Erich J. Plate

Space‐time model for daily rainfall using atmospheric circulation patterns

A multidimensional stochastic model is developed for the space-time distribution of daily precipitation. The rainfall is linked to the atmospheric circulation patterns using conditional distributions and conditional spatial covariance functions. The model is a transformed conditional multivariate autoregressive AR(I) model, with parameters depending on the atmospheric circulation pattern. The model reproduces both the local rainfall occurrence probabilities and the distribution of the rainfall amounts at given locations, and the spatial dependence described with the help of cross-covariances of the transformed series. Parameter estimation methods based on the moments of the observed data are developed. A simulation procedure for the model is also presented. Its link to atmospheric circulation patterns makes it suitable for local precipitation simulation under stationary and nonstationary arrivals of atmospheric circulation patterns such as climate change. The model is applied using the classification scheme of the German Weather Service which is available for the time period 1881–1990. Precipitation data measured at 44 different stations for the time period 1977–1990 in the catchment of the river Ruhr (Germany) are used to demonstrate the model.

Flood risk and flood management

Abstract Risk management has been established as a well defined procedure for handling risks due to natural, environmental or man made hazards, of which floods are representative. Risk management has been discussed in many previous papers giving different meanings to the term—a result of the fact that risk management actually takes place on three different levels of actions: the operational level, which is associated with operating an existing system, a project planning level, which is used when a new, or a revision of an existing project is planned, and a project design level, which is embedded into the second level and describes the process of reaching an optimal solution for the project. The first two levels will be briefly described in the paper. It will be emphasized that the transition from the first to the second level is a dynamic process. As the value system of a nation changes, and as the natural boundary conditions are modified by human actions or global changes, an existing system will be found not meeting the demands of the present society, and actions on the second level are initiated. The decisions for change depend on the changes in options available for handling a flood situation, as well as on the changes in risk perception and attitudes towards risk. On the third level, the actual cost of a design are evaluated and compared with the benefits obtained from the planned project. In particular, on this level the residual risk is considered, i.e. the risk which remains even after a project is completed and fully operational.

Wind-tunnel study of concentration fields in street canyons

Abstract The paper presents results from a case study of gaseous pollutant dispersion in street canyons. Tracer-gas experiments were performed in a neutrally stratified wind tunnel. Vehicle emissions were simulated as line sources. Concentration profiles along building walls were measured. A two-dimensional street canyon was considered as the reference case. The influence of systematic parameter variations on the concentration field is studied and discussed. Building dimensions, upwind building configuration, wind direction and roof geometry were found to be important parameters. Data sets from the study may be used for evaluation of numerical models and for expert estimates of air quality in the urban environment

An analysis of the effects of spatial variability of soil and soil moisture on runoff

Hydrological variables and processes usually exhibit a large spatial variability. Often this variability includes aspects of organization and randomness. Because any hydrological modeling has to deal with the question of spatial variability, methods that quantify the effects of spatial variability are valuable. Moreover, it is important to identify the situations where the spatial variability can be reduced (e.g., by using an “effective” value). For a small and well-instrumented catchment in a loess area in southwest Germany effects of spatial variability of the initial soil moisture and soil hydraulic properties on the runoff are investigated. The analysis is performed with a process-oriented rainfall runoff model. It is shown that organization in spatial patterns of soil moisture and soil properties may have a dominant influence on the catchment runoff. The simulations suggest that spatial variability can result in a complex, event dependent, behavior. It cannot be expected that a model with inputs based on mean parameters or mean initial conditions leads to mean outputs for heterogeneous fields. The analysis of different events shows the changing influence of spatial variability on the runoff with changing storm size. For very small and for large events spatial variability plays a negligible role. A large influence is found for medium-sized events.

The aerodynamics of shelter belts

Abstract The effectiveness of a shelter is determined not only by its total drag but also by the distribution of the drag generated momentum defect in the sheltered area. Many aerodynamic factors affect both the drag and the momentum defect distribution, such as the boundary layer profiles of the approaching flow and the shape and the porosity of the shelter. How the interaction of these factors shapes the velocity distributions in the sheltered region is discussed qualitatively in this paper, on the basis of a flow model which consists of different regions. In each of them a different combination of aerodynamic factors is acting. Emphasis is on the region directly downwind from the shelter. It is shown that separation from the top of the shelter belt gives rise to a separation streamline which divides the low velocity flow below from the high velocity flow aloft. The blending of the flow across this streamline, which determines the recovery of the wind profile and the reduction in sheltering efficiency, is caused by the gradient in velocity across the streamline, while its location is determined by the drag on the shelter and the pressure distribution behind it. It is also shown that more turbulence is produced in a high density shelter than in a porous one. Finally, some conclusions are drawn with regard to research needs for improving our understanding of shelter belt aerodynamics.

Modelling of runoff generation and soil moisture dynamics for hillslopes and micro-catchments

