Evaluation of the Impact of Ecological Water Supplement on Groundwater Restoration Based on Numerical Simulation: A Case Study in the Section of Yongding River, Beijing Plain

Ecological water supplement relies on river channels to introduce surface water, to make a reasonable supplement of groundwater, to repair the regional groundwater environment and urban river ecosystem. Evaluating the degree of groundwater restoration after ecological water supplement (by taking appropriate measures) is a critical problem that needs to be solved. Thus, based on the Yongding River ecological water supplement in 2019 and 2020, we analyzed the groundwater monitoring situates in the ecological water supplement region. We established an unstructured groundwater flow numerical model in the study area through the quadtree grids. The model was calibrated with the measured water level. The simulated results could accurately reflect the real groundwater dynamic characteristics, and it showed that the water level rise was concentrated in the 3–6 km range of the Yongding River after the ecological water supplement. In 2019, the calculated ecological water infiltration amount was 101.28 × 106 m3, the affected area was 265.19 km2, and the average groundwater level rise in the affected area was 2.10 m. In 2020, the calculated ecological water infiltration amount was 102.64 × 106 m3, the affected area was 506.88 km2, and the average groundwater level rise in the affected area was 1.25 m. While the ecological water supplement had a positive impact on groundwater level restoration, the groundwater level around the typical buildings within the study area, including Beijing West Railway Station and Beijing Daxing International Airport, would not be significantly affected.

A coupled dynamic leakage loss and flood routing model for ephemeral rivers with complex sections

Ephemeral rivers commonly occur in regions with a shortage of water resources, and their channel configuration tends to change substantially owing to cultivation, tree planting and sand extraction. There is an urgent need to restore degraded river ecosystems. During short-term water conveyance, water storage in sand pits and leakage in dry riverbeds retards the flow of water, which is detrimental for ecological restoration of the riparian zone. A coupled dynamic leakage loss and flood routing model was established to predict the flow processes in the complex river channel of the Yongding River in China. The model mainly included three sub-models of flow dynamics, dynamic leakage loss, and water balance along multiple cross sections of the river channel. The complex section is reflected in the different infiltration properties for each section, and the existence of sand pits. The water head was dominated by flow velocity and the overflow from sand pits. Owing to the difference in landforms and the deposited sediment size of the riverbed bottom, the river channel was divided into 11 cross sections and a sand pit to ascertain the respective infiltration or leakage loss processes. The input parameters of the model came from field surveys of sand pits, river geometry and hydrogeology. The model was also calibrated and validated using monitoring data from ecological water releases into the Yongding River in 2019 and 2020. This coupled model can predict the water leakage loss and flow process of the water head and also provide important guidance for river reconstruction and ecological restoration.

Simulation of Vegetation Cover Based on the Theory of Ecohydrological Optimality in the Yongding River Watershed, China

During ecological restoration, it is necessary to comprehensively consider the state of vegetation in climate–soil–vegetation systems. The theory of ecohydrological optimality assumes that this state tends to reach long-term dynamic equilibrium between the available water supply of the system and the water demand of vegetation, which is driven by the maximization of productivity. This study aimed to understand the factors that affect the spatial distribution of vegetation and simulate the ideal vegetation coverage (M0) that a specific climate and soil can maintain under an equilibrium state. The ecohydrological optimality model was applied based on meteorological, soil, and vegetation data during the 2000–2018 growing seasons, and the sensitivity of the simulated results to input data under distinct vegetation and soil conditions was also considered in the Yongding River watershed, China. The results revealed that the average observed vegetation coverage (M) was affected by precipitation characteristic factors, followed by wind speed and relative humidity. The M, as a whole, exhibited horizontal zonal changes from a spatial perspective, with an average value of 0.502, whereas the average M0 was 0.475. The ecohydrological optimality theory ignores the drought resistance measures evolved by vegetation in high vegetation coverage areas and is applicable to simulate the long-term average vegetation coverage that minimizes water stress and maximizes productivity. The differences between M and M0 increased from the northwest to the southeast of this area, with a maximum value exceeding 0.3. Meteorological factors were the most sensitive factors of this model, and the M0 of the steppe was most sensitive to the stem fraction, mean storm depth, and air temperature. Whether soil factors are sensitive depends on soil texture. Overall, the study of the carrying capacity of vegetation in the natural environment contributes to providing new insights into vegetation restoration and the conservation of water resources.

