Megadunes are common in large rivers, estuaries, and the continental shelf. They can be hundreds of meters long and several meters tall. Such bedform structures have been proven to play a crucial role in nutrient cycling and the maintenance of aquatic ecosystems. While porewater or hyporheic exchange has been extensively studied in smaller bedforms, these processes have yet to be studied in megadunes. We investigated megadune hyporheic exchange through modeling of surface water and groundwater flows over and through megadunes in a tidal inlet channel. The model was first validated against measured surface flow velocities during both flooding and ebb tides. Then we conducted paired simulations with and without higher-frequency topographic variations to isolate the contribution of smaller dunes. Also, water age was calculated to quantify how these compound dunes affect the pore water travel times. Results showed that the model could effectively capture the flow field over the megadune. The modeled pressure distribution along the sediment surface was dominated by the megadune; however, the superposed smaller dunes caused intense pressure fluctuations. The superimposed small dunes significantly intensify interfacial water flux across the sediment-water interface, with flux magnitudes reaching nearly two times that over smoothed bedforms and strongly influenced by flow direction and bedform asymmetry. Furthermore, water age modeling results showed that compound megadunes generate complex multiscale interfacial exchange zones with residence times spanning minutes to decades. Thus, these megadunes likely host unique ecological and biogeochemical processes that have gone unrecognized.

Understanding the spatial variation of streambed sediment median grain size (d 50) is essential for understanding hydrological, geomorphological, and ecological processes, yet network-scale patterns remain poorly characterized. Here we combined extensive field sampling (249 locations) with Digital Grain Size analysis and laboratory sieving to build a high-precision d 50 database across the 3056 km Xin'anjiang (XAJ) stream network in southeastern China. Using a Spatial Stream Network (SSN) geostatistical model, we produced 1 km resolution predictions of d 50 (R 2 = 0.92) that explicitly incorporate hydrologic connectivity and spatial autocorrelation. Results reveal high spatial heterogeneity (0.22-100.28 mm), with systematic downstream fining punctuated by stepwise anomalies at tributary confluences. d 50 decreases with stream order but increases with flow distance to the outlet, indicating the combined roles of geomorphic scaling and sediment supply contrasts. Sensitivity analysis shows that prediction uncertainty grows as sampling density decreases; a similar to 15 km interval provides reliable estimates, while denser sampling is needed near confluences and morphologically complex reaches. This study establishes a transferable framework for network-scale sediment analysis, advancing understanding of spatial grain-size dynamics and guiding efficient sampling strategies in diverse stream networks.

Increasing global warming and urbanization have significantly altered the urban living environment, exposing more urban residents to the risk of extreme weather disasters, such as extremely high temperatures and heat waves in summer. Although the effect of urbanization on local climate change has been widely considered, human-perceived temperature (HPT) changes, which are jointly influenced by the air temperature (AT), humidity, and wind, remain poorly understood and should be explored further, especially in rapidly urbanizing megacities. In this study, we investigated the spatiotemporal changes in two typical types of HPT factors (apparent temperature (AP) and wet bulb temperature (WBT)) in mainland China from 1960 to 2019. We further examined the effects of urbanization on the HPT and its potential driving factors. The results revealed that (1) the AT and HPT increased significantly (p < 0.05) in most parts of mainland China, especially after 1985. (2) Urban areas demonstrated higher AT and HPT values than did their surrounding rural areas in most mega-provincial capitals. The urban-rural temperature difference (Delta T) exhibited significant increasing trends (p < 0.05), with greater values in northern cities than in southern cities. (3) Urbanization more notably affected the AP than the AT. Vegetation cover reduction dominated the increasing intensification of Delta T in southern cities (southern vs. northern cities: 24 % vs. 6 % for Delta AT, 23 % vs. 5 % for Delta AP, and 20 % vs. 4 % for Delta WBT), whereas anthropogenic heat emissions (represented by the population density) dominated this phenomenon in northern cities (northern vs. southern cities: 48 % vs. 18 % for Delta AT, 53 % vs. 15 % for Delta AP, and 55 % vs. 15 % for Delta WBT). Our findings emphasize the necessity of considering the combined effect of multiple meteorological factors on human health and provide sound support for developing climate adaptation strategies to mitigate heat stress of residents.

