Dam Regulation Research Articles

Spectral Signatures of Flow Regime Alteration by Dams Across the United States

AbstractRiver scientists strive to understand how streamflow regimes vary across space and time because it is fundamental to predicting the impacts of climate change and human activities on riverine ecosystems. Here we tested whether flow periodicity differs between rivers that are regulated or unregulated by large dams, and whether dominant periodicities change over time in response to dam regulation. These questions were addressed by calculating wavelet power at different timescales, ranging from 6 hr to 10 years, across 175 pairs of dam‐regulated and unregulated USGS gages with long‐term discharge data, spanning the conterminous United States. We then focused on eight focal reservoirs with high‐quality and high‐frequency data to examine the spectral signatures of dam‐induced flow alteration and their time‐varying nature. We found that regulation by dams induces changes not only in flow magnitude and variability, but also in the dominant periodicities of a river's flow regime. Our analysis also revealed that dams generally alter multi‐annual and annual periodicity to a higher extent than seasonal or daily periodicity. Based on the focal reservoirs, we illustrate that alteration of flow periodicity is time varying, with dam operations (e.g., daily peaking vs. baseload operation), changes in dam capacity, and environmental policies shifting the relative importance of periodicities over time. Our analysis demonstrates the pervasiveness of human signatures now characterizing the U.S. rivers' flow regimes, and may inform the restoration of environmental periodicity downstream of reservoirs via controlled flow releases—a critical need in light of new damming and dam retrofitting for hydropower globally.

Hydrological impacts of dam regulation for hydropower production: The case of Lake Sibinacocha, Southern Peru

Study regionVilcanota-Urubamba river basin, Southern Peru. Study focusHydraulic infrastructure plays a fundamental role for energy production, drinking and irrigation water storage and flood control in regions with seasonal river flow. The high-Andean Lake Sibinacocha has been regulated since 1988 to increase energy production of the Machupicchu hydropower plant. In this study, river streamflow changes are evaluated by analyzing precipitation and discharge trends using indicators of hydrologic alteration and ecoflow for natural (1965–1987) and altered (1988–2016) flow regimes. New hydrological insights for the regionFor the altered flow regime, an ecodeficit of about 20% (compared to natural river flow) and an ecosurplus > 30% were found during the wet season (December-February) and dry season (June-August), respectively. These changes have reduced the risk of water shortage (dry season) and flood (wet season) and contribute to increasing water use including hydropower production, irrigation and drinking water. However, river alteration might lead to considerable impacts on riverine ecosystems.Despite major limitations related to data scarcity and complex environmental processes in the basin, our results highlight the usefulness of combined methods of hydrological alteration and ecoflow to effectively evaluate water regime changes in regulated basins. An integrated scientific approach is necessary to address uncertainties and develop meaningful future water availability scenarios that guide hydropower projects with improved water and energy security considering minimal impacts on human and natural systems.

Flood Hazard Assessment of Water-Front Geosynthetic Reinforced Soil Wall for Dam Regulation Rule Level

The flood hazard parameter of peak discharge and prolonged inundation due to change in dam regulation is estimated for a water front Geosynthetic Reinforced Soil (GRS) wall on the downstream of large dam. Dam regulation rule level is pre-emptying the reservoir with lower peak discharge to protect the expanding downstream city intruding the flood plain. Present study is to test the hypothesis that rule level has significantly changed peak discharge and inundation duration and also to estimate these two important parameters of flood hazard for different flood return periods. The methodology consists of three parts: first is the categorization of flood data according to major event of flood regulation, second is the distribution test and third is the estimation of design maximum peak discharge and duration of flood. The estimated value of peak discharge and flood duration clearly indicate the implication of dam regulation rule level; the estimated value of peak discharge for 200 years return period is 13.9 lakh Cusecs, which is lower than the flood discharge for year 1968, which was 15 lakh Cusecs, and the estimated flood duration for 10 years return period is 41 days. The most important finding of this study is the substantial increase in duration of flood due to implementation of rule level which will add one more flood hazard parameter for water front geotechnical structure that is prolonged inundation.

Evaluation of river longitudinal connectivity based on landscape pattern and its application in the middle and lower reaches of the Yellow River, China.

