Ecohydrological indicators and environmental flow assessment in the middle and lower reaches of the Huai River, China.

The vitality of river ecosystems is vital for the sustainable development of river basins, with the assessment of environmental flow (EF) playing a pivotal role in eco-informatics. This study delves into the middle and lower reaches (MLR) of the Huai River basin (HRB) in China, utilizing hydrological data spanning from 1950 to 2020. Its principal objective lies in the selection of ecohydrological indicators to refine the estimation of EF in the HRB. Employing principal component analysis (PCA), ecologically relevant hydrological indicators (ERHIs) were discerned and scrutinized for their hydrological characteristics. The analysis extended to evaluating hydrological shifts at different stations using ERHIs, determining suitable EF in the MLR, and delineating the trajectories of appropriate intra-annual flows in different hydrological years through HEC-RPT. Based on a variety of mutation test methods, the change point of runoff sequence was determined in 1991. The PCA analysis identified eight ERHIs, reflecting hydrological changes of 49.79 % and 56.26 % at Bengbu and Sanhezha, respectively, which indicate a moderate alteration. Based on ERHIs, the other stations in the HRB exhibited hydrological alterations ranging from 33 % to 47 %, notably highlighting substantial changes in maximal 30d flow and flow fall rate. The optimal flood pulse discharge in the middle reaches is 4150 m3/s, 3140 m3/s and 2150 m3/s in wet, dry and dry years, respectively. Downstream, flood pulse flow in wet, normal and dry years should exceed 4070 m3/s, 3110 m3/s and 1980 m3/s, respectively. The research contributes significantly to the management of rivers and the sustainable conservation of the ecological milieu.

Dynamic process of ecosystem water use efficiency and response to drought in the Yellow River Basin, China.

Ecosystem water use efficiency (WUE) is a crucial indicator of the impact of climate change on terrestrial ecosystems, reflecting the balance between biological processes (photosynthesis and transpiration) and physical processes (evapotranspiration). However, the response mechanisms and driving processes of WUE to drought remain to be further understood. In this study, we analyzed the spatial and temporal dynamics and response mechanisms of WUE in the Yellow River Basin (YRB) using data on Gross Primary Productivity (GPP), Evapotranspiration (ET) and Standardized Precipitation Evapotranspiration Index (SPEI), which revealed the cumulative effect of drought on WUE and assessed the ecosystem's resilience. The study results showed that (1) GPP, ET and WUE in the YRB exhibited a significant increasing trend, with 63.04 % of the area showing a marked increase in WUE. (2) GPP was the dominant factor influencing WUE in 65.36 % of the area, particularly in cropland and grassland, while ET was more influential in forested areas. Vapor pressure deficit (VPD) was identified as the principal driver affecting vegetation GPP in semi-arid and semi-humid regions of the YRB. In contrast, soil moisture (SM) was the limiting factor in arid areas. (3) 71.00 % of the WUE in the basin was affected by drought cumulative effects, with an average cumulative duration of 4.5 months. Arid regions experienced the most extended duration of 7.29 months, compared to 3.05 months in semi-humid regions. (4) 74.85 % of the regional ecosystems exhibited ecological resilience to drought, particularly in the source areas of the western basin of the YRB. Shrublands have the highest drought resilience among vegetation types, while grasslands have the lowest. The resilience of each climatic zone was in the order of semi-humid, semi-arid, and arid order. This study comprehensively analyzed of the spatial and temporal dynamics and response mechanisms of WUE in the YRB, offering a new perspective and scientific basis for understanding and predicting the ecosystem response to climate change.

Asymmetric response of vegetation GPP to impervious surface expansion: Case studies in the Yellow and Yangtze River Basins.

