An analysis of long-term databases with information on precipitation and discharge records was undertaken to characterize the temporal structure response of four experimental catchments, located in ...the Central Spanish Pyrenees, with a gradient of land-cover (from a relatively pristine forested catchment, through an abandoned cultivated catchment with progressive plant recolonization, to an afforested catchment and ending with a degraded badlands catchment). Precipitation and discharge records are non-stationary and the wavelet transform methodology was thus applied to perform a temporal scale-by-scale analysis of each catchment response to the hydroclimatic characteristics of the area. This temporal decomposition analysis illustrates how land-use and land-cover legacy control the temporal distribution of flow events occurring at different and non-similar time-scales, thus reflecting the timing, variability and physical mechanisms of water storage/transport in each catchment. Intra-annual and annual time-scales are led by climatological characteristics of the catchment sites (seasonal patterns of mountainous Pyrenees catchments). Multi-year scale is mainly shaped by land-cover and land-use legacy. Badlands catchment, with its large proportion of bare land, shows a discharge response closely synchronized with precipitation patterns for all time-scales. On the contrary, for the forested catchment the global hydrological response is mainly governed by the multi-year time-scale. Afforested catchment and abandoned cultivated catchment, which move towards a pristine forest response, are impacted by the former grazing and agriculture activities and intra-annual temporal variability still play a major role on the global discharge response of the catchment. This suggests that vegetated catchments located in the same region can show hydrological responses at different time-scales to the same climatic input. We argue that differences in land-cover and historical land-use changes are not only valuable to understand the current discharge temporal behaviour, but they will also play a significant role in characterizing the future catchment dynamics due to changing climate conditions.
•Long-term hydrological records of four Pyrenean catchments with a gradient of land cover is reviewed.•Wavelet-based technique decomposes in non-similar time-scales the hydrometric time-series.•Time-scales structure unveils the bonds between discharge and catchment landscape traits.•Land-cover and also land-use legacy shape the magnitude of the largest time-scale.
Sediment transport is the main driver of the channel morphology and landscape evolution, with implications for chemical and biological river processes, and human‐related activities. Understanding the ...processes governing the relations between discharge and suspended sediments is essential for the management of river catchments and river networks. Here, we use the method of wavelet transformation to identify the time‐scale dependency of suspended sediment patterns concerning the temporally and spatially uneven transient processes of sediment production, accumulation, and transport. We analyze the temporal variation of concurrent discharge and suspended sediment fluxes for the Upper Changjiang (Yangtze River, China) at Pingshan station by using a long‐term database collected for over 50 years. Furthermore, we bridge the limitations of pure predictive models to learn from temporal data structures with the main purpose of identifying the mechanisms underpinning the suspended sediment patterns (e.g., climatic forces). Intraseasonal‐to‐seasonal, annual, and inter‐annual dominant time‐scales are thus identified. The short time‐scales are driven by the bi‐modal seasonal precipitation pattern specific to the climate of the region and provide a continuous supply of sediments to the river. The large time‐scales, controlled by high magnitude flow events and within‐reach sediment storage, display alternating periods of increasing and decreasing sediment fluxes; ultimately, they maintain the river channel within balance or within a moderate positive sediment accumulation process. This analysis and methodology help to understand temporal sediment dynamics, and ultimately to manage river catchments.
Key Points
Distinct time‐scales of discharge and suspended sediment concentration (SSC) are identified and related qualitatively to seasonal, annual, and short (1.1–5 years) and long‐term (5.5–20 years) variability
Similar dominant time‐scales of variability for discharge and SSC are identified in the Upper Changjiang for a 50‐year study period
Bi‐modal seasonal climatic pattern influence short‐term time‐scales whereas high magnitude flow events control intra‐annual time‐scales
Recent advances in the simulation of free surface flows over mobile bed have shown that accurate and stable results in realistic problems can be provided if an appropriate coupling between the ...shallow water equations (SWE) and the Exner equation is performed. This coupling can be done if using a suitable Jacobian matrix. As a result, faithful numerical predictions are available for a wide range of flow conditions and empirical bed load discharge formulations, allowing to investigate the best option in each case study, which is mandatory in these type of environmental problems. When coupling the equations, the SWE are considered but including an extra conservation law for the sediment dynamics. In this way the computational cost may become unrealistic in situations where the application of the SWE over rigid bed can be used involving large time and space scales without giving up to the adequate level of mesh refinement. Therefore, for restoring the numerical efficiency, the coupling technique is simplified, not decreasing the number of waves involved in the Riemann problem but simplifying their definitions. The effects of the approximations made are tested against experimental data which include transient problems over erodible bed. The simplified model is formulated under a general framework able to insert any desirable discharge solid load formula.