River connectivity plays an essential role in maintaining the health and stability of river basin ecosystems. It is of great significance to protect river ecosystems to clarify the effect of water conservancy project construction and operation on river hydrological connectivity. The longitudinal connectivity is affected by the landscape patterns of river, such as the convergence and dispersion of the mid-channel bars and the river areas. This study aims to analyze the impacts of construction and regulation of Xiaolangdi Dam on the connectivity of the middle and lower reaches of the Yellow River from the perspective of landscape pattern. An improved longitudinal river connectivity evaluation method was proposed by accounting for the influence of the landscape pattern represented by mid-channel bars based on barrier coefficient method, and then was applied to analyze the connectivity pre- and post-dam construction. The results show that the amplitude and frequency of the oscillation of the river were greatly reduced and tended to be stable. The aggregation degree of mid-channel bars was reduced, and the distribution of mid-channel bars was more dispersed. The river longitudinal connectivity before and after the construction of the Xiaolangdi Dam were 1.35 and 1.50 respectively, indicating an increased river longitudinal connectivity. Overall, there are differences in connectivity before and after Xiaolangdi Dam construction, and connectivity fluctuates after dam construction. Because of the dam regulation of water and sediment, the river connectivity during the flood season increased significantly, and was greater than that before and after the flood season. The longitudinal connectivity evaluation method established in this study is accurate and efficient, and provides an intuitive and reliable new method for quantitatively analyzing the changing laws and characteristics of river connectivity.

Transport of particulate organic carbon in the lower Yellow River (Huanghe) as modulated by dam operation

The Yellow River has changed into a highly dam-controlled system over the recent decades. Based on gauging records of water-sediment discharge and particulate organic carbon (POC) datasets, this paper investigates the dam impacts on spatio-temporal variability of hydrology and POC transport. With a large part of suspended sediment and POC normally preserved within the Xiaolangdi (XLD) reservoir, erosion of riverbed downstream the reservoir has now become a primary source of sediment and POC to coastal ocean (172 Mt/yr for sediment and 0.56 Mt/yr for POC, respectively). Sediment accumulation and POC burial within the XLD reservoir are 246 Mt/yr and 0.60 Mt/yr, respectively, globally accounting for ∼10% and ∼1% of the retentions in reservoirs. Downstream the XLD reservoir, the averaged Δ 14 C values of POC was −451 ± 25‰, corresponding to a 14 C age of 4763 ± 366 yr BP, indicating that the POC in the lower reaches of the Yellow River was ∼2400 yr younger than that in the Loess Plateau region where most of the sediment and POC are sourced. Petrogenic POC fluxes upstream and downstream the XLD dam were 0.12 Mt/yr and 0.06 Mt/yr, respectively, clearly indicative of large-scale trapping of petrogenic POC by the XLD dam. At present, the XLD dam regulation intercepts this petrogenic POC from the Loess Plateau and enhances the downstream input of biospheric POC ( Fm = 0.85) to the coast. In contrast to the sediment and POC delivery of the rivers worldwide that are primarily dominated by tectonic, climatic and geographic settings, human activities, especially the dam regulation in the Yellow River, have significantly altered river hydrologic regime and overwhelmed the sediment and POC delivery within the river-coast continuum, which has profound impacts on carbon cycling in the coastal margin. Schematic diagram for POC and sediment budget within the Yellow River-Coast Continuum from 2002 to 2014 based on observed and reconstrcuted datasets. • The XLD reservoir acts as a sink of POC coupled with periodic rapid sediment accumulation. • Downstream riverbed erosion is a dominant source of POC to the coastal margin. • Dam regulation has substantially transformed the POC delivery during the WSRS.

Spatial variations in water chemical components in a coastal zone of northern China: Insights from environmental isotopes

