Research on Seawater Intrusion Suppression Scheme of Minjiang River Estuary.

Seawater intrusion in the Minjiang River estuary has gravely endangered the water security of the surrounding area in recent years. Previous studies mainly focused on exploring the mechanism of intrusion, but failed to provide a scheme for suppressing seawater intrusion. The three most relevant determinants to chlorine level, which represented the strength of seawater intrusion, were determined using Pearson correlation analysis as being the daily average discharge, daily maximum tidal range, and daily minimum tidal level. Considering the lower requirement of sample data and the ability to handle high-dimensional data, the random forest algorithm was used to construct a seawater intrusion suppression model and was combined with a genetic algorithm. The critical river discharge for suppressing estuary seawater intrusion determined using this model. The critical river discharge was found to gradually increase with the maximum tidal range, which in three different tide scenarios was 487 m3/s, 493 m3/s, and 531 m3/s. The practicable seawater intrusion suppression scheme was built up with three phases to make it easier to regulate upstream reservoirs. In the scheme, the initial reading of river discharge was 490 m3/s, and it rose to 650 m3/s over six days, from four days before the high tide's arrival to two days following it, and before falling down to 490 m3/s at the end. Verified with the 16 seawater intrusion events in the five dry years, this scheme could eliminate 75% of the seawater intrusion risk and effectively reduce the chlorine level for the remaining 25% of events.

NosZI microbial community determined the potential of denitrification and nitrous oxide emission in river sediments of Qinghai-Tibetan Plateau.

Denitrification in river sediments is the hotspot of nitrogen removal and nosZI gene is essential for reducing nitrous oxide (N2O) emissions. However, few studies tried to link nosZI communities with variations of denitrification rates in sediments along the high-elevation rivers. Here, we investigated the spatial variation of potential denitrification rates of sediments along a section (hereafter YJ) of the middle reaches of the Yarlung Zangbo River in the Qinghai-Tibetan Plateau. We also used the real-time quantitative PCR (qPCR) and high-throughput sequencing techniques to evaluate the abundance and composition of nosZI-containing microbial groups. The influences of physicochemical factors and denitrifier communities on potential denitrification rates were further revealed through structural equation modeling. The obtained results indicated that potential denitrification rates and N2O/(N2O + N2) ratio in the sediments along YJ section were greatly different. Moreover, the alpha diversity and composition of nosZI-containing microbial community in river sediments differed remarkably, mainly driven by the ammonia nitrogen (NH4+-N), organic matter (OM) and pH in sediments. The relative abundances of Zoogloeaceae, Oxalobacteraceae, Rhodospirillaceae and Bradyrhizobiaceae significantly differed among five groups (P < 0.05). Structural equation modeling further suggested that nitrogen nutrients directly influenced the potential denitrification rates, while total phosphorus (TP) showed indirect effects on potential denitrification rates through modulating denitrifier abundances and nosZI community. The abundance and composition of nosZI community were powerful predictors in regulating denitrification rates and N2O/(N2O + N2) ratio. Our findings highlight that the nosZI-containing microbial groups play a non-negligible role in nitrogen removal and N2O mitigation in high-elevation river sediments.

A framework to assess the impact of ecological water conveyance on groundwater-dependent terrestrial ecosystems in arid inland river basins.

Overexploitation of water resources at middle reaches has threatened downstream oasis in arid inland river basins. To achieve sustainable development, ecological water conveyance is one effective measure to reallocate water resources between socio-economic and natural systems. A comprehensive impact assessment of ecological water conveyance on groundwater-dependent ecosystems that are common in downstream inland river basins is needed. The present study integrated the technologies of remote sensing analysis, trend detection, and numerical simulation into a technical framework that identifies the spatial response and temporal dynamics of vegetation to groundwater changes induced by ecological water conveyance. The Normalized Difference Vegetation Index (NDVI) was used as a proxy to estimate vegetation. The structure of the framework we used is clear and reasonable. We used remote-sensing data and ground truth information, and a decision tree that incorporates an iterative self-organizing data analysis technique. The decision tree classifies the land cover into affected and unaffected areas, and the results indicate the spatial range of impact. The Mann-Kendall algorithm and Sen's slope detect the tendency in NDVI series that indicates the temporal response of vegetation. The Verhulst logistic function, combined with environment carrying capacity function, constitutes a simplified vegetation dynamic model that can be used to predict potential impact through scenario analysis. An application in the Shiyang River basin in Northwest China evaluated the performance and usefulness of the framework; the accuracy of the results suggested that the framework is effective and practical. Additional case studies are required to assess the reliability and applicability of the framework and identify the factors that affect assessment results beyond our case study in the Shiyang River basin.

