Confluence Processes Research Articles

Spectral continuum in the Rabi–Stark model [Invited

The Rabi–Stark model is a nonlinear generalization of the quantum Rabi model including the dynamical Stark shift as a tunable term, which can be realized via quantum simulation on a cavity QED platform. When the Stark coupling becomes equal to the mode frequency, the spectrum changes drastically, a transition usually termed “spectral collapse” because numerical studies indicate an infinitely degenerate ground state. We show that the spectrum extends continuously from a threshold value up to infinity. A set of normalizable states is embedded in the continuum, which furnishes an unexpected analogy to the atomic Stark effect. Bound states and continuum can be obtained analytically through two equally justified but different confluence processes of the associated differential equation in Bargmann space. Moreover, these results are obtained independently using a method based on adiabatic elimination of the spin degree of freedom and corroborated through large-scale numerical checks.

Evaluating the effects of water exchange between surface rivers and karst aquifers on surface flood simulations at different watershed scales

The complex subsurface conditions and spatial anisotropy of water-bearing media in karst basins result in much more variable runoff generation and confluence processes and water circulation than those in nonkarst basins. Therefore, accurate flood forecasting is more difficult in karst areas, and it is challenging to accurately quantify the water exchange relationship between surface rivers and karst aquifers. To provide key technical support for accurate flood forecasts in karst basins, this study improved a distributed physical hydrologic model, the Karst-Liuxihe model, by improving the runoff generation and confluence algorithms and the parameter optimization algorithm to simulate surface floods in three different karst watersheds in southern China. Additionally, in the model calculations, this study focused on the influence of water exchange on surface flood simulations in changing watersheds. The results showed that for the largest basin, Xijiang Basin (353,100 km2), considering or not considering the water exchange between the surface river and karst aquifer had little effect on the surface flood simulation results. However, this water exchange had a great effect on the surface simulations for the smallest basin, the Beijiang Basin (1,700 km2). For instance, the simulated average peak flow showed a 29 % difference, and the total amount of flooding showed a 25 % difference between dry years and normal years. Therefore, when forecasting surface floods in small karst areas, the water exchange relationship between surface rivers and karst aquifers must be considered. The results showed that the impact of groundwater on surface rivers can be disregarded when forecasting surface floods in the large Xijiang karst basin and can be treated directly as surface flood forecasting in nonkarst areas. The application of this strategy will greatly simplify the cost of hydrologic modelling in the Xijiang basin. By improving the Karst-Liuxihe model, the accuracy of its flood simulation and general applicability in karst basins were greatly enhanced. The improvement in the karst hydrological model in this study provides a good tool for flood simulation and forecasting in the karst regions of Southwest China.

Response of Sediment Connectivity to Altered Convergence Processes Induced by Forest Roads in Mountainous Watershed

Forest roads significantly affect sediment connectivity in mountainous catchments by contributing to the production of and disturbing the confluence of sediment-loaded runoff. This study considered forest roads as pathways and sinks of sediment-loaded runoff to understand the effects of forest roads on the confluence characteristics and sediment connectivity in mountainous a catchment using a scenario simulation. In order to determine the contribution and spatial relationship between sediment connectivity and influencing factors, this study utilized buffer analysis, an extremely randomized tree model, and multiscale geographically weighted regression. The results show that the presence of forest roads significantly changes the transport process and connectivity of runoff and sediment in the mountainous catchment. Specifically, flow length increases, but flow accumulation, upslope contributing area, and topographic index decrease with increasing distance from roads and streams. Meanwhile, the effects of roads on convergence characteristics and sediment connectivity are mainly manifested within a certain threshold that varies with different confluence characteristics. Moreover, sediment connectivity increases when considering roads as pathways and sinks of sediment-loaded runoff, especially on the upper hillslopes intercepted by roads and at the road–stream crossings. In addition, the closer the distance to the roads, the greater the impact of road on the confluence characteristics and sediment connectivity. Change in flow length is the most important factor affecting the sediment connectivity among all of the other convergence, terrain, and spatial distance characteristics. The longer the flow length, the lower the sediment connectivity. In conclusion, this study demonstrates that the altered confluence processes by roads increases the possibility that sediment-loaded runoff will be transported to the catchment outlet, which is of significance for the proper management of forest roads in mountainous catchments.

