typical area. The results showed that with enhanced water resource management in the Tarim River Basin, GPP, ET, and WUE all showed patterns of increasing change and could be expected to continue to rise or to remain at a high-level stable state. The longest continuous period of GPP change was 15 years, showing that ecological restoration is a long-term process. The years of GPP mutation were consistent with the implementation periods of major measures in the Tarim River Basin (1990, 2001, and 2011), indicating the reliability of this framework. More importantly, when GPP increased to 216.44 gCm^-2, the maximum WUE threshold of 0.93 gCm^-2mm^-1 occurred. This threshold can be used as a reference criterion for efficient utilization of ecological water in the basin. Among the ecological indicators studied, GPP was the most sensitive to environmental change, but GPP, with 80.60% of pixel area, showed a weak memory effect（α < 0.4). Besides, GPP was the most sensitive to the leaf area index (LAI) and had the strongest correlation with it (p < 0.001). Therefore, LAI can be used as the main control factor for judging plant growth. This research can provide important scientific guidance and reference for the analysis of ecological indicator changes and the sustainable utilization of water resources in arid areas.

Evolution and Climate Drivers of NDVI of Natural Vegetation during the Growing Season in the Arid Region of Northwest China

Vegetation plays an important role in linking water, atmosphere, and soil. The dynamic change in vegetation is an important indicator for the regulation of the terrestrial carbon balance and climate change. This study applied trend analysis, detrended correlation analysis, and the Hierarchical Partitioning Algorithm (HPA) to GIMMS NDVI3g data, meteorological data, and natural vegetation types for the period 1983 to 2015 to analyze the temporal and spatial changes in NDVI during the growing season and its driving factors in the arid region of northwestern China. The results showed that: (1) the growing season length (GSL) was delayed, with a regional trend of 8 d/33 a, due to a significant advancement in the start of the growing season (SOS, −7 d/33 a) and an insignificant delay to the end of growing season (EOS, 2 d/33 a). (2) The regional change in NDVI was mainly driven by temperature and precipitation, contributing to variations in NDVI of forest of 36% and 15%, respectively, and in the NDVI of grassland, of 35% and 21%, respectively. In particular, changes to forested land and medium-coverage grassland (Mgra) were closely related to temperature and precipitation, respectively. (3) The spatial distribution of the mean NDVI of forest was closely related with precipitation, temperature, and solar radiation, with these meteorological variables explaining 20%, 15%, and 10% of the variation in NDVI, respectively. Precipitation and solar radiation explained 29% and 17% of the variation in the NDVI of grassland, respectively. The study reveals the spatial–temporal evolution and driving mechanism of the NDVI of natural vegetation in the arid region of Northwest China, which can provide theoretical and data support for regional vegetation restoration and conservation.

Study on Index of Groundwater Ecological Function Crisis Classification and Early Warning in Northwest China

The natural oases in the plain area of the northwest inland basin strongly depend on the groundwater depth. With the overexploitation and utilization of groundwater, natural oases are faced with the problems of serious degradation and rehabilitation. How to evaluate the degree of the degeneration crisis of groundwater ecological function has become one of the key scientific and technological problems to be solved. In this paper, the Shiyang River basin of Gansu Province was selected as a typical research area. The remote sensing interpretation, groundwater–soil ecology comprehensive investigation, and groundwater in situ monitoring were adopted to carry out the research. Based on the correlation analysis method of natural ecology and groundwater, the interactive relationship between the natural ecological environment and groundwater depth in different ecological types of the region were studied: (1) under the arid climate condition in northwest China, the relationships between the ecological situation and the groundwater depth in different ecological types of the region were obviously different, and as a result, the optimal or limit ecological water level of groundwater in different ecological types was also different; (2) in the natural wetland area, the suitable ecological water level of groundwater was between 0.5 m to 1.5 m, and the limit ecological water level was 8.0 m; in the natural vegetation area, the suitable ecological water level was between 3.0 m to 5.0 m, and the limit ecological water level was 10.0 m; and in the farmland area, the suitable ecological water level was between 2.0 m to 5.0 m, and the limit ecological water level was 2.0 m; (3) in order to effectively protect the natural ecology in different ecological types, a five-level early warning and control index system should be established for the ecological function degeneration crisis of groundwater. It may be beneficial to promote restoration and protection of the groundwater ecological function and natural ecology in the inland area of northwest China.

