Control of mass balance error in a detailed model of surface-subsurface flow interaction

Several process-based catchment-scale hydrologic models have been developed in recent years to describe the interactions and feedbacks between different components of the water cycle, but few studies have considered the sources of coupling error in these models. In this work we analyze the sequential iterative coupling scheme of the distributed model CATHY (CATchment HYdrology) in order to identify the different sources of mass balance error and to examine how these are influenced by topography, hydraulic properties, and atmospheric forcing. A pair of adimensional indices that quantify the degree of coupling and of flux partitioning is presented. Our analysis shows that mass balance errors increase during the flood recession limb because of the exchange of information between surface and subsurface water flow. Surface water propagation is cell centered, while the subsurface flow equation is solved on the vertices of surface cells. Evaluation of surface pressure heads and exchange fluxes is critical on this staggered surface-subsurface mesh, especially during transitions from unsaturated to saturated conditions and vice versa. A modified version of the flux exchange algorithm is introduced that considers the effective availability of water on surface cells. The performance of the model is also improved by introducing a heuristic procedure to control and adapt the time step interval. Starting from numerical stability and convergence constraints, this procedure varies the computational interval as a function of the rate of change of surface saturation via the coupling degree index. A final improvement made to the sequential coupling scheme in CATHY is to solve the surface routing equation after rather than before the subsurface module. We find that the modified version improves the water balance by more than 50% in most of the tests considered for a simple v-shaped catchment. The results so far obtained for the synthetic v-catchment indicate the need for a more comprehensive analysis including real catchments

CONTROL OF COUPLING MASS BALANCE ERROR IN A PROCESS- BASED NUMERICAL MODEL OF SURFACE-SUBSURFACE FLOW INTERACTION

KEY POINTS • Sources of mass balance error in a process-based hydrological model of surface-subsurface flow interaction are investigated to improve the model’s coupling scheme • These sources of mass balance error are identified by using a set of dimensionless indices and the analysis of temporal and spatial patterns of error • A time step control based on a degree of coupling index is proposed and the interpolation algorithm used to pass exchange variables of surface-subsurface flow interaction is improve

Single-file escape of colloidal particles from microfluidic channels

Single-file diffusion is a ubiquitous physical process exploited by living and synthetic systems to exchange molecules with their environment. It is paramount quantifying the escape time needed for single files of particles to exit from constraining synthetic channels and biological pores. This quantity depends on complex cooperative effects, whose predominance can only be established through a strict comparison between theory and experiments. By using colloidal particles, optical manipulation, microfluidics, digital microscopy and theoretical analysis we uncover the self-similar character of the escape process and provide closed-formula evaluations of the escape time. We find that the escape time scales inversely with the diffusion coefficient of the last particle to leave the channel. Importantly, we find that at the investigated {\bf microscale}, bias forces as tiny as $10^{-15}\;{\rm N}$ determine the magnitude of the escape time by drastically reducing interparticle collisions. Our findings provide crucial guidelines to optimize the design of micro- and nano-devices for a variety of applications including drug delivery, particle filtering and transport in geometrical constrictions.Comment: 6 pages, 3 figure

Aging of living polymer networks: a model with patchy particles

Microrheology experiments show that viscoelastic media composed by wormlike micellar networks display complex relaxations lasting seconds even at the scale of micrometers. By mapping a model of patchy colloids with suitable mesoscopic elementary motifs to a system of worm-like micelles, we are able to simulate its relaxation dynamics, upon a thermal quench, spanning many decades, from microseconds up to tens of seconds. After mapping the model to real units and to experimental scission energies, we show that the relaxation process develops through a sequence of non-local and energetically challenging arrangements. These adjustments remove undesired structures formed as a temporary energetic solution for stabilizing the thermodynamically unstable free caps of the network. We claim that the observed scale-free nature of this stagnant process may complicate the correct quantification of experimentally relevant time scales as the Weissenberg number

Synthesis and Structural Characterization of a Tetranuclear Zinc(II) Complex with P,P'-Diphenylmethylenediphosphinate (pcp) and 2,2'-Bipyridine (2,2'-bipy) Ligands

A new tetranuclear complex of zinc(II) with P,P′-diphenylmethylenediphosphinate and 2,2′- bipyridine ligands was synthesized. [(pcp)(2,2′-bipy)Zn (μ3-pcp)Zn (2,2′-bipy)]2 · 6H2O was characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and X-ray diffractometry. The structure consists of tetranuclear complexes connected through water hydrogen-bonding interactions in corrugated 2D layers. Two crystallographically independent zinc ions are in a distorted five-coordinate environment, being surrounded by three oxygen atoms of phosphinate groups (from two pcp ligands) and by two bipy nitrogen donors. Of the two independent pcp anions the first one utilizes all of its oxygen donors to coordinate one metal as bidentate and two metal atoms as a monodentate ligand, whereas the second one is only bidentate for one metal atom

Topological Sieving of Rings According to Their Rigidity

We present a novel mechanism for resolving the mechanical rigidity of nanoscopic circular polymers that flow in a complex environment. The emergence of a regime of negative differential mobility induced by topological interactions between the rings and the substrate is the key mechanism for selective sieving of circular polymers with distinct flexibilities. A simple model accurately describes the sieving process observed in molecular dynamics simulations and yields experimentally verifiable analytical predictions, which can be used as a reference guide for improving filtration procedures of circular filaments. The <i>topological sieving</i> mechanism we propose ought to be relevant also in probing the microscopic details of complex substrates

Modeling coupled surface and subsurface flow at the catchment scale

We will describe a physically-based distributed catchment-scale model for the simulation of coupled surface runoff and subsurface flow. The model is based on coupling Richard’s equation for variably saturated porous media and a diffusion wave approximation for surface water dynamics. The numerical scheme uses a finite element Richard’s equation solver, FLOW3D and a surface DEM-based finite difference module, SURF-ROUTE. Retardation and storage effects due to lakes or depressions are also implemented, to give a complete description of the catchment flow dynamics