Gully erosion on alluvial fans can be mitigated by altering the hydrological connectivity between an alluvial fan and the contributing catchment: a study in the Lhasa River basin

AbstractIn the Lower Lhasa River basin of the Tibetan Plateau, gully erosion poses a threat to alluvial fans (AF), which are the main available land resource for agriculture. Runoff from the whole contributing catchment is concentrated in AF, which makes influencing factors of AF gully erosion more complex than it is for hillslopes. There is a lack of quantitative research about the influencing factors and mechanisms of AF gully erosion. The main objectives of this study were to: (1) quantify gully erosion on AF; and (2) identify the controlling factors of gully erosion on AF. The gullies of 12 AF were investigated by unmanned aerial vehicle (UAV). To help understand better the gully erosion on AF, nine morphological parameters were measured, four gully quantitative characteristics were calculated, and 16 factors affecting the gully erosion on AF were analyzed. The gully width–depth ratio of over 20 on the studied AF is larger than that usually seen in other regions. The mean value of gully amount density (GAD), gully density (GD), the degree of gully dissection (GDD), and gully volume modulus (GVM) were 26.51 km−2, 4.93 km·km−2, 7.16%, and 1.28 × 10−4 km3·km−2, respectively. Linear correlations showed a significant (p < 0.05) relationship between the gully quantitative characteristics of AF and the catchment gully density. So, catchment gully density may indirectly affect gully erosion on AF by influencing hydrological connectivity and then changing generation and confluence processes of runoff in catchment. Therefore, gully erosion on AF can be mitigated by shifting hydrology pathway on AF and reducing hydrological connectivity in contributing catchment.

Stable isotopes of surface water and groundwater in a typical subtropical basin in south‐central China: Insights into the young water fraction and its seasonal origin

AbstractThe young water fraction (Fyw) can be reliably determined in heterogeneous river basins, and the seasonal origin index (SOI) can be used to determine the relative contributions of winter versus summer precipitation to runoff. Fywand SOI are also important descriptors of how a basin controls runoff generation and the transport of contaminants and nutrients. In this study, high‐frequency sampling of precipitation, river water, creek water, and shallow groundwater was conducted in the Xiangjiang River basin, in south‐central China, from January 2010 to December 2012. The samples were subjected to stable isotope measurements to determine the SOI and Fywof surface water and shallow groundwater. The principal findings are: (1) The δ2H variations of the river water, creek water, and shallow groundwater were driven by the input of precipitation but were attenuated during runoff confluence processes. Also, the low lc‐excess values of the river water indicated a strong evaporative effect. (2) 21.4% and 19.7% of the river water and shallow groundwater were younger than 1.5 and 1.7 months, respectively, indicating a quicker renewal cycle than for the creek water, which had a fraction of 8.0% younger than 2.6 months, which originated from the adjacent mountains. (3) Winter precipitation was more prevalent in the river water and creek water, indicating the substantial inter‐seasonal carryover of precipitation that supplied the runoff. The average SOI value of the shallow groundwater was near zero, indicating the mixing of the precipitation in different seasons that supplies the shallow groundwater. Our findings emphasize the importance of vegetation and soil conservation in maintaining the inter‐seasonal carryover of water storage held by soil, plants, and rock fractures, and the necessity to assess the risks of streamflow contamination because of the rapid renewal cycle of river water in this flat and intensively cultivated landscape.

Response of bacterial community in composition and function to the various DOM at river confluences in the urban area