Terrestrial gross primary production (GPP) changes due to impervious surfaces significantly impact ecosystem services in watersheds. Understanding the asymmetric response of vegetation GPP to impervious surface expansion is essential for regional development planning and ecosystem management. However, the asymmetric response of vegetation GPP to the impacts of impervious surface expansion is unknown in different watersheds. This paper selected the Yellow River and Yangtze River basins as case studies. We characterized the overall change in GPP based on changes in impervious surface ratio (ISR), determined impervious surface expansion's direct and indirect impacts on GPP in the two watersheds, and further analyzed the asymmetric response of the compensatory effects of indirect influences on the impervious surface expansion in different watersheds. The results showed that: (1) The vegetation GPP decreased with increasing ISR in the Yangtze River Basin, while that in the Yellow River Basin first increased and then reduced. (2) The direct impacts of increased ISR reduced vegetation GPP, while the indirect impacts both had a growth-compensating effect. Growth compensation stabilized at approximately 0.40 and 0.30 in the Yellow and Yangtze River Basins. (3) When the ISR was 0.34-0.56, the growth compensation could offset the reduction of GPP due to direct impact and ensure that the background vegetation GPP was not damaged in the Yellow River Basin. In contrast, the background vegetation GPP was inevitably impaired with increased ISR in the Yangtze River Basin. Therefore, this study suggests that the ISR should be ensured to be between 0.34 and 0.56 to maximize the impervious surface of the Yellow River Basin without compromising the background vegetation GPP. While pursuing impervious surface expansion in the Yangtze River Basin, other programs should be sought to compensate for the loss to GPP.

Patterns of salt transport and factors affecting typical shrub in desert-oases transition areas.

Soil salinization and water deficits are considered the primary factors limiting economic development and environmental improvement in arid areas. However, there remains limited knowledge of the adaptability of typical shrubs to salinization of desert areas in arid zones. This study was conducted in a desert oasis transition zone (Tarim River, China), aiming to investigate: i) the spatial-temporal changes in soil salinity; ii) the interactions between the pedoenvironment vs typical shrub (Calligonum mongolicum). The van Genuchten soil salinity retention ensemble model (TVGSSREM-3D) was developed to simulate variations in soil water-salt transport in the desert-oasis zone and to accurately explain the main factors influencing Calligonum mongolicum desert-oases transition areas. The results showed that monthly average salinity ranged from 2.0 to 8.0 g kg-1, with a peak in August (9.17 g kg-1). The presence of human activities (Salt Drainage Canal) and the distribution of Calligonum mongolicum resulted in a clear spatial salinity zonation. Moreover, analysis of environmental indicators using the TVGSSREM-3D model revealed strong correlations between the distribution of salinity in Calligonum mongolicum desert-oases transition areas and groundwater depth (GD), minimum relative humidity (MRH), and water vapor pressure (WVP). These findings provide a scientific basis for stabilizing, restoring, and reconstructing the ecosystem of the oasis-desert transition zone.

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.

Attribution Analysis of Runoff Change in Min-Tuo River Basin based on SWAT model simulations, China.

To consummate watershed data and better quantify the impact of climate changes and human activities on runoff, we examined the changes and response mechanisms of runoff in the Min-Tuo River Basin, China. In the examination, the Soil and Water Assessment Tool (SWAT) model was used to simulate possible evapotranspiration, actual evapotranspiration, and runoff in 1980, 1990, 1995, 2000, 2005, 2010, and 2015 under different land-use conditions. SWAT weather generator was used to supplement the missing meteorological data. This study presents a quantitative analysis of the climatic and anthropogenic factors contributing to the runoff alteration in the Min-Tuo River Basin using the Budyko methods. The results suggested that the reduced precipitation was the main cause of runoff reduction. The contributions of precipitation, possible evapotranspiration, and underlying surface alterationsof runoff were 56.18%, 37.08%, and 6.74%, respectively. Sensitivity analysis indicated that the runoff alteration was most sensitive to changes of landscape parameters. The aridity index and all the elasticities showed a spatial variations in the Min-Tuo River Basin. The influence of the three factors on runoff reduction varied with seasons. During the high-flow period, changes of the precipitation and possible evapotranspiration and underlying surface had the greatest effect on runoff reduction, while changes of underlying surfaces had the least effect.

Numerical assessment of the effect of water-saving irrigation on the water cycle at the Manas River Basin oasis, China.

