Distributed Modeling Approach Research Articles

Use of distributed snow measurements to test and improve a snowmelt model for predicting the effect of forest clear-cutting

Summary Modeling the effect of forest clear-cutting with a distributed hydrological model can be used to detect hydrologic changes as an alternative to paired-catchment studies, and also to estimate the hydrologic sensitivity of a catchment to assist in forest management decisions. To model the effect of clear-cutting in a snow dominated forested watershed, a model needs to be able to simulate effects of all of the main controls on snow accumulation and melt. However, most studies that used a distributed modeling approach relied on one or a few sites for model testing. In this study, we employ a stratified nested sample layout that was specifically designed to test distributed models, involving measurement of snow water equivalent (SWE) under forest and in clearcuts over a range of elevations and aspects. To test the ability of a model to simulate the main controls on the spatial distribution of SWE, spatial gradients of observed and simulated SWE in relation to topographic and vegetation controls are computed using regression analysis. Comparison of observed and simulated gradients helps to highlight model weaknesses. The approach is applied to evaluate the snow algorithms in the distributed hydrology soil and vegetation model (DHSVM) using data collected in Cotton Creek, a snow dominated forested watershed in south-eastern British Columbia. SWE measurements were made from 2005 to 2008, covering peak snow accumulation and snow melt. Albedo decay and canopy transmittance were found to be the two processes that DSHVM version 3.0 did not simulate well enough to predict basin average differences between forests and clearcuts properly. After replacing the internal albedo decay functions with functions obtained from snow albedo measurements and changing the canopy transmittance function in the model, DHSVM was able to reproduce the major spatial patterns derived from snow surveys. Model performance is better during winter up to the peak snow accumulation than during snow melt. Spatial patterns of peak snow accumulation in the snow-rich year 2006 can be modeled better than those after the warm winter of 2005. The influence of aspect on snow accumulation and snowmelt is underestimated by DHSVM. While we have focused specifically on DHSVM, the methods developed in this study should be generally useful for model testing purposes and important in the context of interpretation of modeling results, in particular when dealing with large spatial datasets.

Departure headways at signalized intersections: A log-normal distribution model approach

Modeling departure headways at signalized intersections attracts constant research efforts in the last four decades due to its importance. But most previous approaches focus on the mean departure headways only. In this paper, two interesting findings are presented. First, the distributions of the departure headways at each position in a queue are revealed to approximately follow a certain log-normal distribution (except the first one) and the corresponding mean values level out gradually. Second, a car-following model from the driver’s perspective is proposed to explain why the departure headways follow such a log-normal distribution. The consistency between the empirical and simulation results indicates that this new car-following model provides an acceptable description of the unseen interactions between the vehicles in a discharging queue. The new model is also useful for simulation based intersection capacity analysis and traffic signal control.

A new modelling approach to insect reproduction with same-shape reproduction distribution and rate summation: with particular reference to Russian wheat aphid

Same-shape distribution model and rate summation approach are widely used to describe the insect developmental process. In this approach, by integrating a nonlinear deterministic developmental rate model and a probabilistic same-shape distribution model, the proportion of the cohort completing development is quantified as a function of accumulating developmental rates, which themselves are temperature dependent. This method is considered to be more accurate in modelling insect phenology because it can address a well-known biological fact, individual variability, that insect individual developmental rates respond to temperature differently, and because rate-summation essentially simulates developmental rates under variable temperatures instead of constant temperatures. By comparing insect development and reproduction with respect to their responses to temperatures, we argue for the extension of the same-shape and rate-summation approaches to modelling insect reproduction process under variable temperatures. We justify our arguments by the fact that individual variation universally exists in almost all biological characteristics, and the phenomenon that insect development and reproduction respond to temperature very similarly, which is supported by some endocrinological evidences reported in literature. In addition, the approach for testing the applicability of the original same-shape developmental modelling, experimentally verifying the sameness of the same-shape curves or that the shape of the curves is invariant with respect to the temperature regimes, equally applies to our extended version for reproduction modelling. We successfully tested the extension and its applicability with our experimental data of 1800 Russian wheat aphids' (RWA) (Diuraphis noxia (Mordvilko)) reproduction under various temperature and plant growth stage regimes. We also extended Taylor's (1981) nonlinear model for insect development to describe RWA mean (median) nymphal production under different temperatures and barley plant growth stages. Three same-shape distribution models, Weibull distribution, Stinner's model and logistic model, are used to construct the same-shape reproduction distribution models for RWA. The extensions performed in this paper contribute a new modelling approach for predicting insect reproduction under field variable temperatures and plant growth stages. The prediction model can be parameterized with data from typical laboratory demography experiments and further integrated into simulation models for insect population dynamics. Finally, we discussed why the sameness test of the same-shape distribution curves is sufficient in validating the approach and proposed a strategy for dealing with exceptional cases where the sameness test fails.