• Seawater intrusion and weathering determined water quality of coastal zone (CZ). • Dam regulation in CZ rivers affected water chemical components and nitrogen cycle. • N fate in the weak coastal hydrological zone rivers was driven by microorganisms. • Sewage and manure sources were major nitrate contributors in the studied CZ. Coastal zones (CZ) are ecologically and environmentally significant, and it is thus critical to study the interactions among different types of natural CZ waters for better understanding biogeochemical processes. Water samples (n = 101) were analyzed for multiple stable isotopes (δ 18 O-H 2 O, δD-H 2 O, δ 15 N-PON, δ 15 N-NO 3 − , and δ 18 O-NO 3 − ) in seawater, river water, reservoir water and groundwater at Tianjin CZ to demonstrate the spatial variations in the CZ aqueous environment and to identify key influencing processes factors of the N geochemical cycle. This study confirmed that weathering dominated upstream water properties, whereas seawater intrusion determined the downstream and midstream water chemical components, where the seawater fraction was 44% and 11%, respectively. Evaporation and precipitation processes were active across the whole CZ area, but their impacts on water chemical components were less significant than seawater intrusion and weathering. Dam regulation in CZ rivers has significantly affected water chemical components and the nitrogen cycle. The isotopic evidence indicated that bacterioplankton and phytoplankton were the primary forms of PON. The dual nitrate isotopes revealed that animal manure and industrial sewage contributed the leading nitrate to river water. The spatial variation of their contribution was quantitated (from upstream to midstream and downstream: 62%, 91% and 83%, respectively). Considering the potential isotopic fractionation of nitrification reduction from upstream to downstream, 39%, 61% and 57%, respectively. This study proposes a quantitative framework for detecting CZ areas with similar hydrodynamic conditions and climate characteristics found at the Tianjin CZ, which has important implications for CZ water management and environmental protection policies.

Spatial and temporal changes in nutrients associated with dam regulation of the Yellow River

Sediment regulation via manipulating dam operations is an effective solution for dealing with reservoir siltation and downstream channel incision. Large amounts of sediment and thereby nutrients can be discharged within a short period of time. To ascertain the concentration of and transformation between dissolved and particulate nutrients during such periods, we conducted field observations in the Xiaolangdi Reservoir and its downstream reach during the sediment regulation stage of the Water-Sediment Regulation Scheme (WSRS) in 2018. We found that water and sediment characteristics differed significantly pre- and post-WSRS period. Nutrient concentrations varied significantly among the dissolved, suspended, and deposited phases, while no significant differences were observed between the reservoir and downstream reach, except for dissolved total carbon, particulate ammonium nitrogen, and calcium-bound phosphorus. Total nitrogen (TN) and total phosphorus (TP) concentrations of the dissolved phase were significantly positively correlated with suspended sediment concentration (SSC) and electrical conductivity (EC) (p < 0.05). Total organic carbon (TOC) concentration of the suspended phase was significantly negatively correlated with EC and median particle size (D50) (p < 0.05). Nutrient concentration during the deposited phase was less correlated to selected hydrological and physical parameters. Nutrient transformation between the deposited phase and the suspended and dissolved phases were higher than those between suspended and dissolved phases. TP transformation was closely related to hydrological parameters (SSC and D50), while TN transformation was predominantly related to physical parameters (EC and DO). The concentration and transformation of nutrients during the transport process from the reservoir to downstream were likely driven by different factors. In conclusion, reservoir management practices like WSRS could mitigate nutrients retention in reservoir, however, leading to higher nutrients transformation downstream.

Nutrients retention of a series of small dam-impacted urban rivers in northern China

How a series of small rubber dams distributed within urban watersheds impact nutrients transport remains less investigated. To fill this research gap, this study assesses the composition of nitrogen (N) and phosphorus (P) and their retention in five urban river segments across the Qingshui-Yanghe rivers, China from March 2019 to September 2021. The upstream and downstream of the Qingshui river (QR-up and QR-city) were enriched with dissolved N and particulate P, suggesting a greater P contribution of agricultural origin. NH4-N concentrations were lower upstream but increased downstream of the Yanghe river due to the direct discharge of sewage and other human activities. N and P concentrations varied along the urban river and were relatively low at QR-city. A total of 159.2 and 8.4 metric tons/yr, corresponding to 63% and 70% of the TN and TP loading, were trapped within QR-up and QR-city, of which the dissolved N and particulate P load retention accounted for 58% and 78%. N and P retention rates within the river segments sharply decreased from 97 to –320% and were mainly sequestered in the upstream river segments (QR-up- QR-city). The retention rate of nitrate (78%) and dissolved reactive P (67%) were higher than that of TPN (9.75%) and TPP (78%) because of biological assimilation. Further, with retention rates of −320%, the upstream river segments were a net source of dissolved organic P. These results confirm that the small dams have an important nutrient retention function that is highly variable on a spatiotemporal scale similarly to large-scale reservoirs. Due to the cumulative influence of sewage discharge downstream of the Yanghe River, the nutrient load retention is significantly higher than that in the Qingshui river segments. The variations in hydrological regimes due to seasonal events and small dam regulation have a substantial impact on nutrient retention, resulting in higher loads retention in the flood period than in other periods. Overall, the results suggest that management plans must not only focus on decreasing nutrient export from the mountainous area of the Qingshui river basin but also on sewage outfalls and enhancing the hydrodynamic status of the dammed rivers with respect to eutrophication.

