The construction of dams has significant effects on hydrologic regimes downstream. The combined effect of the Danjiangkou Reservoir and the downstream cascade reservoirs on the changes in the hydrologic regime at Huangzhuang station were investigated using the indicators of hydrologic alteration (IHA) and the range of variability approach (RVA) in the middle-lower Hanjiang River. The single operation of the Danjiangkou Reservoir had effects with an overall alteration degree of 40.6%, in which the number of reversals, and flows in March and September were highly altered. The combined operation of Wangfuzhou Reservoir changed the flows in March and August and the extreme minima from high or moderate alteration to low alteration and increased the alteration of the extreme maxima, pulse behavior, and rise rate, with an overall alteration degree of 46.7%. Furthermore, with the combined operation of Cuijiaying Reservoir, the overall alteration degree increased to 63.5%, and 56.2% of the indicators were of high alteration. These changes are mainly caused by the reservoir regulations, which have profound effects on the river aquatic environment and biological habitats.
Channel bars are essential landforms and their evolution is crucial to aquatic and riparian biodiversity, river’s water-sediment process, and economic development. With the development of water conservation facilities and hydropower projects, numerous changes have been taken place in hydrological regimes and morphology. There have been many changes on channel bars in the middle reaches of Hanjiang River due to the combined effects of cascade reservoirs. However, little was known about such dynamics and their linkages to cascade dams across the entire downstream area. Using Landsat remote sensing images from 1986–2018 and the threshold binary Otsu extraction method, this study completed comprehensive monitoring of nine mid-channel bars (DX1–DX7, XZ1, and XZ2), and three shoal group (XZ3–XZ5) dynamics. Results showed that the mid-channel bars’ area in the reach from Danjiangkou to Xiangyang (DX) decreased over the past 33 years, with the exception of DX4, while the total area decreased by 23.19%, this channel bars’ area change was mainly influenced by backwater from the Cuijiaying Reservoir with high water level after 2010 (r = −0.93, p < 0.01). The total channel bar area from Xiangyang to Huangzhuang (XZ) decreased by 16.63% from 1986 to 2018. The total channel bar area in XZ had a strong negative correlation with runoff at Huangzhuang hydrologic station (r = −0.79, p < 0.05), which was partly attributed to upstream precipitation according to the high correlation between runoff and precipitation (R2 = 0.65). In general, the DX section was under equilibrium between scouring and deposition compared to downstream Xiangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited. In addition, vegetation cover, revetments, flood events, sand mining, land use, and over-exploitation may cause channel bar area dynamics. Hence, more continuous investigations are suggested to focus on effects of cascade reservoir operation on hydrological regime, as well as the changing morphology of channel bars in the middle reaches of the Hanjiang River.
Currently, efficient and cost-efficient methods are needed to manage fields in order to maximize economic return, minimize environmental impact, and improve soil and crop site-specific management. Management zones (MZs) of uniform production potential may offer a solution to the problems associated with grid soil sampling while still effectively describing variability in soil properties. In this study, a hillside field of 4 ha was selected as the study site and 111 soil samples were taken from the topsoil (0–20 cm), on an ∼20-m grid. Soil samples were analyzed for pH, organic matter, active soil organic matter, total nitrogen, alkalytic nitrogen, available phosphorous, available potassium and cation-exchange capacity. Their spatial variability was analyzed and spatial distribution maps were constructed using geostatistical techniques. Principal component analysis (PCA) and fuzzy c-means clustering algorithm were then performed to delineate MZs, and eigenvalues were used to select the principal components (PCs) for cluster analysis. Fuzzy performance index and normalized classification entropy were used to determine the optimum cluster number. The results revealed that the optimum number of MZs for this study area was three and analysis of variance indicated that the MZs were reasonable for the area. Based on the description of MZs, it was concluded that the area could be managed in a site-specific way.
Operation of the middle route of the South-to-North Water Diversion Project in the Hanjiang River, China, has made the environmental health downstream of the Danjiangkou Reservoir an important issue for people living along the river. This study used the Planktonic Index of Biotic Integrity (P-IBI) including phytoplankton and zooplankton to evaluate riverine ecological health in an area with cascade reservoirs. Plankton and water samples from 19 sampling sites were collected along the middle-lower reaches of the Hanjiang River in Nov. 2017 and July 2018. To further verify the reliability of the P-IBI results, they were compared with the Saprobic Index (SI) and Comprehensive Water Quality Identification Index (CWQII). Redundancy analysis (RDA) was used to evaluate correlations and variability between P-IBI, SI, CWQII, candidate metrics, and primary environmental factors in the dry and wet seasons. The results showed that the ecological health of the aquatic ecosystem was 'fair' in status, oligotrophic, and class II (wet season)-class III (dry season) as evaluated by the IBI, SI, and CWQII, respectively. The ecosystem during the wet season was in better health than that in the dry season: based on CWQII, water quality at 63% of sites was in class III during the dry season, while 79% of sites were in class II in the wet season; and 11% more sites had IBI-A values (the mean values of Phyto-IBI and Z-IBI) less than 3 IBI scores (indicating "fair" status) in the dry season compared to the wet season. Ecological conditions were also poorer in the reservoir backwater area. The main driving forces impacting aquatic ecosystem in the study area were pollutant emissions and dam impacts. This study demonstrated that the P-IBI is a potential tool for evaluating riverine ecological health in areas with cascade reservoirs.
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