River confluences result in mixture and transformation of dissolved organic matter (DOM), influencing the phylogeny of microbial community, furthermore, the integrity and function of river systems. The relationship between the microbial community and DOM is complex, especially in the confluence zone. Previous reports focused on shifts in the different bacterial community in response to exposure to the same terrestrial DOM. However, the transformation of bacterial community induced by convergent DOM remains unknown. This study showed the shifts of DOM components at the junction via excitation–emission matrices parallel factor analysis. Metabolic differences were also determined via phylogenetic investigation of communities by reconstruction of unobserved states. The results demonstrated a direct link between the microbial metabolism and DOM biodegradation during the heterotrophic process. In response to diverse DOM conditions, the taxonomic composition and metabolic function of the microbial community presented significant differences. Different taxa may be involved in metabolizing various DOM components. As indicative bacteria that are closely associated with DOM components, Proteobacteria (Sphingomonas) are significant for microbial utilization and were important during the DOM-degrading process. Compared with other conditions, the abundance of carbon metabolism was higher in convergences where urban rivers joined with estuary or source water. Furthermore, humic-like DOM, converging in the confluence zone, induced a more active lipid metabolism. This study applied techniques that capture the diversity and complexity of bacterial communities and DOM, and provides new insight on the basis of the interaction between bacterial communities and DOM in confluence processes of biogeochemical significance.

Pure spin current in antiferromagnetic insulators

We theoretically investigate the transport properties of the two coupled antiferromagnon modes in a uniaxial antiferromagnetic insulator. The exchange-induced magnon-magnon scattering is found to give rise to strong mode splitting and confluence processes, which conserve the total angular momentum but change the magnon number. These processes can dramatically affect the antiferromagnon dynamics by driving the two oppositely polarized antiferromagnon species to opposite chemical potentials, resulting in a counterflow of the two species with the net magnon current being significantly suppressed. Our results suggest that the antiferromagnetic insulators could be a suitable platform for studying bosonic pure spin currents in insulators.

Time-resolved study of nonlinear three-magnon processes in yttrium iron garnet films

In this work, the temporal aspects of two nonlinear spin-wave processes, i.e., three-magnon splitting and confluence, were investigated in a yttrium iron garnet film. Three-magnon splitting involves the conversion of a magnon at the microwave pumping frequency ${f}_{p}$ into two magnons, each with a frequency near ${f}_{p}/2$, while confluence refers to the subsequent combination of two of the split magnons into one with frequency ${f}_{c}<{f}_{p}$. Time-resolved Brillouin scattering measurements confirm that these processes occur close to the driving antenna ($<1.7\phantom{\rule{0.16em}{0ex}}\mathrm{mm}$ away), which is expected. The data indicate, however, that magnons with larger group velocities are more likely to undergo confluence, which is not predicted by existing theory. Understanding the details of the splitting and confluence processes may have important implications for the use of short-wavelength spin waves for spintronic devices.

A field study of the confluence between Negro and Solimões Rivers. Part 1: Hydrodynamics and sediment transport

Confluences are a common feature of riverine systems, where are located converging flow streamlines and potential mixing of separate flows. The confluence of the Negro and Solimões Rivers ranks among the largest on Earth and its study may provide some general insights into large confluence dynamics and processes. An investigation was recently conducted about that confluence in both low and high-flow conditions using acoustic Doppler velocity profiling (ADCP), water quality sampling and high-resolution seismic data. First, the study gained insights into the characterization of the basic hydrodynamics parameters about the confluence as well as of those affecting sediments transport. Second, the analysis of the results showed that common hydrodynamic features noted in previous confluence studies were herein observed. Finally, some differences between low-flow and relatively high-flow conditions about the transfer of momentum from the Solimões to the Negro side of the Amazon Channel were identified.

Three-magnon splitting and confluence processes for spin-wave excitations in yttrium iron garnet films: Wave vector selective Brillouin light scattering measurements and analysis

Brillouin light scattering (BLS) has been used to observe and confirm the existence of nonlinear three magnon splitting and confluence processes for propagating spin waves in the magnetostatic backward volume wave configuration. Wave vector and frequency selective BLS techniques were also used to provide a quantitative map of the wave vector make-up for the parametrically excited half-frequency dipole-exchange spin wave (DESW) split magnons and the confluence magnons that result from the recombination of these DESW modes. The experimental wave vector maps for the product splitting and confluence magnons matched nicely with those expected from spin-wave theory. The data were obtained with (1) a strip line excitation/detection transducer structure, (2) forward-scattering BLS optics, (3) a fixed magnetic field of 352 Oe applied along the propagation direction, (4) pumping frequencies from 2.5 down to 2.1 GHz, (5) and cw input powers from $200\text{ }\ensuremath{\mu}\text{W}$ to 6 mW. The wave vector selective measurements utilized variable diameter circular diaphragms, rotatable slit apertures, and circular light blocks to access spin waves with wave numbers from about 100 to $3.6\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{rad}/\text{cm}$ and the full $360\ifmmode^\circ\else\textdegree\fi{}$ range of propagation angles.