Mulch drip irrigation is widely used in the arid areas of Northwest China. Consequently, the Manas River Basin has developed into the fourth largest irrigated agricultural area in China. In this study, a groundwater model of the regional water cycle was developed to quantitatively assess the groundwater balance in response to different irrigation schemes, including traditional irrigation, conventional water-saving irrigation, and high-efficiency water-saving irrigation schemes. Our results reveal that 1) The water-saving irrigation technology has affected the water cycle process in farmlands. The higher the degree of water conservation, the lower the infiltration into groundwater, the higher the deficit of the groundwater balance, and the more significant the decline of the groundwater level. 2) The groundwater at the Manas River Basin remains in a negative equilibrium state. To achieve an equilibrium state of the groundwater at the Manas River Basin, the catchment management agencies should restrict the scale of oasis development and the utilization of groundwater.

Assessment of changes in oasis scale and water management in the arid Manas River Basin, north western China.

Modern water-saving irrigation technology has expanded the scale of agricultural oases in arid and semi-arid regions of China. In this study, we used Landsat MSS and Landsat TM/ETM remote sensing data to assess changes in oasis scale and water availability with reference to differing water management practices in the Manas River Basin of north-western China from 1975 to 2015. We used the water-heat balance index H0 to determine oasis stability over time and constructed a suitable-scale calculation model for arid and semi-arid regions to assess the suitable development scale and cultivated land area in the study area. The implementation of water-saving technology in 2000 effectively improved the utilization efficiency of water resources and accelerated the formation of artificial oases; these expanded by 3873.3 km2 while natural oasis area was reduced by 3485.0 km2. The oasis stability index H0 was less than the critical stability index of 0.5 throughout the study period, implying that these areas were in a metastable state and unsuitable for further development. Therefore, in order to improve oasis stability, both scale and agricultural area should be further controlled. At present, actual oasis scale exceeds appropriate scale by 1.1 times and agricultural area exceeds suitable area by 2.5 times. To ensure the stability of the oasis, its area should be maintained at 3942.28-4481.06 km2 and the cultivated land should be maintained at 1576.91-1792.42 km2.

Spatiotemporal analysis of ecological vulnerability and management in the Tarim River Basin, China.

The Tarim River Basin (TRB) is an extremely arid area in China, suffering from dry climate and intense human activities, which have brought about significant changes in ecological processes and then, led to serious ecological vulnerability (EV). This study proposes an assessment framework to evaluate EV and analyze its dynamic change in the TRB during 2005-2015. An integrated method is developed with the Fuzzy Analytic Hierarchy Process (FAHP) and the Pressure-State-Response (PSR) framework, which highlights impacts of nature and anthropogenic interference on the ecology. Specific management strategies are put forward based on the spatial recognition of ecologically vulnerable areas in the TRB. The EV is divided into four vulnerability levels including Light I, Medium II, Heavy III and Very heavy IV. Results show that the average EV is at Heavy III vulnerability level in the TRB in the last 2005-2015, and there has been an increasing trend in EV, which even has come up to the Very heavy IV vulnerability level in the year 2013-2015. As a whole, the EV displays a high-to-low gradient from east to west during the study period. Heavy III and Very heavy IV vulnerability levels, distributed in the East, mainly in the mainstream areas with characterization of frequent human interferences, tend to increase persistently. In contrast, Light I vulnerability level, mainly in the west source areas, shows a significant decline after 2010. Based on the results, some suggestions targeted at different vulnerable areas were proposed to help restore ecological environments by integrating legal managements with public efforts. The proposed methodology, reflecting the nature and human interaction on the EV is of practical use for the ecological restorations in the TRB.

Dynamic projection of ecological risk in the Manas River basin based on terrain gradients.