Simulating short‐circuiting flow in a constructed wetland: the implications of bathymetry and vegetation effects

AbstractShort‐circuiting flow, commonly experienced in many constructed wetlands, reduces hydraulic retention times in unit wetland cells and decreases the treatment efficiency. A two‐dimensional (2‐D), physically based, distributed modelling approach was used to systematically address the effects of bathymetry and vegetation on short‐circuiting flow, which previously have been neglected or lumped in one‐dimensional wetland flow models. In this study, a 2‐D transient hydrodynamics with advection‐dispersion model was developed using MIKE 21 and calibrated with bromide tracer data collected at the Orlando Easterly Wetland Cell 7. The estimated topographic difference between short‐circuiting flow zone and adjacent area ranged from 0·3 to 0·8 m. A range of the Manning roughness coefficient at the short‐circuiting flow zone was estimated (0·022–0·045 s m−1/3). Sensitivity analysis of topographical and vegetative heterogeneity deduced during model calibration shows that relic ditches or other ditch‐shaped landforms and the associated sparse vegetation along the main flow direction intensify the short‐circuiting pattern, considerably affecting 2‐D solute transport simulation. In terms of hydraulic efficiency, this study indicates that the bathymetry effect on short‐circuiting flow is more important than the vegetation effect. Copyright © 2009 John Wiley & Sons, Ltd.

Solid Oxide Fuel Cell Modeling

This paper discusses the modeling of a solid oxide fuel cell using both lumped and distributed modeling approaches. In particular, the focus of this paper is on the development of a computationally efficient lumped-parameter model for real-time emulation and control. The performance of this model is compared with a detailed distributed model and experimental results. The fundamental relations that govern a fuel cell operation are utilized in both approaches. However, the partial pressure of the species (fuel, air, and water) in the distributed model is assumed to vary through the length of the fuel cell. The lumped model approach uses the partial pressure of the species at the exit point of the fuel cell. The partial pressure of the species is represented by an equivalent RC circuit in the lumped model.

Effects of spatial grid resolution on river flow and surface inundation simulation by physically based distributed modelling approach

AbstractGrid‐based distributed hydrological models are considered to be a very effective flood modelling tool for basin‐wide flood risk analysis because of their capabilities of simulating river and surface inundations at high spatio‐temporal resolutions by taking advantages of grid‐based data from meteorological models, radar and satellite remote sensing. Selecting an appropriate grid size is critically important for any application of a grid‐based model, which requires proper understanding of effects of grid sizes on simulated outcomes. The paper presents the outcomes of a study conducted to analyse the effects of grid resolution on simulated river peak flows and surface inundation in two selected river basins using a process‐based distributed hydrological model.The outcomes show that grid resolution significantly affects the simulated river peak flows and surface inundation characteristics. In both cases, it has been found that the effects are mainly caused by changes of the topographic parameters as a result of changes of grid sizes. The reduction of average surface slope with the increase of grid size affects the simulated surface inundation extents and heights. There is a threshold resolution of digital elevation model (DEM) in the simulated flood inundation and beyond that the model outcomes become arbitrary. Averaged topographic values at coarse resolution beyond this threshold level do not represent any characteristics of locally elevated topographic features such as dykes, highways, etc. and their influence on flood inundation characteristics can be no more captured by the model. Copyright © 2008 John Wiley & Sons, Ltd.

A Distributed Millimeter-Wave Small-Signal HBT Model Based on Electromagnetic Simulation

In this paper, a novel distributed small-signal heterojunction bipolar transistor (HBT) model at millimeter-wave frequencies is proposed. This new approach integrates the electromagnetic (EM) simulation of the outer extrinsic passive part of an HBT, the coupled transmission lines for the fingers, and the Gupta multiport connection into an efficient global distributed modeling approach. For the first time, the values of the entire HBT intrinsic model elements used in the active elementary cells (AECs) can subsequently be extracted through the explicit analytical expressions derived through the multiport connection method. Good agreement between the measured and the simulated results has been demonstrated. This model has several unique advantages for microwave transistor optimization and synthesis in that the derived explicit analytical expressions are in terms of the effect of the extrinsic parts, allowing the designer to have better control over the whole transistor design. Furthermore, it serves as one of the valuable steps toward global modeling of millimeter-wave devices and circuits.

