Attribution of climate change and human activities to streamflow variations with a posterior distribution of hydrological simulations

Hydrological simulations are a main method of quantifying the contribution rate (CR) of climate change (CC) and human activities (HAs) to watershed streamflow changes. However, the uncertainty of hydrological simulations is rarely considered in current research. To fill this research gap, based on the Soil and Water Assessment Tool (SWAT) model, in this study, we propose a new framework to quantify the contribution rate of climate change and human activities based on the posterior histogram distribution of hydrological simulations. In our new quantitative framework, the uncertainty of hydrological simulations is first considered to avoid the phenomenon of "equifinality for different parameters", which is common in hydrological simulations. The Lancang River (LR) Basin in China, which has been greatly affected by human activities in the past two decades, is then selected as the study area. The global gridded monthly sectoral water use data set (GMSWU), coupled with the dead capacity data of the large reservoirs within the LR basin and the Budyko hypothesis framework, are used to compare the calculation result of the novel framework. The results show that (1) the annual streamflow at Yunjinghong station in the Lancang River Basin changed abruptly in 2005, which was mainly due to the construction of the Xiaowan hydropower station that started in October 2004. The annual streamflow and annual mean temperature time series from 1961 to 2015 in the LR Basin showed a significant decreasing and increasing trend at the α = 0.01 significance level, respectively. The annual precipitation showed an insignificant decreasing trend. (2) The results of quantitative analysis using the new framework showed that the reason for the decrease in the streamflow at Yunjinghong station was 42.6 % due to climate change, and the remaining 57.4 % was due to human activities, such as the construction of hydropower stations within the study area. (3) The comparison with the other two methods showed that the contribution rate of climate change calculated by the Budyko framework and the GMSWU data were 37.2 % and 42.5 %, respectively, and the errors of the calculations of the new framework proposed in this study were within 5 %. Therefore, the newly proposed framework, which considers the uncertainty of hydrological simulations, can accurately quantify the contribution rate of climate change and human activities to streamflow changes. (4) The quantitative results calculated by using the simulation results with the largest Nash-Sutcliffe efficiency coefficient (NSE) indicated that climate change was the dominant factor for streamflow reduction, which was in opposition to the calculation results of our new framework. In other words, our novel framework could effectively solve the calculation errors caused by the “equifinality for different parameters” of hydrological simulations. (5) The results of this case study also showed that the reduction in the streamflow in June and November was mainly caused by decreased precipitation and increased evapotranspiration, while the changes in the streamflow in other months were mainly due to human activities such as the regulation of the constructed reservoirs. In general, the novel quantitative framework that considers the uncertainty of hydrological simulations presented in this study has validated an efficient alternative for quantifying the contribution rate of climate change and human activities to streamflow changes.

Evaluations of Remote Sensing-Based Global Evapotranspiration Datasets at Catchment Scale in Mountain Regions

Evapotranspiration (ET) is essential for connecting ecosystems and directly affects the water consumption of forests, grasslands, and farmlands. Eight global remote sensing-based ET (RS_ET) datasets generated using satellite imagery and ground-based observations were comprehensively assessed using monthly ET time series simulated by the water balance (WB) method at the catchment scale in the Hengduan Mountain (HDM) region, including the Nu River, Lancang River, and Jinsha River basins. The complementary relationship (CR) model, which derives ET from meteorological data, was also evaluated against WB-based ET (WB_ET). In addition, WB_ET, RS_ET, and CR-based ET (CR_ET) data were used to investigate ET spatial and temporal variations at the catchment, grid, and site scale, respectively. Most RS_ET datasets accurately simulated monthly ET with an average index of agreement ranging from 0.71–0.91. The Operational Simplified Surface Energy Balance dataset outperformed other RS_ET datasets, with Nash–Sutcliffe efficiency coefficient (NSE) and Kling–Gupta efficiency values of 0.80 and 0.90, respectively. RS_ET datasets generally performed better in northern semiarid areas than in humid southern areas. The monthly ET simulation by the CR model was consistent with that of the WB_ET in the HDM region, with mean values of correlation coefficient (cc) and NSE at each site of 0.89 and 0.68, respectively. The model showed better performance in simulating monthly ET in the Lancang River Basin than in the Nu River and Lancang River basins, with mean cc and NSE of 0.92 and 0.83, respectively. Generally, annual ET trends were consistent at the catchment, grid, and site scale, as estimated by the WB method, RS_ET datasets, and CR model. It showed a significant decreasing trend in the northern semiarid region of the HDM while exhibiting an increasing trend in the humid southern region.