Abstract The modelling of hillslope hydrology is of great importance not only for the reason that all non-plain, i.e. hilly or mountainous, landscapes can be considered as being composed of a mosaic of hillslopes. A hillslope model may also be used for both research purposes and for application-oriented, detailed, hillslope-scale hydrological studies in conjunction with related scientific disciplines such as geotechnics, geo-chemistry and environmental technology. Despite the current limited application of multi-process and multi-dimensional hydrological models (particularly at the hillslope scale), hardly any comprehensive model has been available for operational use. In this paper we introduce a model which considers most of the relevant hillslope hydrological processes. Some recent applications are described which demonstrate its ability to narrow the stated gap in hillslope hydrological modelling. The modelling system accounts for the hydrological processes of interception, evapotranspiration, infiltration, soil-moisture movement (where the flow processes can be modelled in three dimensions), surface runoff, subsurface stormflow and streamflow discharge. The relevant process interactions are also included. Special regard has been given to consideration of state-of-the-art knowledge concerning rapid soilwater flow processes during storm conditions (e.g. macropore infiltration, lateral subsurface stormflow, return flow) and to its transfer to and inclusion within an operational modelling scheme. The model is “physically based” in the sense that its parameters have a physical meaning and can be obtained or derived from field measurements. This somewhat weaker than usual definition of a physical basis implies that some of the sub-models (still) contain empirical components, that the effects of the high spatial and temporal variability found in nature cannot always be expressed within the various physical laws, i.e. that the laws are scale dependent, and that due to limitations of measurements and data processing, one can express only averaged and incomplete data conditions. Several applications demonstrate the reliable performance of the model for one-, two- and three-dimensional simulations. The described examples of application are part of a comprehensive erosion and agro-chemical transport study in a loessy agricultural catchment in southwestern Germany, and of a study on the sealing efficacy of capillary barriers in landfill covers.

Modeling daily rainfall using a semi-Markov representation of circulation pattern occurrence

Abstract The daily rainfall occurrence process is modeled as a process coupled to atmospheric circulation. Atmospheric circulations are classified into a finite number of circulation patterns. Time series of circulation patterns are modeled with the help of a semi-Markov field. Rainfall is linked to the circulation patterns using conditional probabilities. The model is applied using the classification scheme of the German Weather Service for the time period 1881–1988. Precipitation data measured at different locations for a period of 34 years are linked to the circulation patterns. Using the model several series of circulation patterns and corresponding rainfall occurrences are simulated. Statistics of the simulated and the observed sequences are similar. The model is also applicable for the simulation of nonstationary atmospheric conditions like climate change.

Capillarity-induced resonance of oil blobs in capillary tubes and porous media

We investigate the excitation by sound waves of capillary trapped oil blobs. The three-phase contact lines either remain pinned to the heterogeneities of the solid surface or slide if sound waves are applied. We derive approximate, analytical expressions for the resonance of oil blobs in capillary tubes for both types of contact line behavior. Based upon these simple model systems, we conclude that resonance of oil blobs is significant for coarse-grained but not fine-grained media.

Modeling Water Flow and Mass Transport in a Loess Catchment

Abstract For more than ten years, scientists of the Universities of Karlsruhe, Heidelberg and Bayreuth have investigated the water dynamics as well as transport of matter in the Weiherbach catchment, a loess area of about 6.3 km2 located in the “Kraichgau” of Southwest Germany. The investigations had the following scientific objectives: to identify the processes governing water and solute dynamics on different spatial and temporal scales in a small rural loess catchment, to develop a physically based numerical model to cope with all hydrological situations, i.e. simulating the extreme cases of very small runoff and flood runoff events as well as simulating soil water flow and transport, based on the obtained understanding of processes as well as to provide a data set for hydrological research for the intensively used rural loess areas of Central Europe. Modeling approaches for the processes dominating water and mass balance on the plot, hillslope and small catchment scale were developed based on process studies, tested against observations and implemented into the physically based, distributed model CATFLOW. During the observation period rainfall-runoff activity was low. The distribution of soil types was found to be strongly influenced by erosion leading to a typical hillslope soil catena. Preferential flow in earthworm burrows turned out to be crucial for solute transport in the soil on the plot and the hillslope scale, especially for leaching of surface applied pesticides, but also for runoff generation on the catchment scale. However, pesticide as well as phosphate loads in the Weiherbach creek during rainfall-runoff events were strongly determined by losses of sewer pipelines, which drain paved farm courtyards and pesticide loss due to runoff from courtyards. Those entrance paths are difficult to model in physical terms. A simplified, effective model approach for preferential flow based on field and laboratory measurements yielded simulation results in good accordance with short term observations of tracer transport on the plot scale as well as long term observations of tracer transport on the hillslope scale. Assuming that the soil catena of each hillslope may be substituted by the same typical soil catena a long term simulation of catchment scale water dynamics yielded results in acceptable agreement with the observed rainfall-runoff dynamics, soil water dynamics as well as evapo-transpiration. However, a falsification of the developed modeling approach was not possible, because of the uncertainty of the obtained parameter values due to measurement errors and due to the enormous variability of state variables and parameters in the Weiherbach soils.

Modelling of Vehicle Induced Turbulence in Air Pollution Studies for Streets

Vehicle-induced turbulence can be an important factor of pollutant dispersion in urban areas, especially under conditions of low wind speeds which are typical for street canyons. An experimental concept (Plate, 1982) for modelling the effects of vehicle-induced turbulence was applied in the present study. The movement of vehicles was simulated in a boundary-layer wind tunnel by small metal plates mounted on two belts moving along a modelled street canyon. The scaling factor was based on the ratio of turbulence production by cars to that by wind flow. The traffic was represented by the velocity, density, frontal area and drag coefficients of the vehicles. The velocity and traffic density were varied, and the influence of the vehicle-induced turbulence on concentration patterns at the canyon walls was studied. It was found that concentration decreases with an increasing ratio of vehicle to wind velocity and with an increase of traffic density. A dimensionless combination of vehicle to wind-velocity ratio and density factor was proved to be a universal parameter describing the dependence of the concentration on vehicle-induced turbulence. The wind tunnel measurements were compared with predictions by the numerical Operational Street Pollution Model (OSPM; Hertel and Berkowicz, 1989a). Differences between the wind tunnel and numerical results regarding effects of vehicle-induced turbulence are discussed. The comparison revealed general agreement between wind tunnel and numerical data. Turbulence and concentration measurements in a street canyon in Copenhagen have been additionally employed for analysis of the model results.