Dynamic spatially explicit mass-balance modeling for targeted watershed phosphorus management: I. Model development

Surface waters are frequently impaired by excessive phosphorus (P) from nonpoint sources, especially in regions of intensive livestock agriculture. Despite concerted efforts to apply new management measures, reductions in nonpoint source P loads have been difficult to accomplish. Watershed management to reduce P export could be more cost-effective if treatments were targeted to critical source areas at high risk for excessive P export. These critical source areas can be defined as the intersection of P source areas and active runoff contributing areas; such areas vary in space and time due to watershed characteristics and management practices. We developed an approach to identify, analyze, and map high-risk areas for P export by integrating spatial data with land use and agronomic data. We evaluated changes over time and space in soil P concentration and P export in response to changes in inputs and outputs with a dynamic mass-balance simulation model running in grid cells across a watershed. The temporal and spatial relationships that define the risk of P export are captured simultaneously using a raster-based distributed dynamic modeling approach and related to management interventions. Simulated responses to management interventions are analyzed and displayed spatially through a geographic information system (GIS). This approach allows the spatial distribution of P runoff risk to be tracked through time in response to long-term P input/output balance, evolving from either continuation of current practices or from management changes specifically targeted to areas of high P loss risk. Baseline simulations show that if present-day management continues, both soil test P and P export will increase dramatically in some parts of a test watershed; critical P source areas in a watershed will evolve over time and are likely to occur in limited areas that can be identified and tracked. Model results can contribute to improved targeting of scarce resources by focusing management interventions on those areas at highest risk of nutrient loss. This paper describes the underlying principles of the model, discusses the process of model development, and presents the final modeling system. Application of the model to alternative management scenarios is discussed in a subsequent paper.

A framework for classifying and comparing distributed hillslope and catchment hydrologic models

The term distributed model is widely applied to describe hydrologic models that can simulate broad classes of pathways of water through space, e.g., overland flow, saturated groundwater flow, and/or unsaturated flow in the vadose zone. Because existing distributed modeling approaches differ substantially from one another, we present a common framework from which to compare the many existing hillslope‐ and catchment‐scale models. To provide a context for understanding the structure of the current generation of distributed models, we briefly review the history of hydrologic modeling. We define relevant modeling terms and describe common physical, analytical, and empirical approaches for representing hydrologic processes in the subsurface, surface, atmosphere, and biosphere. We then introduce criteria for classifying existing distributed models based on the nature of their process representation, solution scheme, coupling between the surface and subsurface, and treatment of space and time. On the basis of these criteria we describe 19 representative distributed models and discuss how process, scale, solution, and logistical considerations can be incorporated into model selection and application.

Hydrologic response of upland catchments to wildfires

To which extent do wildfires affect runoff production, soil erosion and sediment transport in upland catchments? This transient effect is investigated here by combining data of long term precipitation, sediment yield and wildfire records with a fine resolution spatially distributed modeling approach to flow generation and surface erosion. The model accounts for changes in the structure and properties of soil and vegetation cover by combining the tube-flux approach to topographic watershed partition with a parsimonious parametrization of hydrologic processes. This model is used to predict hydrologic and sediment fluxes for nine small catchments in Saint Gabriel mountains of southern California under control (pre-fire) and altered (post-fire) conditions. Simulation runs using a 45 years record of hourly precipitation show the passage of fire to significantly modify catchment response to storms with a major effect on erosion and flood flows. The probability of occurrence of major floods in the post-fire season is shown to increase up to an order of magnitude under same precipitation conditions. Also, the expected anomaly of sediment yield can increase dramatically the desertification hazard in upland wildfire prone areas. One should further consider the role of firefloods produced by the combined occurrence of wildfires and storms as a fundamental source of non-stationarity in the assessment of hydrologic hazard.

Poverty, income distribution and CGE micro-simulation modeling: Does the functional form of distribution matter?

This paper explores income distribution modeling approaches for poverty analysis in a CGE micro-simulation context. Income distribution functional forms such as the lognormal, Pareto, beta distribution and empirical methods are currently used in CGE models in parallel with the estimation of FGT poverty indices. The particular methods or functional forms used in this context are not always clearly defined and justified. In this paper, we investigate and provide better criteria for selecting a functional distribution for poverty analysis. To achieve this, we apply parametric estimation to seven functional forms and compare the results to a purely “empirical” method. The results showed that no single form is more appropriate in all instances or for all household subgroups. The choice of a modeling approach should be motivated by a search for best fit and should be based on appropriate statistical tests. Selecting inappropriate distributional forms can lead to biased results in terms of poverty analysis. Introducing functional forms in the empirical approach can also provide greater confidence in the results obtained.