Using Ensemble Data Assimilation to Estimate Transient Hydrologic Exchange Flow Under Highly Dynamic Flow Conditions

AbstractQuantifying dynamic hydrologic exchange flows (HEFs) within river corridors that experience high‐frequency flow variations caused by dam regulations is important for understanding the biogeochemical processes at the river water and groundwater interfaces. Heat has been widely used as a tracer to infer steady‐state flow velocities through analytical solutions of heat transport defined by the diurnal temperature signals. Under sub‐daily dynamic flow conditions, however, such analytical solutions are not applicable due to the violation of their fundamental assumptions. In this study, we developed a data assimilation‐based approach to estimate the sub‐daily flux under highly dynamic flow conditions using multi‐depth temperature observations at a 5‐min resolution. If the hydraulic gradient is measured, Darcy's law was used to calculate the flux with permeability estimated from temperature responses below the riverbed. Otherwise, flux was estimated directly by assimilating multi‐depth temperature data at 1‐ or 2‐hr time intervals assuming one‐dimensional flow and heat transport governing equation. By comparing estimated fluxes with model‐generated synthetic truth, we demonstrated that both schemes have robust performance in estimating fluxes under highly dynamic flow conditions. This data assimilation‐based flux estimation method was able to capture the vertical sub‐daily fluxes using multi‐depth high‐resolution temperature data alone, even in the presence of multi‐dimensional flow. This approach has been successfully applied to real field temperature data collected at the Hanford site, which experiences highly dynamic HEFs. Our study shows the promise of adopting distributed 1‐D temperature monitoring to capture spatial and temporal exchange dynamics in river corridors at a watershed scale or beyond.

Effects of seasonal hydrological regulation of cascade dams on the functional diversity of zooplankton: Implications for the management of massive reservoirs and dams

The construction of cascade dams has led to a lack of longitudinal connectivity in rivers, fragmentation of habitats, slowing of the spread of biota, and loss of biodiversity. Here, we examined the zooplankton functional diversity in a typical river with cascade dams and compared the differences between the upstream and downstream parts of the river during the dry and wet seasons. The findings were as follows: (1) during the dry season, the nutrient load in the lower reaches of the river increased, and the water quality deteriorated, while the conditions during the wet season did not change significantly; (2) during the dry season, the functional beta diversity of downstream zooplankton caused by cascade dams was significantly (p < 0.05) lower than that upstream, but this phenomenon was not prominent during the wet season; (3) during the dry season, environmental factors and dam numbers significantly affected the functional diversity of zooplankton, and the number of dams directly impacted the formation of the distribution pattern of zooplankton functional traits. These findings highlight the effects of the hydrological regulation of cascade dams in different seasons on the functional diversity of zooplankton and suggest that: 1) water retention times and pollution source inputs should be reduced during the dry season, 2) dedicated biological channels should be established to reduce the harm caused by clustered pollution to zooplankton, and 3) thermal pollution and strong scouring caused by large-scale flood discharge simultaneously should be avoided during the rainy season. Zooplankton functional diversity is sensitive to dam hydrological regulation, and we advise it be included as a critical indicator during ecosystem assessment in the dam management process, along with additional biological indicators are recommended.

A study of supercooling in rivers

The process of supercooling, where turbulent water is cooled below the freezing point, is a critical condition for the formation of suspended frazil and anchor ice in northern rivers. The behaviour of supercooling has been investigated in numerous laboratory studies but there is a comparatively small number of field observations reported in literature. In this study, high-precision water temperature loggers were deployed on three regulated rivers in Alberta, Canada over multiple winters. A total of 696 supercooling events were detected in the recorded temperature time series. The properties of each supercooling event were calculated (e.g., peak supercooling temperature and duration) and analyzed to increase our understanding of the timing, duration and magnitude of supercooling in rivers. The median peak supercooling temperature, duration and cooling rate averaged of all events was −0.013 °C, 4.8 h and − 2.5 × 10−4 °C/min, respectively. Supercooling events with durations of 2 to 14 days were observed to occur relatively frequently which was surprising since the longest event reported in the literature had a duration on the order of 50 h. Preliminary analysis of the longitudinal distribution of supercooling events along the study reaches demonstrated that dam regulation and urbanization may have a significant impact on supercooling in rivers. Comparison of supercooling observed near the left and right banks in one river showed that there were significant differences in the frequency and duration of supercooling events across the channel.