Analysis on the Variation of Hydro-Meteorological Variables in the Yongding River Mountain Area Driven by Multiple Factors

In recent decades, strong human activities have not only brought about climate change including both global warming and shifts in the weather patterns but have also caused anomalous variations of hydrological elements in different basins all around the world. Studying the mechanisms and causes of these hydrological variations scientifically is the basis for the management of water resources and the implementation of ecological protection. Therefore, taking the Yongding River mountain area as a representative watershed in China, the changes of different observed and simulated hydro-meteorological variables and their possible causes are analyzed on an inter-annual scale based on ground based observations and remotely sensed data of hydrology, meteorology and underlying surface characteristics from 1956 to 2016. The results show that the annual natural runoff of Guanting hydrological station in the main stream of the Yongding River, Cetian hydrological station and Xiangshuibao hydrological station in the tributary of the Yongding River all have a significant decreasing trend and abrupt changes, and all the abrupt change points of the annual natural runoff series of the three hydrological stations appear in the early 1980s. On the inter-annual scale, the water balance model with double parameters is unable to effectively simulate the natural surface runoff after the abrupt change points. The annual average precipitation after the abrupt change points decreases by no more than 10%, compared with that before the abrupt change points. However, the precipitation from July to August, which is the main runoff-production period, decreases by more than 25%, besides the intra-annual temporal distribution of precipitation becoming uniform and a significant decrease in effective rainfall, which is the source of the runoff. Meanwhile, the NDVI in the basin show an increasing trend, while the groundwater level and land water storage decrease significantly. These factors do not lead only to the continuous reduction of the annual natural runoff in the Yongding River mountain area from 1956 to 2016, but also result in significant changes of the hydro-meteorological relationship in the basin.

Spatiotemporal Evolution Trajectory of Channel Morphology and Controlling Factors of Yongding River, Beijing, China

It is necessary to understand the evolution of a river channel when reconstructing its evolution process and analyzing the controlling factors essential for river management and ecological restoration. In the past 50 years, the ecological environment around the Yongding River has deteriorated considerably, and the downstream has been completely cut off. Despite this, few have studied its morphology. In this study, we analyze the morphology of the Yongding River (Beijing, China) stretching for 92 km in four different periods between 1964 and 2018. A data treatment is carried out based on GIS, and the morphological evolution trajectory of the river channel at the overall and reach scales is reconstructed. The results show that the river morphology has undergone significant changes: the channel width has narrowed by 31%, and the temporal and spatial patterns show significant differences. By analyzing the impacts of human activities and climate change in various periods, we find human intervention to be the most important controlling factor. Based on our results, we proposed a set of river restoration strategies and protection measures for the Yongding River to guide watershed management and land planning.

Trend and attribution analysis of water and sediment variations in sandy rivers

Human activities and climate change have led to significant changes in the flow and sediment of sandy rivers in northern China. The key work to reveal the changes of river water and sediment conditions is to quantitatively study the changes of precipitation, water and soil conservation in river basins, and the effect of reservoirs on sediment containment. Taking the Yongding River as a case study, we analyze the changing trend of the water and sediment into the Yongding River and find that their amount has greatly decreased. In particular, the sediment yield has decreased by more than 90% and its trend has changed, and the turning point occurred in the 1980s. Based on the statistical data analysis model, the influences of human activities on the sediment inflow of the Guanting Reservoir were quantitatively evaluated. The results show that sand retention of the upper cascade reservoirs is the main reason for the sharp reduction in sediment loads, but the sand retention effect of reservoirs has a certain time limit. Water and soil conservation played a vital role in the sediment loads reduction during the present stage. The present studies may provide insights into understanding the integrated reclamation of the river basin.

Vegetation Properties in Human-Impacted Riparian Zones Based on Unmanned Aerial Vehicle (UAV) Imagery: An Analysis of River Reaches in the Yongding River Basin

Riparian zones, transitional areas between aquatic and terrestrial ecosystems, have high plant species diversities. However, they are extremely vulnerable to natural factors, such as changes in river hydrological conditions (floods, droughts) and disturbances from human activities (dams, farmland encroachment, etc.). The distribution of plant life forms and variations in the degree of vegetation coverage in a riparian zone can reflect changes in the environmental conditions. In this study, we analyzed eight reaches from the four main tributaries (Dongyang River, Yang River, Sanggan River, and Yongding River) of the Yongding River Basin, which were selected based on their climate, terrain, and degree of human disturbance. One reach was located on the Dongyang River (DYR), two reaches on the Yang River (YR1 and YR2), three on the Sanggan River (SGR1, 2, and 3), and two on the Yongding River (YDR1 and YDR2). Unmanned aerial vehicle (UAV) technology was used to obtain high-resolution, true-color, multispectral images. The distributions of the plant life forms and the differences in the vegetation coverage were analyzed in the eight selected riparian zones. The results showed that grasses dominated the riparian zone and shrubs and trees were sparsely distributed along both banks of all streams, excluding SGR2 and YDR1. The areas with an extremely high vegetation coverage classification accounted for the highest proportion in the DYR (29.3%), YR2 (48.1%), SGR1 (32.9%), SGR2 (39.9%), SGR3 (85.1%), YDR1 (36.7%), and YDR2 (51.1%) reaches. Extremely low vegetation coverage accounted for the highest proportion in the YR2 reach, reaching 37.4%. This study indicated that natural factors and human activities have a serious impact on the distribution of different plants life forms and vegetation coverage classifications in the riparian zones of the Yongding River Basin. We hope that this research can provide practical assistance in the efforts of ecological restoration and the management of riparian vegetation in the Yongding River Basin.