GIS-BASED WATER QUALITY MODELING IN THE SANDUSKY WATERSHED, OHIO, USA

This study focused on the Sandusky Watershed (SW) in Ohio, located within the Great Lakes Basin, with emphasis on two of its subwatersheds, namely Honey Creek (HC) and Rock Creek (RC). The goal was to assess the capabilities of the Soil and Water Assessment Tool (SWAT) to simulate suspended sediment (SS), phosphorus (P) and nitrogen (N) yield in the SW that contribute major sediment and nutrient loads into Lake Erie. The model was calibrated using water flow and water quality parameters for water years 1998 to 1999 and validated model simulations covering the period of water years 2000 to 2001 for monthly conditions. The validation of SS showed correlation coefficients of 0.29 (SW), 0.75 (HC) and 0.69 (RC). Correlation coefficients for P were 0.68 (SW), 0.78 (HC) and 0.37 (RC); for NO 2 -N 0.84 (HC) and 0.38 (RC); for NO 3 -N 0.27 (HC) and 0.76 (RC); for NH 3 -N 0.57 (SW), 0.49 (HC), and 0.13 (RC). In addition, mean errors, root mean square errors, Nash-Sutcliffe coefficients, and graphs were used to compare simulated to measured data. Simulation success was variable with poor and good simulations, but in most cases, simulated water quality values followed the trend of measured data except for extreme (or intense) rainfall/runoff events. Reviews of 17 applications indicated that the SWAT is suitable for long term continuous simulations but not for storm events. A spatially distributed modeling approach generated maps showing the spatial distribution of SS, P, and N for each simulation element across the Sandusky Watershed.

Effect of layout parasitics on the current distribution of power MOSFETs operated at high switching frequency

The internal electrical characteristics of an 800-V 9-A 0.7-Ω silicon power MOSFET are investigated using a distributed modelling approach. A cad tool has been developed to automatically extract layout parasitics and create an equivalent spice-like netlist of the device under investigation. The distributed model is employed to investigate the performance of the device during a turn-off switching at different values of the gate fall time. Simulations show that reducing the gate fall time contributes to increase the internal current imbalance leading to current density overshoots, referred to as hot spots. Moreover, faster switching operation forces current density to crowd towards the slowest parts of the device which in turn degrades the overall device ruggedness. These results demonstrate that the proposed modelling approach allows accurate simulation of the internal current distribution identifying the regions where hot spots are more likely to occur.

Test of a distributed modelling approach to predict flood flows in the karst Suoimuoi catchment in Vietnam

The major obstacles for modelling flood processes in karst areas are a lack of understanding and model representations of the distinctive features and processes associated with runoff generation and often a paucity of field data. In this study, a distributed flood-modelling approach, WetSpa, is modified and applied to simulate the hydrological features and processes in the karst Suoimuoi catchment in northwest Vietnam. With input of topography, land use and soil types in a GIS format, the model is calibrated based on 15 months of hourly meteorological and hydrological data, and is used to simulate both fast surface and conduit flows, and groundwater discharges from karst and non-karst aquifers. Considerable variability in the simulation accuracy is found among storm events and within the catchment. The simulation results show that the model is able to represent reasonably well the stormflows generated by rainfall events in the study catchment.

Global environmental change and the uncertain fate of biodiversity

Global environmental change and the uncertain fate of biodiversity

Rainfall – runoff modelling in the urban catchment of El Batan, Quito, Ecuador

The El Batan urban watershed covers about 50?km2 in Quito, Ecuador. Rainfall and stream flow data were collected between 1982 and 1984 and then again in 1998. Although both geometric and hydraulic data were available, rainfall?–?runoff modelling was carried out using a conceptual rainfall?–?runoff model in order to avoid the very tedious work that defining the topology (pipes, channels, nodes, subcatchments) of such a large area implies. Considering both the physiographic heterogeneity and the high variability of the rainfall in this mountainous context, a distributed modelling approach was chosen that develops and generalizes the concept of the unit hydrogram to a grid-cell structure. For each cell, the contribution at a given time is computed by using an instant runoff coefficient. Then the elementary contributions are transferred through the drainage system by means of a translation and storage combination, which uses two parameters. The runoff coefficient primarily depends on the land-use characteristics, so this coefficient changes appreciably between 1982?–?1984 and 1998 due to urban growth and the development of drainage infrastructure. Discharge parameters are stable for the whole set of events and for both observation periods, and can be related to hydraulic characteristics of the drainage network. On the basis of simplified assumptions, it is shown that the distributed model is able to simulate the observed floods. Finally, the model was applied to the systematic detection of overloading in the Quito sewage network.