Turbulent shallow flows are characterized by the presence of horizontal large-scale vortices, caused by local variations of the velocity field. Apart from these 2D large vortices, small scale 3D ...turbulence, mainly produced by the interaction of the flowing water with the solid boundaries, is also present. The energy spectrum of turbulent shallow flows shows the presence of a 2D energy cascade at low wave numbers and a 3D energy cascade at high wave numbers, with a well-defined separation region between them. Horizontal flow movements (e.g. 2D large-scale vortical structures) at low wave numbers mostly determine the hydrodynamic behavior of these flows. Moreover, the generation of standing waves often occurs closely associated to the interaction of 2D horizontal flows with lateral boundaries, this is the case of seiches. To adequately reproduce these phenomena, a mathematical and numerical model able to resolve 2D turbulence is required. We herein show that depth-averaged (DA) unsteady Reynolds averaged Navier Stokes (URANS) models based on the Shallow Water Equations (SWE) are a suitable choice for the resolution of turbulent shallow flows with sufficient accuracy in an affordable computational time. The 3D small-scale vortices are modeled by means of diffusion terms, whereas the 2D large-scales are resolved. A high order numerical scheme is required for the resolution of 2D large eddies. In this work, we design a DA-URANS model based on a high order augmented WENO-ADER scheme. The mathematical model and numerical scheme are validated against observation of complex experiments in an open channel with lateral cavities that involve the presence of resonant phenomena (seiching). The numerical results evidence that the model accurately reproduces both longitudinal and transversal resonant waves and provides an accurate description of the flow field. The high order WENO-ADER scheme combined with a SWE model allows to obtain a powerful, reliable and efficient URANS simulation tool.
•Augmented WENO-ADER scheme applied to the resolution of turbulent shallow flows.•Non-linear limiting prevents an accurate approximation of the diffusion terms.•2D turbulence is mostly resolved and 3D turbulence is modeled.•The subgrid models and the high order scheme allow relatively coarse meshes.•Complex transient resonant wave phenomena are accurately reproduced.
In this work approximate augmented Riemann solvers are formulated providing appropriate numerical schemes for mathematical models of granular flow on irregular steep slopes. Fluxes and source terms ...are discretized to ensure steady state configurations including correct modeling of start/stop flow conditions, both in a global and a local system of coordinates. The weak solutions presented involve the effect of bed slope in pressure distribution and frictional effects by means of the adequate gravity acceleration components. The numerical solvers proposed are first tested against 1D cases with exact solution and then their results are compared with experimental data in order to check the suitability of the mathematical models described in this work. Comparisons between results provided when using global and local system of coordinates are presented. The obtained results point out that both the global and the local system of coordinates can be used to predict faithfully the overall behavior of the phenomena considered in this work.
River discharge has experienced diverse changes in the last decades due to modification of hydrological patterns, anthropogenic intervention, re‐vegetation or annual and interannual climatic and ...atmospheric fluctuations. Assessing the recent changes in river discharge and understanding the main drivers of these changes is thus extremely important from theoretical and applied points of view. More specifically, here we want to draw attention toward the impacts of streamflow changes on reservoir storage and operation. We describe the hydrological dynamics of the Yesa reservoir draining catchment, located in the central Spanish Pyrenees, and characterize the reservoir operation modes over the last 60 years (1956–2020). We analyze concurrent climatic (precipitation, air temperature, drought index), atmospheric mechanisms, land cover (Normalized Different Vegetation Index) and discharge (inlet and outlet of Yesa reservoir) time‐series. By using the wavelet transform methodology, we detect historical breakpoints in the hydrological dynamics at different time‐scales. Distinctive periods are thus identified. More regular seasonal flows characterized the catchment's dynamics during the first decades of the study period, while the last decades were characterized by a high inter‐annual variability. These changes are primarily attributed to the natural re‐vegetation process that the catchment experienced. Furthermore, we related changes in atmospheric circulation with a decline of the long‐term discharge temporal features. This research contributes to the understanding of long‐term river discharge changes and helps to improve the reservoir management practices.
Key Points
Incoming river flow to Yesa reservoir dwindled as vegetation cover increased
Inter‐annual time‐scales increased their control of the annual discharge
The is a growing influence of Mediterranean atmospheric dynamics on the river flow discharge
The dynamics of blue and green water partitioning under vegetation and climate change, as well as their different interactions during wet and dry periods, are poorly understood in the literature. We ...analyzed the impact of vegetation changes on blue water generation in a central Spanish Pyrenees basin undergoing intense afforestation. We found that vegetation change is a key driver of large decreases in blue water availability. The effect of vegetation increase is amplified during dry years, and mainly during the dry season, with streamflow reductions of more than 50%. This pattern can be attributed primarily to increased plant water consumption. Our findings highlight the importance of vegetation changes in reinforcing the decrease in water resource availability. With aridity expected to rise in southern Europe over the next few decades, interactions between climate and land management practices appear to be amplifying future hydrological drought risk in the region.