Changes in streamflow regimes and their responses to different soil and water conservation measures in the Loess Plateau watersheds, China

AbstractInvestigating the changes in streamflow regimes in response to various influencing factors contributes to our understanding of the mechanisms of hydrological processes in different watersheds and to water resource management strategies. This study examined streamflow regime changes by applying the indicators of hydrologic alteration method and eco‐flow metrics to daily runoff data (1965–2016) from the Sandu, Hulu and Dali Rivers on the Chinese Loess Plateau, and then determined their responses to terracing, afforestation and damming. The Budyko water balance equation and the double mass curve method were used to separate the impacts of climate change and human activities on the mean discharge changes. The results showed that the terraced and dammed watersheds exhibited significant decreases in annual runoff. All hydrologic metrics indicated that the highest degree of hydrologic alteration was in the Sandu River watershed (terraced), where the monthly and extreme flows reduced significantly. In contrast, the annual eco‐deficit increased significantly, indicating the highest reduction in streamflow among the three watersheds. The regulation of dams and reservoirs in the Dali River watershed has altered the flow regime, and obvious decreases in the maximum flow and slight increases in the minimum flow and baseflow indices were observed. In the Hulu River watershed (afforested), the monthly flow and extreme flows decreased slightly and were categorized as low‐degree alteration, indicating that the long‐term delayed effects of afforestation on hydrological processes. The magnitude of the eco‐flow metrics varied with the alteration of annual precipitation. Climate change contributed 67.47% to the runoff reduction in the Hulu River watershed, while human activities played predominant roles in reducing runoff in the Sandu and Dali River watersheds. The findings revealed distinct patterns and causes of streamflow regime alteration due to different conservation measures, emphasizing the need to optimize the spatial allocation of measures to control soil erosion and utilize water resources on the Loess Plateau.

Anthropogenic flash floods and climate change in rural Zimbabwe: Impacts and options for adaptation

Zimbabwe has been affected by numerous floods-related disasters in the recent past. These disasters often left rural communities in difficult socioeconomic situations. Floods are among the major water-related hazards and natural disasters worldwide. They are associated with excess rainfall, resulting in river overflow due to climate change . Developing countries constitute a large number of losses caused by water hazards. Thereby, in these countries, the vulnerabilities of households that depend on rain fed agriculture and livestock production for their livelihoods increases. This review paper seeks to establish the association between anthropogenic flash floods and climate change and unearth the impacts; vulnerabilities and adaptation options associated with anthropogenic flash floods in rural Zimbabwe. The study found that numerous anthropogenic activities perpetuate an increase in flood risk. These include littering, river or dam regulation measures, intensified land use and emissions of greenhouse gases, which subsequently result in global climate change. The impacts of flash floods established in the study include drought, food insecurity, displacements, malnutrition and severe effects on subsistence economy. The study concluded that even though most rural communities in Zimbabwe are vulnerable to the effects of floods, they have devised numerous diverse adaptation strategies to cope with the changes in the environment.

Impact of Artificial Floods on the Quantity and Grain Size of River‐Borne Sediment: A Case Study of a Dam Regulation Scheme in the Yellow River Catchment

AbstractThe quantity and grain size of river‐borne sediment are critical to geomorphological evolution and ecosystem health of river deltas. Compared to quantitative aspects of fluvial systems, however, studies describing the physical properties of river sediment are still quite limited. This work examines how an unprecedented engineering effort (the Water‐Sediment Regulation Scheme, WSRS) within the lower Yellow River system has caused important changes in the volume and grain size patterns of transported sediment. After the implementation of the WSRS, artificial floods dominated the delivery of water and sediment to the delta region—the natural (seasonal) rhythm of the hydrological cycle has been replaced by impulsive delivery. Compared to natural floods, the duration of the artificial flood due to WSRS is substantially shortened, and timing has changed to early summer (June to July), which is two to three months ahead of the usual flood season. Now, as a result of WSRS, there are high sediment loads over a shorter than usual time frame. The Yellow River's sediment load rebounded and coarsened, because artificial floods have strongly scoured the river channel. These changes turned the Yellow River delta from a recession phase to one of progradation, and caused impulsive delivery and rapidly buried of terrestrial organic carbon and pollutants. For other rivers worldwide, the lower Yellow River system provides an illustrative example of how transporting coarse‐grained sediment, otherwise deposited on river bed, may be used to mitigate coastal land loss on intra‐decadal time scales, however with the potential for adverse ecological impacts.