Bedform-induced variations in pressure along streambeds can stimulate vertical hyporheic exchange in the hyporheic zone (HZ). The role of HZ in supporting riverine ecosystems depends on the spatial extent of HZ and the magnitude of mass and energy exchange rates. However, the quantification of the spatial extent of bedforminduced HZ remains less constrained, especially for the heterogeneous subsurface settings. This study therefore develops a streamline-based method to map the HZ. The identification process is fulfilled by analyzing the subsurface flow field based on simulating both of surface and subsurface fluid flow. Given that there is no flux exchange across the interface of HZ and underflow, the interface can be interpreted as the global minimum positive value of vertically-integrated flux for the entire domain. Like traditional particle tracking method, our method is tested to be effective to delineate the HZ for homogeneous cases, where the volume, mean residence time, and maximum depth of HZ can be directly estimated. Notably, our approach outperforms traditional methods for the heterogeneous porous and fractured streambeds, since the new method is viable to cover the entire stagnation zone within the HZ, which is critical for anaerobic reactions.

Urban lakes contribute to global greenhouse gas (GHG) emissions driven by both natural processes and anthropogenic activities. In this study, we conducted seasonal sampling and analysis of GHG concentrations and water chemistries in Xuanwu Lake, Nanjing, China. Concurrently, we observed pore water chemistry within the lake bottom sediments. Radon isotope activity in lake water was also measured. Then, this study expanded to a broad understanding of urban lake GHG emissions by conducting a meta-analysis of over 100 lakes of similar size but various types (urban vs. non-urban). Xuanwu Lake is a net source of GHG, with mean annual diffusive fluxes of 10.2 ± 9.3 mmol∙m−2∙d−1 for CO2, 3.1 ± 0.3 mmol∙m−2∙d−1 for CH4, and 12.4 ± 1.0 μmol∙m−2∙d−1 for N2O. The lake emitted 614.9 tons of CO2, 68.6 tons of CH4, and 0.84 tons of N2O throughout the year. CO2 levels were positively correlated with dissolved organic and inorganic carbon, while CH4 peaked in winter due to increased anaerobic decomposition. N2O concentrations were strongly linked to nutrient levels. Furthermore, statistical analysis showed that urban lakes demonstrated significantly greater CH4 and N2O emissions compared to non-urban lakes. These findings emphasize the need for further research and targeted mitigation strategies to address GHG emissions from urban lakes, especially in the context of increasing anthropogenic pressures and climate change.

Understanding the dynamics of pCO2 and pCH4 is important for evaluating carbon emissions from the aquatic environment. While temporal dynamics of pCO2 and pCH4 have been extensively studied, there is a noticeable gap in the literature concerning their spatial characteristics. In this study, we used boat-mounted sensors to directly measure pCO2 and pCH4 with high spatial resolution across seasons in Xuanwu Lake (XWL), an urban lake in Nanjing, China. Additionally, water chemistries were measured at selected sites for correlation analysis. Sensitivity analysis was performed to assess the effects of measurement location and density on carbon flux estimates. Results show that continuous on-board measurements innovatively captured the spatial distribution of pCO2 and pCH4. Both gases varied significantly across seasons, exhibiting pronounced spatial heterogeneity. Peak emissions occurred in summer, with the lowest CO2 in spring and CH4 in fall. Both pCO2 and pCH4 increased from the lake center to the shoreline, with the largest fluctuations near the shore. In total, XWL acted as a net source of CO2 and CH4, with mean diffusive fluxes of 11.4 +/- 10.1 and 3.0 +/- 0.3 mmol center dot m- 2 center dot d- 1, respectively. Furthermore, sensitivity analysis showed that lake-wide flux calculations depend on measurement locations and sample size. pCO2 exhibited greater heterogeneity than pCH4, necessitating different sampling thresholds. The optimal threshold for capturing lake-wide CO2 flux was 18-30 samples per km2, while this density was sufficient for CH4 flux estimates. Our study highlights that both CO2 and CH4 exhibit significant spatial and temporal heterogeneity, necessitating high-resolution sampling for accurate flux assessments. The sampling strategy presented could guide future studies, as sampling in the intermediate zone effectively reduces the number of samples needed while maintaining accuracy.