Plain Language Summary
Forest recovery, due to human land abandonment, has been observed in several regions worldwide. To improve integrated land and water management, it is crucial to explore how these changes affect resource availability in water‐stressed areas. Forest regeneration has resulted in a large decrease in streamflow in a natural catchment in southwestern Europe, which cannot be explained by climate change. During the dry season, forest regeneration amplifies the impact of drought and water availability, with less impacts in the wet season. Therefore, the effects of vegetation recovery on water resources differ based on water availability, with the most serious implications for water resources occurring during dry periods.
Key Points
Forest secondary succession is the main driver of streamflow trends in mountain Mediterranean areas
The effects of vegetation changes on water availability strongly differ between dry and humid periods
Trends in streamflow in response to vegetation changes are mostly recorded during the dry and warm season
Mediterranean mountain forests play a significant role in hydrological regulation. In this study, hydrological dynamics was examined at different temporal scales in a small mountain forest catchment ...in the Central Spanish Pyrenees (San Salvador), based on a 20-year dataset (1999–2019). Mean annual runoff coefficient is 0.21, and ranged from 0.02 to 0.58. The catchment has a bi-modal hydrological behavior with two hydrological periods: a dry-period between July and December, and a wet-period between January and June. During the study period, only 108 floods were recorded, suggesting a low responsiveness of the catchment, with a high variable response. Spearman correlation analysis and stepwise multivariate regression suggest that the hydrological response in the San Salvador catchment is mainly depending on water table, with antecedent moisture conditions and rainfall depth as secondary factors. Seasonal differences were also observed: during dry season, the response was mainly related to rainfall depth and rainfall intensity; in contrast in wet season, the response was mainly related to antecedent conditions (previous rainfall and base flow). Thus, the already challenging water resources management in the Mediterranean basin is magnified by the key function of forests as natural modulators of water cycle. Consequently, the study of natural forested catchments is needed and long-datasets have to be analysed to understand the role of natural Mediterranean forest in the hydrological dynamics and its evolution and adaptation in a context of Global Change.
Display omitted
•Long-term hydrological records of a small mountain forest catchment are analysed.•Differences in catchment response are identified at seasonal time-scales.•A low responsiveness has been observed due to the forest cover and deeps soils.•A complex response has been observed mainly depending on antecedent moisture conditions and rainfall depth.•Slow rise and prolonged recession limbs suggest that subsurface flow is the dominant process.
•In-cavity sediment deposits are highly resilient to the high flow events.•The mobilization of the in-cavity sediments depends on the geometry of the cavities and on the hydrograph applied.•The ...magnitude of the disruptive phase is the most relevant parameter for the in-cavity sediments mobilization.
Lateral cavities are built in the banks of rivers for several purposes: to create harbors, to capture sediment, to keep a central navigable channel (i.e., Casiers de Girardon in the Rhone river) or to promote the formation of aquatic habitats if a limited amount of sediment is captured, providing hydraulic and morphologic diversity (i.e., the case of Japanese Wandos). This work is focused on this latter purpose: promotion of hydraulic and morphologic diversity. In these scenarios, an increase in the flow discharge in the main channel may, however, re-mobilize the deposit of sediment inside these lateral embayments and cause a sudden increase of the sediment concentration and turbidity in the main channel. It is thus of interest to characterize the resistance and resilience of these sedimentary deposits when the main channel is subjected to high flow or flushing events. Laboratory tests were carried out for five different normalized geometries of the cavities installed in the banks of an open channel and for five hydrographs with different levels of unsteadiness. Water depth, sediment deposit mass, sediment concentration and area covered by the settled sediments were recorded throughout each experiment. Although sediment deposits established at equilibrium before the flushing events are different depending on the geometry of the cavities, generally, they are recovered after being flushed by the high flow events. It is shown that the resistance and resilience of the sediment deposits are strongly dependent on the flow field and the mass exchange between the main channel and the cavities. This mass exchange is governed by the geometry of the cavities and the magnitude of the hydrographs applied.
Large Eddy simulations of lock-exchange gravity currents propagating over a mobile reach are presented. The numerical setting allows to investigate the sediment pick up induced by the currents and to ...study the underlying mechanisms leading to sediment entrainment for different Grashof numbers and grain sizes. First, the velocity field and the bed shear-stress distribution are investigated, along with turbulent structures formed in the flow, before the current reaches the mobile bed. Then, during the propagation of the current above the erodible section of the bed the contour plots of the entrained material are presented as well as the time evolution of the areas covered by the current and by the sediment at this section. The numerical outcomes are compared with experimental data showing a very good agreement. Overall, the study confirms that sediment pick up is prevalent at the head of the current where the strongest turbulence occurs. Further, above the mobile reach of the bed, settling process seems to be of minor importance, with the entrained material being advected downstream by the current. Additionally, the study shows that, although shear stress is the main mechanism that sets particles in motion, turbulent bursts as well as vertical velocity fluctuations are also necessary to counteract the falling velocity of the particles and maintain them into suspension. Finally, the analysis of the stability conditions of the current shows that, from one side, sediment concentration gives a negligible contribution to the stability of the front of the current and from the other side, the stability conditions provided by the current do not allow sediments to move into the ambient fluid.
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