Exploring conceptual models of infiltration and groundwater recharge on an intermittent river: The role of geologic controls

Study regionThis study is along an intermittent reach of the Alamosa River in the San Luis Valley of south-central Colorado, a river that is typical of the semi-arid southwestern United States with respect to climate, land use, and the impacts of upstream dam regulation. Study focusWe use conceptual steady-state models to identify geologic factors that may control water loss through infiltration. These conceptual models are parameterized according to a range of conditions observed from stream discharge, topographic data, geologic data, and drone magnetometer data. New hydrological insightsThe introduction of a fault, variation in the alluvial aquifer hydraulic conductivity, and presence or absence of a confining unit in the numerical models were the primary geologic controls that affected infiltration across the study reach. Conversely, variation in the thickness of the streambed had little impact. This information may help determine future data collection within this and similar semi-arid regions where rivers are controlled by a combination of surface water availability (e.g. through dam regulation) and complex subsurface geology, which are often not well constrained.

Representing Human Water Management in a Land Surface Model Using a Supply/Demand Approach

AbstractThe impact of human water management on river discharge is increasingly viewed as a missing process in Earth system modeling. Models which have attempted to include it are generally at coarse resolution and uncoupled to the atmosphere. We propose to describe human water management at high spatial resolution using ORganizing Carbon and Hydrology In Dynamic EcosystEms concept of hydrological‐transfer‐units in the routing parametrization. Irrigated areas are linked to river abstraction points using a minimization process. The directed graphs of river flows and adduction network for irrigation is transposed to propagate water demands upstream. Reservoirs and dams are placed along the graph to balance supply and demands in the four chosen water value classes. Dam regulation is assumed to maximize demand satisfaction and dampen floods while respecting the properties of the infrastructure. The developed human water management module is applied to the Yellow River where irrigation and dam regulation are known to have a strong impact. Results show that the impact of human water management is strongly heterogeneous over space. It propagates along the river channels and can be mitigated by the confluence of tributaries. Moreover, the human impact has a strong seasonality due to time varying irrigation demands and the response of dam regulation. A number of uncertainties still remain and affect the simulated river discharge. Nevertheless, the representation of human water management improves the model's behavior in terms of magnitude and intra‐annual variations of river discharge, and offers the opportunity to implement anthropogenic processes in the water cycle of Earth System Models.

Assimilation of Ground‐Based GPS Observations of Vertical Displacement into a Land Surface Model to Improve Terrestrial Water Storage Estimates

AbstractGround‐based Global Positioning System (GPS) observations of vertical surface displacement can be used to study terrestrial water storage (TWS) change after properly accounting for nonhydrological loading effects. This study systematically merged ground‐based GPS observations of vertical displacement into a land surface model in order to better estimate TWS. Assimilation was conducted across two snow‐dominated watersheds in the western United States using a one‐dimensional ensemble Kalman filter (EnkF). Modeled estimates were compared against TWS retrievals derived from the Gravity Recovery and Climate Experiment (GRACE) mission and in situ measurements of snow water equivalent (SWE), soil moisture, and runoff. The GPS data assimilation (GPS DA) technique improves prediction skill of TWS anomalies relative to the Open Loop (OL; model run without assimilation) when compared against GRACE TWS retrievals, especially during an extended drought period post‐2011 (e.g., correlation coefficient ROL = 0.46 and RGPSDA = 0.82 in the Great Basin). Furthermore, GPS DA improves SWE estimation with improved R values found over 76% and 69% of all pixels collocated with in situ stations in the Great Basin and Upper Colorado watersheds, respectively. GPS DA estimates of surface soil moisture and runoff, however, exhibit degraded performance relative to the OL due to the limited sensitivity of GPS observations to surface soil moisture variations and the lack of a dynamic surface routing scheme as well as other missing model physics (e.g., river and dam regulation, irrigation‐related water withdrawals, surface water impoundments) that are not included in the Catchment Land Surface Model.