Vegetation Response to Groundwater Variation in Arid Environments: Visualization of Research Evolution, Synthesis of Response Types, and Estimation of Groundwater Threshold.

Groundwater depth is an important environmental factor affecting vegetation growth and landscape dynamics in arid environments. This study applied a science mapping approach to visualize the development of groundwater-vegetation-related research, synthesized the vegetation response to changes in groundwater depth, and analyzed the change rate of the response curve to identify the groundwater threshold that is essential to conserve the groundwater-dependent terrestrial ecosystems. These ecosystems emerged as a research hotspot due to climate change, groundwater overexploitation, and the recognition of these ecosystems' importance for sustainable development. There are two main types of response functions of vegetation to changes in groundwater depth-monotone and bell-shaped functions-among which the monotone function includes linear, curvilinear, and stepwise response. The shape of a response curve is mainly determined by the combined effects of oxygen stress, salinization, and water stress; oxygen stress and salinization dominate in shallow groundwater depth, while water stress dominates in deep groundwater depth. On a non-linear vegetation metric-groundwater depth response curve, the change rate analysis method is effective to identify the breakpoint that can be taken as a candidate threshold of groundwater depth. The results will add insight into the intellectual structure of the groundwater-vegetation interactions and provide practical reference for groundwater resource management, ecological conservation, and sustainable development in arid environments.

Environmental Groundwater Depth for Groundwater-Dependent Terrestrial Ecosystems in Arid/Semiarid Regions: A Review.

Groundwater in arid/semiarid regions plays crucial roles in providing drinking water supply, supporting irrigated agriculture, and sustaining important native terrestrial ecosystems. Groundwater depth controls water availability to vegetation and is essential for conserving groundwater-dependent terrestrial ecosystems. Environmental groundwater depth can be defined as a mean depth or a range of depths, satisfying the growth of natural vegetation that is not under stress, either due to lack of water or anoxia or soil salinization. Five methodologies have been reported to estimate environmental groundwater depth: the direct ones rely on response functions that relate vegetation condition, e.g., physiological parameters, appearance frequency, community structure, and remotely sensed physical indexes, to changes in groundwater depth; the indirect one estimates environmental groundwater depth based on the threshold of soil moisture content. To fill a knowledge gap of unique recognized methodology, a conceptual framework was proposed, which involves initial estimation (data collection, response assessment, and estimation) and feedback adjustment (implementation and modification). A key component of the framework is to quantify the linkage between ecological conditions and geohydrological features. This review may provide references for groundwater resources management, ecological conservation, and sustainable development in arid/semiarid regions.

Assessing the Ecological Effects of Water Transport to a Lake in Arid Regions: A Case Study of Qingtu Lake in Shiyang River Basin, Northwest China.

With the continuous growth of economic water consumption in arid regions, many endorheic rivers and terminal lakes have desiccated. As an important ecological engineering measure, water transport in arid regions has vital ecological significance for protecting the regional ecological environment and delaying desertification. In this study, Qingtu Lake, the terminal lake of Shiyang River, was selected to analyze the ecological effects of water transport by means of remote sensing interpretations and current year field investigations. The results demonstrated that, in July 2018, the water surface had formed and recovered to 5.68 km². Additionally, Qingtu Lake formed a spatial gradient distribution in groundwater depth. The depth increased in gradient from the waterside to the desert edge. There was a significant increase in the overall regional vegetation coverage, which mainly occurred in the water areas because of the extensive growth in Phragmitesaustralis, which reached 10.54 km² in area in 2018. Furthermore, the regional vegetation formed a gradient distribution, which transitioned from hygrophytes to xerophytes. This study can provide guidelines for the protection and restoration of lakes in arid regions.

To identify the important soil properties affecting dinoseb adsorption with statistical analysis.

Investigating the influences of soil characteristic factors on dinoseb adsorption parameter with different statistical methods would be valuable to explicitly figure out the extent of these influences. The correlation coefficients and the direct, indirect effects of soil characteristic factors on dinoseb adsorption parameter were analyzed through bivariate correlation analysis, and path analysis. With stepwise regression analysis the factors which had little influence on the adsorption parameter were excluded. Results indicate that pH and CEC had moderate relationship and lower direct effect on dinoseb adsorption parameter due to the multicollinearity with other soil factors, and organic carbon and clay contents were found to be the most significant soil factors which affect the dinoseb adsorption process. A regression is thereby set up to explore the relationship between the dinoseb adsorption parameter and the two soil factors: the soil organic carbon and clay contents. A 92% of the variation of dinoseb sorption coefficient could be attributed to the variation of the soil organic carbon and clay contents.