Many rivers are subject to ice-covered flow conditions in cold climate regions or during the winter months. The presence of ice cover can change stream flow hydrodynamics and thus affect the hyporheic exchange involved in many biogeochemical processes in aquatic environments. Among the various features of ice cover, its underside roughness is a main factor modifying the surface–subsurface flow hydrodynamics. To date, there has been little research on how ice cover affects the hyporheic exchange, even less on the effect of the ice cover roughness. This leaves open questions on the mechanical changes initiated at the ice-water interface and transferred to the water–sediment interface. In this study, a coupled surface–subsurface computational approach was used to model conjunctively the channel flow over dune bed and the underlying porous flow under different ice cover conditions. The results show that ice cover can enhance the hyporheic flux and reduce the hyporheic zone depth when compared to open channel flow at equivalent discharge. As the river flow depth increases to 4 times the height of the dune, the effect of the ice cover on the hyporheic flow becomes negligible. Within this effective range, the hyporheic flux via the channel water depth follows a power-law function. Ice cover roughness greatly augments the exchange rate while compresses the exchange space, therefore reduces flow residence time in the hyporheic zone. Based on the modeling results, prediction models were proposed for evaluating the impacts of ice cover roughness on the hyporheic exchange flux and depth. Our study indicates that hyporheic flow exchange is significantly impacted by the roughness of the ice cover which in turn the hyporheic exchange is likely to affect the river ice processes as well as the water quality and ecological functions throughout the river corridors.

The release of carbon dioxide (CO2) from lakes is a critical element of carbon (C) emissions from inland waters. Within the realm of climate change, the inquiries surrounding whether lakes on the Tibetan Plateau (TP) function as C sources or sinks and the magnitude of CO2 exchange flux from these lakes have garnered significant attentions. Nevertheless, accurately assessing the lakes' contribution to the C budgets poses challenges due to data scarcity and methodological inaccuracies. By amalgamating data from literature reviews and field measurements for different sizes of lakes during the ice-free (IF) and ice-covered (IC) periods from 2016 to 2021, this study offers a refined estimate of the CO2 exchange flux and flux rate for lakes on the TP by including lakes ranging in size from 0.01 to 1 km(2) (small lakes) in the C budgets. Findings revealed that the annual CO2 exchange flux of TP lakes amounted to 7.10 Tg C yr(-1), with 6.56 Tg C yr(-1) and 0.54 Tg C yr(-1) during the IF and IC periods, respectively. Notably, small lakes contributed 0.76 Tg C yr(-1), representing 10.65 % of the total lake CO2 emissions on the TP, which indicates the significant role of small lakes in estimating CO2 emissions from TP lakes. The CO2 exchange fluxes of small lakes showed significant variability during the IF period, with the origins of lake water replenishment possibly explaining this diversity, where glacial meltwater replenishment is likely a key contributing factor. In contrast, CO2 emissions from small lakes increased during the IC period. The view of this study is that the groundwater recharge with higher CO2 concentrations and the shallow nature of small lakes may be the main reasons for the increase in CO2 emissions from small lakes during this period. The study underscores that the contribution of small lakes to the CO2 budgets of TP lakes is substantial and warrants attention, particularly in elucidating the mechanisms driving CO2 emissions from small lakes.

Groundwater discharge flux into rivers (riverine groundwater discharge or RGD) is essential information for the conservation and management of aquatic ecosystems and resources. One way to estimate area‐integrated groundwater discharge into surface water bodies is to measure the concentration of a groundwater tracer within the water body. We assessed groundwater discharge using 222Rn, a tracer common in many surface water studies, through field measurements, surface water 222Rn mass balance model, and groundwater flow simulation, for the seldom studied but ubiquitous setting of a flooding river corridor. The investigation was conducted at the dam‐regulated Lower Colorado River (LCR) in Austin, Texas, USA. We found that 222Rn in both the river water and groundwater in the river bank changed synchronously over a 12‐hour flood cycle. A 222Rn mass balance model allowed for estimation of groundwater discharge into a 500‐m long reach of the LCR over the flood. The groundwater discharge ranged between negative values (indicating recharge) to 1570 m3/h; groundwater discharge from groundwater flow simulations corroborated these estimates. However, for the dynamic groundwater discharge estimated by the 222Rn box model, assuming whether the groundwater 222Rn endmember was constant or dynamic led to notably different results. The resultant groundwater discharge estimates are also highly sensitive to river 222Rn values. We thus recommend that when using this approach to accurately characterize dynamic groundwater discharge, the 222Rn in near‐stream groundwater should be monitored at the same frequency as river 222Rn. If this is not possible, the 222Rn method can still provide reasonable but approximate groundwater discharge given background information on surface water‐groundwater exchange time scales.