Runoff response to spatial variability in precipitation: an analysis of observed data

We examine the hypothesis that basins characterized by (1) marked spatial variability in precipitation, and (2) less of a filtering effect of the input rainfall signal will show improved outlet simulations from distributed versus lumped models. Basin outflow response to observed spatial variability of rainfall is examined for several basins in the Distributed Model Intercomparison Project. The study basins are located in the Southern Great Plains and range in size from 795 to 1645 km 2. We test our hypothesis by studying indices of rainfall spatial variability and basin filtering. Spatial variability of rainfall is measured using two indices for specific events: a general variability index and a locational index. The variability of basin response to rainfall event is measured in terms of a dampening ratio reflecting the amount of filtering performed on the input rainfall signal to produce the observed basin outflow signal. Analysis of the observed rainfall and streamflow data indicates that all basins perform a range of dampening of the input rainfall signal. All basins except one had a very limited range of rainfall location index. Concurrent time series of observed radar rainfall estimates and observed streamflow are analyzed to avoid model-specific conclusions. The results indicate that one basin contains complexities that suggest the use of distributed modeling approach. Furthermore, the analyses of observed data support the calibrated results from a distributed model.

Shallow landslides in pyroclastic soils: a distributed modelling approach for hazard assessment

An effective assessment of shallow landslide hazard requires spatially distributed modelling of triggering processes. This is possible by using physically based models that allow us to simulate the transient hydrological and geotechnical processes responsible for slope instability. Some simplifications are needed to address the lack of data and the difficulty of calibration over complex terrain at the catchment's scale. We applied two simple hydrological models, coupled with the infinite slope stability analysis, to the May 1998 landslide event in Sarno, Southern Italy. A quasi-dynamic model (Barling et al., 1994) was used to model the contribution to instability of lateral flow by simulating the time-dependent formation of a groundwater table in response to rainfall. A diffusion model [Water Resour. Res. 36 (2000) 1897] was used to model the role of vertical flux by simulating groundwater pressures that develop in response to heavy rainstorms. The quasi-dynamic model overestimated the slope instability over the whole area (more than 16%) but was able to predict correctly slope instability within zero order basins where landslides occurred and developed into large debris flows. The diffusion model simulated correctly the triggering time of more than 70% of landslides within an unstable area amounting to 7.3% of the study area. These results support the hypothesis that both vertical and lateral fluxes were responsible for landslide triggering during the Sarno event, and confirm the utility of such models as tools for hazard planning and land management.

Estimating and interpreting hydrological drought indices using a selected catchment in Zimbabwe

A distributed modeling approach is applied at a scale that permits visualization of hydrological drought conditions in different tertiary catchments. Hydrological drought sequences are calculated from runoff time-series data. Statistics of runoff and drought impact information from literature are applied to classify hydrological droughts on magnitude, duration, recurrent interval and impact to define a set of hydrological drought indices for specific conditions. The indices are applied on a sample of hydrological drought seasons in a selected primary catchment and visualized in a geographical information software. Scenarios are created to identify worst cases.

A distributed system simulation modelling approach

The employment of network-based technologies, such as the WWW and middleware platforms, significantly increased the complexity of distributed application, as well as the Quality-of-Service requirements for the underlying network. Distributed application modelling is nowadays far more demanding than network modelling, where numerous solutions are already employed in commercial tools. We introduce a simulation modelling approach for distributed systems, giving emphasis to distributed applications. The proposed scheme enables the in-depth description of application functionality, the accurate estimation of network load and the extension of existing application models to support further customisation. It supports widely employed architectural models, such as the client–server model and its variations, and is based on multi-layer decomposition. Application functionality is described using pre-defined operations, which can be further decomposed into simpler ones, ultimately resulting into elementary actions corresponding to primitive network operations, such as transfer and processing. Even if realisation of this scheme proves to be time demanding, individual application modelling is performed with consistency and considerably lower overhead. The distributed system simulation environment built to realise the proposed modelling scheme and a case study indicating key features of the overall approach are also presented.