Review and improvement of conventional models for reservoir sediment trapping efficiency

Linking reservoir annual residence time to nitrogen deposition using paleolimnological techniques

In river networks, reservoirs are hotspots for nutrient transformations, providing multiple pathways for nitrogen processing. One of the less measured pathways is nitrogen deposition. Here, we investigated the decadal relationship between water residence time and nitrogen deposition using sediment cores from eight mainstem reservoirs within a river system containing two contrasting watersheds. One watershed was significantly urbanized with regulated flow and the other watershed was unregulated with extensive rural land use. We explored the relationship of sediment nitrogen concentrations across a range of residence times, land uses, and other parameters throughout this linked river-reservoir system. Results show that average annual residence time had the strongest relationship to nitrogen deposition when compared to reservoir volume, mean depth, surface area, outflow, and land use. Pigment analysis revealed that residence time influences nitrogen by allowing for longer periods of algal uptake, followed by deposition in particulate organic form. Supporting this mechanism, sedimentary C:N, with low values representing greater algal influence, expressed a strong and negative relationship with average annual residence time, as well as a positive relationship between residence time and photosynthetic pigments diagnostic of cyanobacteria, diatoms, and a combination of green algae+cyanobacteria. Furthermore, we investigated how drought conditions could alter residence times and intensify nitrogen cycling through primary productivity in reservoirs. Drought increased residence time by 45-60 %. This increase was estimated to raise sediment nitrogen concentrations by roughly 2.5-4 %.

Seasonal and annual variations of sediment trapping and particulate organic carbon burial in Yellow River reservoirs

The Yellow River is distinguished by the highest sediment load in mainland China and significant siltation in artificial reservoirs along its main channel, reducing water availability, sediment trapping, and carbon burial in the hydrological project. Since 2002, the Water Sediment Regulation Scheme (WSRS) has been progressively implemented as a hydraulic management strategy to mitigate reservoir sedimentation in the middle-lower basin reservoirs. However, this substantial release of sediment and water has also affected river morphology, carbon burial, and sediment trapping. This research assesses the evolution of particulate organic carbon burial and sediment trapping in eight Yellow River reservoirs from 2002 to 2018, covering the upper and middle-lower basins and two reservoirs in the Yiluo River sub-basin. We calculated the annual and seasonal variations using hydrological data from 11 stations along the Yellow River. A hydrological framework was developed to calculate sediment trapping, and a Monte Carlo analysis was performed to estimate particulate organic carbon burial across all the evaluated reservoirs. We found that sediment trapping and carbon burial in upper basin reservoirs are most influenced by shifts in the precipitation regime, particularly by natural events such as severe droughts or heavy rainfall. A strong correlation was observed between annual precipitation variations and sediment load. In middle-lower basin reservoirs, the major artificial sediment regulation is strongly linked to a significant reduction in sediment trapping and particulate organic carbon burial. This impact is especially notable in the Yiluo River Sub-basin reservoirs, which have experienced a >90 % reduction compared to levels before the WSRS implementation. Finally, we highlight the consequences of climate change and artificial water management strategies in reservoirs, demonstrating how both affect their capacity for sediment trapping and impact their role as significant carbon sinks.

Variation in sediment sources and the response of suspended sediment grain size in the upper Changjiang River Basin following the large dam constructions

The amount, patterns, and particle size composition of suspended sediment in the upper Changjiang Basin has been altered significantly due to the cascade reservoirs construction. A dam breach may disrupt sedimentation in the reservoir, channel erosion downstream of the dam, and the cycle of nutrients and contaminants adherent to the dam. This study is based on the long time-series field data of water discharge, sediment flux, and suspended sediment grain size of the upper Changjiang River Basin from 1973 to 2019. Four significant stepwise reduction periods in sediment load were identified by employing the M-K test, namely, 1973-1992, 1993-2002, 2003-2012, and 2013-2019. Based on the results, sediment load reduction in the upper Changjiang River (CJR) is remarkably correlated with an increase in the capacity of large reservoirs. Jialingjiang River became the largest sediment source area for upper CJR in 2013-2019. Moreover, the variation range of suspended sediment median particle size gradually narrowed with decreasing sediment load during the four periods in the upper CJR. The silt content of suspended sediment increased while clay and sand contents decreased which would maybe a factor of decreasing of P flux in the upper CJR. After the impounding of the Xiangjiaba and Xiluodu Reservoirs in the Jinshajiang River, the fining trend of suspended sediment along the main stem of upper CJR was cut off in 2013-2019, which may be ascribed to sediment management of the cascade reservoirs regulation and the sediment resuspension from the channel erosion in the river downstream. These observations can also serve as a reference for future studies about the effects of cascade reservoirs on aquatic ecology and environmental changes.

Prediction research on sedimentation balance of Three Gorges Reservoir under new conditions of water and sediment

Influenced by climate change and human activities, especially the completion and operation of cascade reservoirs in the middle and lower reaches of Jinsha River since 2012, new changes have taken place in the water and sediment characteristics of the Three Gorges Reservoir (TGR) in recent years. In this paper, a one-dimensional unsteady water and sediment mathematical model of the main and tributary rivers of the TGR is established, and the main calculation parameters of the model are calibrated with the measured water and sediment data from January 1, 2008 to December 31, 2017. In view of the different combinations of inflow water and sediment that may occur in the TGR under the condition of new water and sediment, the long-term changes of sediment erosion and deposition and the balance of reservoir deposition in the TGR are studied using the model. The results show that: (1) Under the new conditions of water and sediment, the amount of sediment in the TGR accounts for only 14.8% and 35.8% of that in 1956–1990 and 2003–2012, respectively; (2) The variation process of water level, discharge and sediment concentration of each station along the route calculated by the model is basically consistent with the measured results, and the calculated values of total deposition amount and deposition distribution are also basically consistent with the measured results. The verification results of the model are in accordance with the measured values; (3) Under the water-sediment conditions during 1961–1970 and 1991–2000, the model predicted the estimates of 320 and 430 years for the TGR to reach a sedimentation balance, respectively. Under the new water-sediment conditions, it takes 560 years at most and 450 years at least to reach the sedimentation balance for the TGR, and the corresponding condition is the typical year with less water-less sediment and more water-more sediment, respectively. The research results of this paper can provide a new reference for the long-term safe operation and operation optimization of the TGR.