Study region: The upper Brahmaputra River, China. Study focus: The long-term water level and storage variations of the upper Brahmaputra River (UBR) are not yet clear due to the low accessibility of gauged data in high-altitude and harsh environments. The main objective of this study is to facilitate investigations of long-term river water level and storage variations over the UBR. Here we developed a hypsometric method to reconstruct temporally resolved long-term river water level observations based on remote sensing-derived accumulative water inundation area information and short-term altimetric water level data at 24 virtual stations (VSs) of the UBR. We further analyzed the intra-annual and inter annual variations of the water level (storage) changes of the studied river reaches. New hydrological insights: The results reveal that the reconstructed water levels of the UBR are quite consistent with the water levels from the Hydroweb with an average R-2 of 0.79 and the reconstructed water levels are generally consistent with the variation of the gauged runoff. The ascending amplitudes of the annual water level cycle of VSs were observed from upstream to downstream of the UBR, varying from 0.58 m to 6.92 m. Additionally, we find that the seasonal change in river water storage approximately accounted for one-tenth of the net mass seasonal amplitude of the UBR basin, as revealed from the Gravity Recovery and Climate Experiment (GRACE) data.

It has been known that groundwater-lake water interaction plays an important role in mass exchange between land and lakes. However, most of the lakes in the world freeze during the winter, and few studies of groundwater and surface water interaction (groundwater discharge tracing) focus on ice-covered lakes. This study investigated groundwater discharge into a large ice-covered lake, Xiao Qaidam Lake in the Tibetan Plateau (TP), using sat-ellite remote sensing and chemical and isotopic tracing. Analysis of chemical components, stable isotopes (D, O-18) and Rn-222 revealed that groundwater discharge occurred in the western and southern portions of Xiao Qaidam Lake. Remote sensing data (the distribution of and variation in lake surface temperature, LST) revealed these locations of groundwater discharge and indicated additional groundwater discharge in areas of springs. Based on the regression model between LST and Rn-222 activity in lake water and 222Rn mass balance model, the groundwater discharge into Xiao Qaidam Lake was estimated to be (2.77 +/- 0.26) x 10(5) m(3)/d, which is slightly higher than river water input at summer. In addition, this study also suggests that ice melting process should be considered when tracing groundwater discharge into an ice-covered lake, and that the LST distribution during the icing period is better than the LST variation for tracing groundwater discharge because the ice eliminated noise from wind and surface water. The study provides insight into the tracing of groundwater discharge into large ice-covered lakes in the TP and elsewhere.

Dynamic hydrologic exchange flows in river beds and banks are important for many ecosystem functions throughout river corridors. Here we test whether the exchanges and the associated mixing between a flooding river and groundwater within the river’s bank can be effectively traced by Radon‐222 ( 222 Rn), a naturally occurring, inert, radiogenic, and radioactive gas that can be analyzed and monitored in situ. The assessment was done by simulation of groundwater flow and reactive transport of 222 Rn in the bank following a single, relatively rapid (hours long) flood wave and auxiliary field observations of 222 Rn, temperature and total dissolved solids (a surrogate for any ionic conservative tracer). Results illustrate that 222 Rn is more effective than temperature and total dissolved solids in tracing dynamic hyporheic exchange. 222 Rn variations in space and time are larger than the analytical uncertainty of common measurement methods. The individual effects of aquifer hydraulic conductivity, dispersivity, river water 222 Rn concentration, and bank topography were analyzed through sensitivity analysis. Larger hydraulic conductivity and dispersivity, lower 222 Rn concentration in river water relative to groundwater, and gentler bank slopes resulted in a more prominent and traceable 222 Rn signal. The transport and residence time of exchanged water may be estimated and interpreted using reactive transport models such as those implemented here. However, such application is sensitive to fluctuations in river water 222 Rn, requiring it to be well characterized. The assessment provides guidance for using 222 Rn as a tracer for groundwater and surface water interactions in dynamic settings.