With large-scale developments, the Manas River Basin (MRB) is in an extreme imbalance especially in land use, thus causing a series of ecological problems. A reliable dynamic ecological risk assessment is expected to provide useful information for the economic development. Through coupling spatial Cellular Automaton-Markov (CA-Markov) model and Landsat satellite images in 2000, 2008 and 2016, we forecasted the land use maps in 2024 and 2032. Based on the ecological risk model, we evaluated the ecological risk at landscape level from 2000 to 2032. More importantly, an improved evaluation of ecological risk was proposed based on terrain gradients and the correlation between terrain niche index (TNI) and future ecological risk was analyzed. The results showed that the artificial oases and urban are expanding, while the natural grassland is shrinking. Corresponding to the rapid development stage and stable consolidation stage, farmland will be followed by a slower increase (2016-2032) after a rapid increase (2000-2016), and water decreases first but then is projected to recover. As the overall spatial diversity increasing, the ecological risk in the whole basin is growing, especially in grassland. Compared with the stable critical state in artificial landscape, the future ecological risks in natural landscape tend to increase due to the cumulative effects of human activities. Also, we found that the great ecological risk mainly happens in "high altitude and complex terrain" or "low altitude and flat terrain" areas. The future ecological risk in medium terrain niche index (TNI) gradient will increase, while it will decrease in the lowest. Above all, the proposed framework can do well in forecasting ecological risk at landscape level, and can help simply infer the changes of ecological risk based on terrain.

Hydrological simulation and uncertainty analysis using the improved TOPMODEL in the arid Manas River basin, China.

Understanding the mechanism of complicated hydrological processes is important for sustainable management of water resources in an arid area. This paper carried out the simulations of water movement for the Manas River Basin (MRB) using the improved semi-distributed Topographic hydrologic model (TOPMODEL) with a snowmelt model and topographic index algorithm. A new algorithm is proposed to calculate the curve of topographic index using internal tangent circle on a conical surface. Based on the traditional model, the improved indicator of temperature considered solar radiation is used to calculate the amount of snowmelt. The uncertainty of parameters for the TOPMODEL model was analyzed using the generalized likelihood uncertainty estimation (GLUE) method. The proposed model shows that the distribution of the topographic index is concentrated in high mountains, and the accuracy of runoff simulation has certain enhancement by considering radiation. Our results revealed that the performance of the improved TOPMODEL is acceptable and comparable to runoff simulation in the MRB. The uncertainty of the simulations resulted from the parameters and structures of model, climatic and anthropogenic factors. This study is expected to serve as a valuable complement for widely application of TOPMODEL and identify the mechanism of hydrological processes in arid area.

Change in Land Use and Evapotranspiration in the Manas River Basin, China with Long-term Water-saving Measures.

Widespread application of water-saving measures, especially advanced drip irrigation technologies, may significantly impact on the land use, and further potentially alter regional ecological environments in an arid area. In this study, the remote sensing and geographic information system technology were used to analyze the LANDSAT images (1976-2015) and the MOD16 evapotranspiration data (2000-2015) in the Manas River Basin (MRB), China where the water-saving technologies have experienced the past 40 years. Our results show that the area of the cultivated land was approximately doubled from 1976 to 2015 with a dynamic degree of cultivated land ranging from 1.7% to 4%. The reclamation rates were estimated at 9.5% in 1976 and 21.8% in 2015 and the comprehensive index of land use degree shows an increasing trend in the MRB. The evapotranspiration in the MRB suggests that the cultivated land is becoming more humid while the other regions are becoming more arid. Long-term change in the land use is mainly promoted due to the multiple years' efforts on development of the water-saving technologies. This study greatly improves our understanding of the interactions between change in ecological environments and human activities and may provide policy makers guidance of sustainable development at an arid area.

Study on the characteristics of future precipitation in response to external changes over arid and humid basins.

The simulation abilities of the Coupled Model Inter-comparison Project Phase 5 (CMIP5) models to the arid basin (the Tarim River Basin, TRB) and humid basin (the Yangtze River Basin, YRB) were evaluated, determining the response of precipitation to external changes over typical basins. Our study shows that the future temporal and spatial variation characteristics of precipitation are different in different regions with the CMIP5. The annual and seasonal changes in precipitation were analyzed for the RCP2.6, RCP4.5 and RCP8.5 during 2021~2100 compared to those during 1961~2005. Precipitation shows an increasing trend in the TRB, but which decreases and then increases in the YRB, with a turning point in the middle of twenty-first Century. The ranges in annual precipitation increase with the increase in the scenario emissions in the future. Note that the Tarim River Basin is more vulnerable to the impact of emissions, especially for annual or spring and winter precipitation. Based on the uncertainty of CMIP5 data, the links between future precipitation changes and the elevation and relief amplitude were evaluated. The change of precipitation decreases with elevation, relief amplitude in the TRB, while it increases with elevation but decreases with relief amplitude in the YRB.