Hierarchy of Bäcklund transformation groups of the Painlevé systems

For each Painlevé system PJ except the first one, we have a Bäcklund transformation group which is a lift of an alfine Weyl group. In this PaPer, we show that the Bäcklund transformation groups for J=V,IV,III,II are successively obtained from that for J=VI by the well known degeneration or confluence processes.

Confluence processes in defining manifolds for Painlevé systems

For each Painlevé system, we have a manifold, called the defining manifold, on which the system defines a uniform foliation. In this paper, we describe confluence processes in these manifolds as deformations of manifolds compatible to those in Painlevé systems.

Structural theory of special functions

A block diagram is suggested for classifying differential equations whose solutions are special functions of mathematical physics. Three classes of these equations are identified: the hypergeometric, Heun, and Painleve classes. The constituent types of equations are listed for each class. The confluence processes that transform one type into another are described. The interrelations between the equations belonging to different classes are indicated. For example, the Painleve-class equations are equations of classical motion for Hamiltonians corresponding to Heun-class equations, and linearizing the Painleve-class equations leads to hypergeometric-class equations. The “confluence principle” is stated, and an example of its application is given.

Fine structure and critical modes at the first-order Suhl instability in YIG spheres

The FMR fine structure of the first-order Suhl instability in YIG spheres, previously observed by Jantz and Schneider (1975), has been re-investigated both for the conditions of coincidence and subsidiary absorption (3 and 9 GHz) from room temperature down to 1.6K. The observed fine structure consists of several series of closely spaced resonances, indicating a critical wave vectork≠0 even for half the pumping frequency lower than the 45° spin-wave branch, which is in contrast to the prediction of Suhl's theory. The spacing of these resonances was analyzed in terms of spherical modes. We were able to determine the indices and the effective wave numbers (≈105 cm−1) of the critical modes. The very different dynamic behaviour observed at room temperature and at 1.6K can be attributed to a competition of different damping mechanisms—mainly surface pit scattering and three-magnon confluence processes — and their dependences on field, frequency, and temperature.

The magnon-magnon interactions in easy-plane antiferromagnets

In the framework of the spin-operator diagram technique, the processes of relaxation of lower branch magnons in a two-sublattice easy-plane antiferromagnet are investigated. The procedure of the spin Hamiltonian diagonalization allowing to obtain compact expressions for magnon interaction amplitudes is proposed. The relaxation frequencies due to three- and four-magnon processes, processes of elastic scattering and interbranch transformation by thermal and concentrational magnetization fluctuations as well as the three-magnon confluence processes with rescattering of thermal magnons by fluctuations are calculated. The results obtained are consistent with the experimental data on parametric excitation of spin waves in the low-temperature antiferromagnets MnCO3 and CsMnF3.

Kinetic equation for three-magnon confluence processes in ferromagnets

We show that the usual kinetic equation is not valid for the calculation of the three-magnon confluence relaxation time of spin waves with small wave vectors. The results of our calculations for small wave vectors considerably differ from previous calculations by Sparks and Sparks, Loudon and Kittel.

Spin-wave relaxation and phenomenological damping in ferromagnetic resonance

Relaxation rates for the uniform precession mode in ferromagnetic resonance, with general elliptical polarization, have been calculated for several microscopic scattering processes using the spin-wave formalism. These results are compared with the widely used phenomenological formulations for ferromagnetic resonance. The results demonstrate in relatively general terms the specific features of the Landau-Lifshitz and Gilbert phenomenological formulations on the one hand, and of what may be called "intrinsic" confluence processes in the microscopic formulation. These formulations are consistent with the assumption of an intrinsic damping parameter describing the motion of the magnetization vector under sufficiently general conditions. The two-magnon process and the Bloch-Bloembergen phenomenological description of damping in ferromagnetic resonance are not consistent with such an assumption.