Flow‐vegetation interaction affects fluid flow hydraulics and associated material transport in river corridors. Concomitant changes in pressure within the flow field due to the presence of vegetation may act as a driver for the formation of hyporheic flow across the sediment‐water interface. This potentially important process, however, has yet to be studied. In order to investigate vegetation‐induced hyporheic exchange, a series of numerical models of interlinked surface‐subsurface flow modified by plant stems was conducted. Periodically staggered plant stem arrays on a flat sediment bed were considered within a coupled multiphysics computational fluid dynamics approach. Plants were idealized as rigid cylinders and arranged in different streamwise and spanwise spacing distances. Each vegetation array was then subjected to a broad range of flow Reynolds Numbers (Re). The results showed that hyporheic flow occurs in all conditions with the presence of vegetation. The vegetation‐induced hyporheic flux is found to be a function of Re via a power law. The flux increases with interstem space until the space reaches the distance that rigid stems no longer affect the flow structures in the vicinity of each other. Larger intervegetation distances lead to a larger hyporheic zone. A direct comparison with bedform‐induced hyporheic flow showed that vegetation can induce higher hyporheic flux through relatively shallower exchange zones. The results of all the simulations were synthesized into predictive models for hyporheic flux, bulk residence time and exchange depth based on drag coefficient, vegetation density, and Reynolds Number.

River-floodplain connectivity is vital to hydrological and biogeochemical processes in river corridors over multiple spatiotemporal scales. In the present study, an approach of particle tracking in conjunction with two-dimensional hydrodynamic modeling was developed to study the dynamics of river-floodplain connectivity during flood periods and applied to the study of McCarran Ranch in the lower Truckee River, Nevada, U.S. The hydrodynamic model was calibrated and validated for flood events of different magnitudes of peak flow discharge. The outputs of the hydrodynamic model were further applied to determine the lateral transboundary flux during high flows. Lagrangian particle trajectories were then performed to quantify the flow path length and residence time on floodplains. The exchange frequency between the river mainstem and the floodplain were also examined. The results show that the hydrodynamic model reproduce hydrographs well. The river-floodplain exchange is sensitive to the magnitudes of the flood events. Larger floods cause shorter residence times and flow path lengths on the floodplains but lead to larger transboundary flux within the river reaches. The particle analysis results also show that higher discharge introduces less frequent exchange between the river channel and floodplains. Furthermore, comparison of the river-floodplain exchange properties among river reaches with different levels of sinuosity show that meandering reaches can introduce considerably higher exchange flux than a straight reach, leading to complex exchange behaviors within the river-floodplain system.

针对源区河流氮素时空分布规律,选择位于安徽省休宁县境内上溪口地区新安江源头段作为试验研究对象,通过野外采样、水质监测和统计分析方法,研究试验区域洪水前后各项监测指标的时空分布特征及其影响因素,揭示洪水对新安江源区河流氮素的影响.结果表明:流域水体总氮浓度沿水流路径逐渐增加;洪水后总氮、氨氮和硝氮浓度比洪水前均有所增大,洪水期雨水对流域的氮素进行了"冲刷";相关性分析表明洪水前硝氮和总氮受植被影响较大,洪水后硝氮与汇水面积相关,总氮浓度受高程、坡度和地质等因素影响;所构建的预测模型能够较好地预测洪水前硝氮和总氮以及洪水后氨氮和硝氮的分布.

The hyporheic exchange plays a crucial role in maintaining fluvial ecological processes. In order to examine the influence of streambed heterogeneity on the hyporheic exchange,a surface water-groundwater coupling model is constructed for solving flow interactions over a sand dune. By generating different permeability random fields,the influences of hydrodynamic processes and streambed heterogeneity on the hyporheic exchange flux,exchange space and mean residence time are addressed. The results showed that the coupling model has good performance and it can accurately depict the flow fields near the sediment-water interface. Under homogeneous or heterogeneous scenarios,the mean exchange flux and bulk residence time via Reynolds number are representing power law relationships,and the mean exchanged depth tends to be stable after the surface flow transitions into highly turbulent conditions. Results further shown that higher heterogeneity can effectively enhance the hyporheic exchange flux and spatial exchange frequency along the sediment-water interface. However,it will restrict the exchange volume and shorten the mean residence time of flow in the hyporheic zone.