Groundwater Dynamic Characteristics with the Ecological Threshold in the Northwest China Oasis

Suitable groundwater level is an important foundation for the stability of the ecological environment, and the healthy development of the social economy, in the arid area of Northwest China. The Manas River Basin is a typical oasis in an arid area, where the problems of salinization and desertification are prominent. By analyzing the variation characteristics of groundwater in the study area from 2013 to 2019 combined with remote sensing technology—according to the theory of capillary water rise and phreatic evaporation—a mathematical calculation model of the ecological threshold is established to determine the ecological groundwater level. The results show that (1) the groundwater level in the study area fluctuates by 0.2–18 m throughout the year, and the variation of groundwater drawdown is 5–35 m from 2013 to 2019; (2) the upper threshold of the ecological groundwater level is 0.82–4.05 m and the lower threshold is 3.35–10.23 m; (3) the ecological water shortage area in the study area is 9755.36 km2, and the groundwater ecological deficit is 105.741 × 108 m3. This study can provide a theoretical basis for the determination of the ecological groundwater level, the optimal allocation of water resources, and ecological environment management in the arid area of Northwest China.

Application of Ecological Restoration Technologies for the Improvement of Biodiversity and Ecosystem in the River

With global warming, urbanization, and the intensification of human activities, great pressures on river ecosystems have caused ecosystem degradation, the decline in habitats and biodiversity, and the loss of function. Ecological restoration technologies (ERTs) in rivers are effective measures for improving habitat and biodiversity, which has the advantage of recovering ecosystems and biodiversity and promoting the formation of healthy rivers. Several applications of ERTs, including ecological water transfer, fish passage construction, dam removal/retrofit, channel reconfiguration, river geomorphological restoration, natural shoreline restoration, floodplain reconnection, revegetation, etc., are summarized. The classifications of ERTs are highlighted, aiming to distinguish the difference and relationship between structure and the processes of hydrology, physics, geography, and biology. The pros and cons of these technologies are discussed to identify the applicability and limitations on the river ecosystem. In the dynamic processes in the river, these interact with each other to keep ecosystem balance. ERTs are more helpful in promoting the restoration of the natural function of the river, which contribute to the management of river ecological health. Some proposals on river management are suggested. Establishing a unified river health evaluation system will help promote positive feedback on rivers and the further development of ERTs.

Research on Vegetation Coverage Dynamics and Prediction in the Taitema Lake Region

The Tarim River is the largest inland river in China, which plays a crucial role in maintaining regional ecological security and carbon cycle/dynamic. However, the “green corridor” in the Taitema Lake region at the lower reaches of the Tarim River has unclear environmental changes and future dynamics due to the influence of the ecological water conveyance. Hence, protecting the “green corridor” at the lower reaches of the Tarim River in China is strategically important not only ecologically but also socially and economically. In this paper, the temporal and spatial features of the fractional vegetation coverage (FVC) dynamics in the Taitema Lake region at the lower reaches of the Tarim River in 2000–2018 are analyzed and calculated using Landsat TM/OLI remote sensing images and MODIS data products. Additionally, the future trend of FVC dynamics in the study region are predicted using trend analysis and the pixel-based Hurst index. The results show that FVC in the Taitema Lake region exhibit a positive development after the implementation of ecological water conveyance. Specifically, from 2000 to 2018, the areas of low, medium, and high FVC expanded from 1.28 km2 to 179.87 km2, resulting in an increase of 140.52%. Spatially, the regions around the lake entrance channel of the Tarim River saw a significant increase in FVC of 9.71%. The middle part of the study region, accounting for only 1.96% of the area, displayed relatively high and high fluctuations in FVC. In the future, the regions at the middle part of the lake and around the lake entrance channel of the Tarim River, accounting for 11.33% of the area, will likely show an increasing trend in FVC. The regions with either extremely low or low FVC are predicted to decrease to 14.16% of the overall area. Because the positive effects of ecological water conveyance were more significant on FVC in the study region than the influences of either temperature or precipitation, ecological water conveyance should remain the primary means of ecological restoration for Taitema Lake.

Influence of Multi-Layered Structure of Vadose Zone on Ecological Effect of Groundwater in Arid Area: A Case Study of Shiyang River Basin, Northwest China

The natural vegetation in arid areas of northwest China is strongly dependent on the availability of groundwater. Significantly, capillary water plays an essential role in regulating the ecological groundwater level in the multilayered structure of the vadose zone. The soil-column test and field survey in the lower reaches of the Shiyang River Basin were conducted to investigate the influence of the multi-layered structure of the vadose zone on maintaining the ecological effect of groundwater. Based on the field survey, the results show that the depth of groundwater is 3.0 m, and the rising height of capillary water is 140 cm. In the soil-column test, the height of the wetting front of the column was 125 cm. During the water releasing test, the water held by the vadose zone was 182.54 mm, which would have maintained Haloxylon’s survival in a growing season. Therefore, the multi-layered structure of the vadose zone extends the ecological groundwater depth and consequently enhances the ecological function of groundwater. Importantly, with a lower groundwater level, the clay soil layer within the rising height range of the original capillary water would hold more water and maintain a higher water content for a certain period to supply surface vegetation.