Identification of potential impacts of climate change and anthropogenic activities on streamflow alterations in the Tarim River Basin, China.

Understanding contributions of climate change and human activities to changes in streamflow is important for sustainable management of water resources in an arid area. This study presents quantitative analysis of climatic and anthropogenic factors to streamflow alteration in the Tarim River Basin (TRB) using the double mass curve method (DMC) and the Budyko methods. The time series (1960~2015) are divided into three periods: the prior impacted period (1960~1972) and the two post impacted periods, 1973~1986 and 1987~2015 with trend analysis. Our results suggest that human activities played a dominant role in deduction in the streamflow in TRB with contribution of 144.6% to 120.68% during the post impacted period I and 228.68% to 140.38% during the post impacted period II. Climatic variables accounted for 20.68%~44.6% of the decrease during the post impacted period I and 40.38% ~128.68% during the post impacted period II. Sensitivity analysis indicates that the streamflow alteration was most sensitive to changes in landscape parameters. The aridity index and all the elasticities showed an obvious increasing trend from the upstream to the downstream in the TRB. Our study suggests that it is important to take effective measures for sustainable development of eco-hydrological and socio-economic systems in the TRB.

Quantitative Assessment of Hydrological Alteration Caused by Irrigation Projects in the Tarim River basin, China.

The Tarim River is the longest inland river at an arid area in China. Deterioration in its ecohydrological system has received much attention world widely. This study presents quantitative assessment of hydrological alterations in the hydrological regime of the Tarim River caused by reservoir irrigation and channel irrigation over a period of over a half century. The improved indicators of hydrologic alteration and range of variability approach were applied to the daily flow rates at the two representative hydrological stations. Our study shows that the annual extreme water conditions (1-, 3-, 7-day annual minimum and extreme low timing) have been altered, compared with the pre-impact period. The average flow rate in July, the 30-day annual maximum flow rates, the date for the maximum rate, the rise rate, and the fall rate show a significant decreasing trend. The improved overall degree of hydrological alteration for the two stations are approximately 68.7% and 61.8%, suggesting a high degree of alteration. This study greatly improved our understanding of impacts of irrigations on the ecohydrological characteristics in the Tarim River.

Nutrient exchange and release experiment and its simulation study in lake water-sediment interface.

The sediment distributed and insolated under lake was collected for experiments. The nutrient layer distribution conditions of sampled sediment and its physical and chemical characteristics were analyzed to simulate and assess the influence degree to lake water quality. Based on the dynamic water exchanging experiments the nutrient release process in sediment and influence mechanism to substance exchanging on water-sediment interface was studied, and the correlation between the changing content of total phosphors and total nitrogen in sediment and covered water were analyzed for setting up a simulation model. At the same time the influence degree is explained in detail. The experimental results indicated that even if clean water without nutrient contents was used for water exchangement so as to decrease pollution or prevent eutrophication, however owing to the vertical nutrient distribution in lake sediment, it will lead to the increasing release amount greatly especially when the organic nutrient contained in sediment turns into inorganic status because of isolation. Besides the release process of total phosphate (TP) and total nitrogen (TN) were modeled and each nutrient's exchanging equation at interface caused by covered water nutrient concentration changing was set up. According to the simulating prediction, TP and TN content of cover water will also sustain a steady higher level in a long period. The nutrient release amount of sediment is not only affected by the covered water concentration but also connects with accumulative time. The experiments provide the fundamental theoretical and practical basis for taking ecological restoration project. And research is helpful to prevent or restore lake eutrophication.