Hydrological Processes最新文献

Sap flow dynamics are critical for understanding how vegetation consumes water and adapts to environmental stress. The response of sap flow in boreal birch secondary forests to rainfall variations during the rainy season, however, has been inadequately explored. Our study indicated that photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) are the primary drivers of sap flow density in birch trees across different diameter classes (F_ds: small trees, F_dm: medium-sized trees, F_dl: large trees). Soil water content (SWC) significantly reduces sap flow when it falls below the 0.18 cm³/cm³. Sap flow density increased with PAR and initially with VPD but plateaued at higher VPD levels due to saturation. A hierarchy of sap flow density was observed, with F_dl > F_dm > F_ds, each responding differently to PAR, VPD and SWC. With decreasing rainfall across rainy seasons, the influence of PAR on F_ds and F_dm weakened, while the influence of VPD strengthened. For F_dl, the impact of VPD peaked and then declined, while the influence of PAR showed an inverse pattern. In the dry season, F_dl was primarily driven by PAR and influenced by VPD and SWC, whereas F_ds was mainly controlled by VPD, with minimal effects from PAR and SWC. The response of F_dm to SWC was similar to that of F_dl, but it mirrored the response of F_ds to PAR and VPD. These findings suggest that sap flow in boreal birch forests may become increasingly susceptible to SWC stress as global climate change intensifies.

Glacier meltwater runoff during extreme heat waves is crucial for overall runoff replenishment; however, studies on the characteristics and mechanisms of extreme meltwater runoff on the Tibetan Plateau (TP) are relatively scarce. In this study, we combine field observations (hydrological, meteorological, and glaciological) with a precipitation runoff modelling system and glacier model (PRMSglacier) to investigate the characteristics of extreme glacier meltwater runoff and the associated energy balance and hydrological processes from October 2018 to September 2022 in the Sangqu Basin on the central TP. Good agreement was shown between observed and modelled total runoff and glacier-wide mass balance, with a mean Nash–Sutcliffe efficiency (NSE) of 0.74 and root-mean-square error (RMSE) of 22 mm w.e. The mean glacial meltwater runoff contributed 14% of the total runoff and snowmelt runoff 72.5% during the study period. Contributions of 21.3% and 59% for glacier meltwater and snowmelt runoff, respectively, during a heatwave from June to September 2022 thus indicated anomalously high glacial meltwater and snowmelt runoff in association with hot and dry meteorological conditions. Basin-scale energy balance results suggest that extremely low albedo and extremely high surface temperatures control the net shortwave and longwave radiation, leading to anomalously high melting of glaciers and snow. The hot and dry meteorological conditions from June to September 2022 primarily affected the source regions of the Yangtze River and Selincuo in Geladandong. This study highlights the importance of extreme glacial meltwater runoff to terrestrial water resources in association with frequent extreme heat waves.

Oxbow lakes are iconic fluvial landforms found in the floodplains of meandering rivers around the world. Their formation is associated with meander cutoff, a process that excises sections of river channel to optimise the downstream transmission of water and sediment. Overbank floods and conveyance through tie channels maintain some hydrological connectivity, but lakes are generally considered to passively infill until they are terrestrialised. Here, a suite of 64 lakes across two meandering rivers in the Bolivian Amazon Basin are used to demonstrate the hydrological dynamism of oxbow lakes by quantifying interannual variations in lake water surface area (WSA), using the modified Normalised Difference Water Index (mNDWI) on an archive of Landsat images, and evaluating the mechanisms controlling these changes using remotely sensed rainfall data and geospatial analysis. The majority of lakes (75%) decreased in size over the study period, while 25% increased in size. The results suggest that WSA variations are controlled by proximity to the active channel, with the magnitude of these variations being set by mechanisms of connectivity. Lakes connected by tie channels experienced WSA changes up to 3.9 times larger than lakes with no visible connection mechanisms. Incursion lakes displayed similar WSA changes to those with tie channels, while isolated lakes were found furthest from the mainstem and had the smallest range of WSAs. Chute lakes experienced a wider range of WSA change (−95% to +281%) and were more strongly controlled by mainstem proximity than neck lakes. Connectivity between the river and oxbow lakes is essential for governing lake hydrodynamics, and tie channels provide the critical conduit by which water can be transmitted deep into the floodplain.