This study analyses the changes in sediment transport regimes in the middle Yellow River basin (MYRB) using sediment rating parameters. Daily streamflow and suspended sediment concentration data were collected at 35 hydrological stations from the 1950s to 2016, which can be divided into three periods based on the type and intensity of human activities: the base stage before 1970, the restraining stage from 1971 to 1989, and the restoration stage after 2002. Data within each period were fitted by log‐linear sediment rating curves and the sediment rating parameters were utilized to analyse the spatial and temporal variations in sediment transport regimes. The results show that sediment rating parameters are indicative of sediment transport regimes. In the base stage and the restraining stage, the hydrological stations can be categorized into four groups based on their locations on the rating parameter plot. The stations with small drainage basins were characterized by the highest sediment transport regime, followed by those located in the coarse‐particle zone, the loess zone, and the mountainous/forest zone. In the restoration stage, the difference in sediment transport regimes between different geomorphic zones became less distinguishable than in previous stages. During the transition from the base stage to the restraining stage, sediment rating parameters showed no significant changes in sediment transport regimes in all four geomorphic groups. During the transition from the restraining stage to the restoration stage, significant changes were observed in the coarse‐particle zone and the mountain/forest zone, indicating that the revegetation programme and large reservoirs imposed a stronger influence on sediment transport regimes in these two zones than in the rest of the MYRB. This study provides theoretical support for evaluating sediment transport regimes with sediment rating parameters.

We examined the dynamic processes of hyporheic exchange and temperature distribution in a riparian zone in response to low-temperature water fluctuations downstream of the Xin’an River Dam, China, using analytical and mainly hydrodynamic methods. For this purpose, we installed six HM21 piezometers (R, P1–P5) between the river water and the groundwater at an interval of approximately 2 m perpendicular to the flow path. We also installed 20 PT100 thermistors (T1–T20) along the transect at depths of 1.19 m to 3.58 m and monitored the temperatures of river and air. Water levels and temperatures were automatically logged every 5 min by the real-time system from November to December 2014 and sent to the remote platform through the remote terminal unit. Results revealed that the intensity and direction of the hyporheic exchange ( Q ) between the river water and the groundwater varied periodically (t = 1 d) with the water level of the river. In each cycle, the Q was in a counterclockwise loop curve with the water level of the river and with the non-uniform distribution along the transect perpendicular to the river, which showed that the farther the lateral exchange was away from the river, the lower its intensity and the more hysteretic the alteration of its direction. The daily exchange width and residence time had no necessary connection with the average river stage, but mainly depended on the amplitude of the fluctuating river stage and the duration of river infiltration and established a strong linear relationship with their product. The temperature distribution of the riparian aquifer was mainly affected by the surface radiation and river water infiltration. It was characterized as “cool on the surface and warm at the bottom” in the vertical direction and could be divided into low-, medium-, and high-temperature zones along the horizontal direction. The horizontal infiltration distance ( L ) increased by power functions with the increase in infiltration rate ( v ) and decrease in river temperature ( T ).

土壤水力参数是表征土壤水力性质的重要参量,是准确预测土壤水分及溶质运移等过程的基础.基于GML非线性参数优化算法,构造了三个不同的目标函数,同时反演van Genuchten-Mualem模型中三个土壤水力参数(进气吸力倒数α、孔隙体积大小分布指数n和饱和导水率Ks),并分析了同一目标函数下不同降雨条件对反演结果的影响.结果表明,选取不同的目标函数对反演结果有显著影响,以累积入渗量作为目标函数能有效反演参数Ks,以压力水头和累积入渗量作为多目标函数则能同时有效反演α、n、Ks三个参数;此外,降雨条件对反演结果也有一定影响,相对于均匀降雨,在非均匀降雨条件下以压力水头作为目标函数对反演结果的精确度和准确度有较大的提升;以累积入渗量作为目标函数对反演结果的精确度略有提升但降低了反演结果的准确度.