Simulating Crack Development and Failure Characteristic of Toppling Rock Slope under Seismic Loading on Lancang River in China

Toppling rock slopes, induced by rapid and continuous downcutting of Lancang River, are widely distributed in the mountainous area of southwest China. To investigate the instability mechanism of 1# toppling rock slope of Huangdeng Hydropower Station under seismic loading, particle flow code (PFC) is applied to simulate the dynamic response and failure mode. The study considers the particle characteristics of displacement, velocity, energy, and cracks. According to numerical results, the potential failure mechanism of toppling rock slope is identified: multisliding surfaces form at the interfaces between the highly and moderately toppled rock mass and between the highly/moderately and weak toppled-crept rock mass; intersecting faults cut rock mass at the toe, leading to shear-toppling deformation; tension cracks develop, penetrate, and coalesce in the weak toppled-crept rock mass, resulting in tension-toppling-bending deformation. During the 2 to 5 s of strong seismic intensity, crack increases sharply and energy of particles fluctuates greatly. The impacts of the amplitude of seismic loading and loading method in PFC are investigated. This study will provide a practically useful reference for seismic design of rock slopes.

The Impact of Cascade Large Deep Reservoir on the Migration and Deposition of Cadmium in Lancang River

Metal pollution is a global environmental problem. In order to understand the effects of the cascade hydropower development on Cd in reservoir sediments, this study investigated the migration and deposition mechanisms of Cd in reservoir sediments by using the film diffusion gradient technique (DGT) for the cascade large deep reservoirs of the Lancang River (LCR), Nuozhadu (NZD) and Xiaowan (XW). The results show that: (1) Strong correlations amongst the deposition processes of Cd, Mn and S were found in the sediments of the NZD reservoir; weak correlations between the deposition processes of Cd and Mn were found in the sediments of the XW reservoir. (2) Part of labile Cd in the pore water of the NZD reservoir was supplemented by silt and sand, and another part was inherent in the pore water; most of labile Cd in the pore water of the XW reservoir was inherent in the pore water, and minimal replenishment of sediment was noted. (3) Mn is the main factor controlling the migration of Cd in the NZD reservoir. This paper provides an effective resource for understanding the migration and deposition of Cd in the sediments of large deep reservoirs along the LCR.

Synchronous Cycle of Available Phosphorus, Iron, and Sulfur in the Sediment of Lancang River Reservoirs

Large-scale deep reservoirs associated with hydropower cascade development are known to influence the cycle of phosphorus (P). However, there is scarce information on the fractions and availability of P in sediments of large-scale deep reservoirs constructed due to hydropower cascade development. In this study, we researched the fractions and release mechanism of P in the sediments of large-scale deep reservoirs by analyzing the fractions and availability of P in the sediments of the Xiaowan (XW) and Nuozhadu (NZD) reservoirs in the middle and lower reaches of the Lancang River (China). According to the results, there is a significant difference in the P fractions in the sediments of the XW and NZD reservoirs, but not for the available P in the sediments. Compared to the NZD reservoir, there was less solid bioavailable phosphorus (BAP) in the sediments of the XW reservoir, but the replenishment degree of active solid phase P into pore water was higher in the XW. There was a significant positive correlation between the available P and the BAP; the Fe/P ratio measured by the diffusive gradients in thin films reflects the control of active iron (oxyhydr) oxides over labile P in the sediments. In addition to the reductive dissolution of iron-bound P, the release of P into the large deep reservoirs may be related to factors such as the sulfate reduction and the degradation of organic materials. The P cycling in deep reservoir sediments is mainly controlled by the Fe, and there is a clear spatial distribution of this mechanism in deep reservoirs.