Hydrological changes along the Jing River and its causal analysis

The hydrological response of rivers to upstream reservoir regulation is one of the increasingly important research issues of watershed management. In this study, a hydrodynamic coupling model was used to simulate water level and flow processes of the Jing river and diversion flow of three outlets before and after the operation of TGP (Three Gorges Projects). The results indicated that water level and flow of three diversion channels both decreased in flood and dry seasons: the compensation regulation of the TGP was significant to maintain the water level and flow of three diversion outlets in the Jing river, but it also increased zero-flow days in the diversion channels of the Yangtze River. The riverbed erosion of channels in the upper reaches as well as lower reaches caused hydraulic gradient varying so much that it can’t be ignored in the water exchange process between the Yangtze and Jing river. This mechanism cooperates with the operation of TGP to change the natural process of the Yangtze-Jing river’s hydrologic process. This work provides essential information for future water resources and environmental management of the river-lake system and facilitates the effective flow regulation of dams, which has significant implications not only for the Dongting river-lake system but also the similar lakes and rivers elsewhere.

Changes in planktonic and sediment bacterial communities under the highly regulated dam in the mid-part of the Three Gorges Reservoir.

Bacterial communities play an important role in the biogeochemical cycle in reservoir ecosystems. However, the dynamic changes in both planktonic and sediment bacterial communities in a highly regulated dam reservoir remain unclear. This study investigated the temporal distribution patterns of bacterial communities in a transition section of the Three Gorges Reservoir (TGR) using Illumina MiSeq sequencing. Results suggested that in comparison to the planktonic bacteria, sediment bacteria contributed more to the reservoir microbial communities, accounting for 97% of the 7434 OTUs. The Shannon diversity index in the water (3.22~5.68) was generally lower than that in the sediment (6.72~7.56). In the high water level period (January and March), Proteobacteria, Actinobacteria, Cyanobacteria, and Firmicutes were the most abundant phyla, whereas in the low water level period (May, July, and September), the dominant phyla were Proteobacteria, Actinobacteria, and Bacteroidetes. Sediment samples were dominated by Proteobacteria, Chloroflexi, and Acidobacteria. Principal coordinate analysis of the bacterioplankton communities showed greater sensitivity to monthly changes than that of the sediment bacterial communities. Network analysis suggested that in comparison to planktonic bacterial communities, sediment bacterial communities were more complex and stable. The linear relationship between the CH4/CO2 ratio, water level, and relative abundance of methanotrophs highlighted the potential methane-oxidizing process in the mid-part of the TGR. Moreover, the potential impact of dam regulation on the bacterial communities was revealed by the significant relationship between abundant phyla and the inflow of the TGR. KEY POINTS: • Bacterioplankton communities showed great sensitivity to monthly changes. • Potential methane-oxidizing process was revealed in this representative area. • Water inflow regulated by dam has significant effects on dominant bacterioplankton.

Landward shifts of the maximum accretion zone in the tidal reach of the Changjiang estuary following construction of the Three Gorges Dam

Impacts from anthropogenic activities have substantially modified the geomorphology of most of the world’s large rivers. While many studies have focused on the fluvially-dominated and estuarine/delta segments of these rivers, their tidal reaches that links the river to the estuarine delta is much less extensively documented. Yet, morphological variations in these key transition zones, however, directly affect the transfer of water and sediment to the sea, and have a significant influence on the delta environment. Here, we analyze the morphological variation of the Datong-Xuliujing Reach (DXR), the tidal reach of the Changjiang River, following the closure and operation of the Three Gorges Dam, using a unique dataset combining surveys in 1992, 2002, 2008 and 2013. The results demonstrate that the DXR exhibits three different morphological development phases. When sediment supply is high (at 3.18 × 108 t/y), the DXR experienced deposition (1992–2002) with the maximum accretion zone located in the middle portions of the reach. Thereafter (2002–2008) the channel underwent a major period of erosion coincident with a substantial decline of fluvial sediment supply (to 1.72 × 108 t/y). More recently (i.e., during 2008–2013), the entire reach experienced deposition, but with the maximum accretion zone shifting around 100 km landward (compared to its position in 1992–2002), while the riverine sediment supply was further reduced to 1.30 × 108 t/y. Our analytical modelling further reveals that a damped high fluvial discharge, induced by Three Gorges Dam regulation, and a relatively strong water level fluctuation induced by tidal forcing in the wet season, are responsible for the upstream shift in the maximum accretion zone. In addition, local variations caused by sand mining and dredging generate spatial nonuniformity in the observed patterns of erosion and deposition along the DXR. Such knowledge is of vital importance for the sustainable management of large alluvial rivers and their tidal reaches as they respond to natural and anthropogenic effects.