Karst landforms interfere with the runoff generation and confluence process, resulting in generally poor hydrological simulation accuracy in karst watersheds. We proposed a new karst hydrological module, which has two cores. One is the karst water storage capacity distribution curve that represents the distribution of runoff generation thresholds in karst areas, and the other is the underground nonclosure coefficient that represents the nonclosure phenomenon of underground watersheds in karst areas. The new module was further coupled with the Xinanjiang rainfall–runoff (XAJ) model to establish a complete hydrological model for karst areas (referred to as XAJ-karst model). The sensitivity of the XAJ-karst model parameters was analysed using the Sobol method, and applied to a typical karst watershed in Guizhou Province, China, to test the model performance on daily and hourly time scales. In addition, we also explored the impact of dynamic changes in the nonclosure coefficient of underground watershed area in karst watersheds on model results. Results showed that the average value of Kling–Gupta efficiency (KGE) of the XAJ-karst model on the daily and hourly time scales was 0.85 and 0.77, respectively. In comparison with the XAJ model, the average KGE value of the XAJ-karst model on both daily and hourly scales improved by 10.8% and 6.4%, respectively, demonstrating better simulation accuracy. In addition, there is a underground nonclosure phenomenon in the Xiangyang watershed, and the actual area of underground watershed expands abruptly as the antecedent-precipitation increases to the critical value. Moreover, the water storage and hysteresis effects of the karst landform result in a certain hysteresis in water exchange between the underground watershed and adjacent watersheds.

Vegetation plays a crucial role in mitigating and controlling soil erosion caused by overland flow. However, variations in the hydraulic characteristics of overland flow induced by the spatial distribution of vegetation with different row and column spacings are often overlooked in existing literature, potentially leading to significant deviations in predicting these characteristics. In this study, 180 lab-scale runoff tests were conducted to clarify the hydraulic characteristics of overland flow considering six α (the ratio of the lateral distance of vegetation to stem diameter) levels, six β (the ratio of the slope distance of vegetation to stem diameter) levels, and three slope angles (θ) under five flow discharges (Q) conditions. The results show that the observed flow regime of overland flow belongs to the transition flow regions, shifting from slow to rapid as α and/or β increase. The friction coefficient and the proportion of frictional resistance in the total flow resistance increase with increasing α and β. The local resistance dominates the total flow resistance of bare glass slopes. The local resistance coefficient ξ decreases with increasing α and β, however, it initially increases and then decreases with increasing θ. The impact of β on the local resistance is greater for gentle slopes, whereas the impact of α is more significant for steep slopes. ξ exhibits a negative correlation with Re and the ξ-Re curves gradually level off as α or β increases, while they become steeper with increasing θ. A prediction model for the total flow resistance was established taking into account the combined effects of Re, α, β and θ, which provides better prediction performance than two other relevant models. The results obtained from this study provide valuable insights into the hydraulic characteristics of overland flow and offer clear guidance for vegetation management in controlling soil erosion on slopes with heterogeneous vegetation coverage.

Managed aquifer recharge (MAR) is a strategy within water resources management. However, issues related to clogging have hindered its implementation. The change in permeability of the medium is significantly influenced not only by the macroscopic characteristics of infiltration sand, such as heterogeneity and anisotropy, but also by its microstructural features, including pore structure, morphology and connectivity. Nevertheless, the interactions between fluid flow, particle migration and changes in permeability remain unclear. This study investigates the pore-scale response mechanisms between fluid flow and pore clogging using a non-destructive x-ray computed tomography approach. Our findings indicate that the decrease in permeability due to particle deposition occurs in stages, with particles preferentially accumulating in irregularly shaped pores. The changes in the permeability of the sand column exhibit a negative correlation with alterations in shape factor and tortuosity, while showing a positive correlation with the fractal dimension. As pores become clogged with particles, the increase in tortuosity leads to a longer flow path. Once the sharp edges of the irregular pores are filled with particles, the pore space becomes smoother and more uniform, and the fractal dimension of the pores gradually decreases with further clogging. Based on numerical modelling of particle movement and the clogging process in porous media, it was determined that pressure is greatest in clogged pores. When this pressure reaches a certain threshold, the particles that were previously trapped in the pores are flushed out, leading to uneven changes in normalised hydraulic conductivity and normalised concentration at the outlet. If the pressure is insufficient to dislodge the clogging particles, the water flow path is compelled to change, resulting in a gradual stabilisation of the clogging.