Explicit Numerical Model Research Articles

Landscape gene flow, coexistence and threshold effect: The case of genetically modified herbicide tolerant oilseed rape ( Brassica napus)

Globally there have been a number of concerns about the development of genetically modified crops many of which relate to the implications of gene flow at various levels. In Europe these concerns have led the European Union (EU) to promote the concept of ‘coexistence’ to allow the freedom to plant conventional and genetically modified (GM) varieties but to minimise the presence of transgenic material within conventional crops. Should a premium for non-GM varieties emerge on the market, the presence of transgenes would generate a ‘negative externality’ to conventional growers. The establishment of maximum tolerance level for the adventitious presence of GM material in conventional crops produces a threshold effect in the external costs. The existing literature suggests that apart from the biological characteristics of the plant under consideration (e.g. self-pollination rates, entomophilous species, anemophilous species, etc.), gene flow at the landscape level is affected by the relative size of the source and sink populations and the spatial arrangement of the fields in the landscape. In this paper, we take genetically modified herbicide tolerant oilseed rape (GM HT OSR) as a model crop. Starting from an individual pollen dispersal function, we develop a spatially explicit numerical model in order to assess the effect of the size of the source/sink populations and the degree of spatial aggregation on the extent of gene flow into conventional OSR varieties under two alternative settings. We find that when the transgene presence in conventional produce is detected at the field level, the external cost will increase with the size of the source area and with the level of spatial disaggregation. On the other hand when the transgene presence is averaged among all conventional fields in the landscape (e.g. because of grain mixing before detection), the external cost will only depend on the relative size of the source area. The model could readily be incorporated into an economic evaluation of policies to regulate adoption of GM HT OSR.

Initial Application of Unequal Step Grid and Explicit Method in the Simulation of Nonlinear Propagation of Gravity Wave

采用3阶TVD型显式时间积分方法,建立了2维可压缩大气中重力波非线性传播的非等间距显式数值模式.对小振幅重力波传播过程的模拟结果表明,该模式能够很好地再现小振幅重力波的传播过程,并能保持能量守恒关系和各扰动量之间的相位关系,与线性重力波理论预测的结果吻合很好.对有限振幅重力波的模拟结果表明,与格点数相同的等间距网格中的模拟结果相比,采用非等间距网格能够以更高的分辨率模拟重力波的不稳定传播直到破碎的整个过程;在非等间距网格中的模拟结果与在加密一倍的等间距网格中的模拟结果相同,说明了用非等间距网格模拟重力波的饱和与破碎过程是正确的,并且采用非等间距网格能够大大减少计算量.

Explicit Simulations of the Intertropical Convergence Zone

Abstract The intertropical convergence zone (ITCZ) is one of the most important components of the global circulation. In order to understand the dynamical processes that regulate its formation, latitudinal preference, and structure, explicit two-dimensional numerical modeling of convection on an equatorial beta plane was conducted with a nonhydrostatic cloud-system-resolving model. The model was forced by energy fluxes associated with constant sea surface temperature (SST) and by horizontally homogeneous radiative cooling. Two distinct patterns were identified for the spatial distribution of convective activity in the Tropics. The first was characteristic of enhanced off-equator convection, namely, a double ITCZ-like morphology (one more salient than the other) straddling the equator during the early period of the integration. The second featured enhanced equatorial convection, namely, a single ITCZ-like morphology on the equator during the later quasi-equilibrium period. Diagnostic analysis and two addit...

Effect of fracture aperture variations on the dispersion of contaminants

Dispersion of dissolved contaminants in fractured rocks is strongly dependent on the variability of fracture aperture. Large fracture aperture regions result in significant channeling of the fluid flow, accelerating the movement of solutes in a particular direction, which may differ locally with respect to the main flow direction from high to low hydraulic potential. This may result in very early breakthrough of the contaminants, significantly earlier than in the conventional “parallel plate” simplification of fluid flow in fractured systems. In addition, flow and transport in the low aperture regions can be orders of magnitude smaller, which has major implications for remediating contaminated fractured media. In this study we observed the transport of a nonsorbing solute in a naturally fractured rock which was characterized at very high resolution using computed tomography scanning to determine the fracture aperture distribution. We compared the observed breakthrough curve of the solute at various flow rates with the predicted breakthrough curve using the fracture's geostatistics and small perturbation analysis, as well as direct numerical simulation of flow through the fracture using the measured fracture aperture distribution. The experimentally determined fracture transmissivity and dispersivity are within a factor of 2 of those predicted using either the small perturbation approach (using the fracture geostatistics) or explicit numerical modeling of breakthrough, which differ by around 20%, indicating that the geostatistics of the fracture aperture can be used to predict the approximate shape of the breakthrough curve for a fracture.

A Finite-element Model of Radial and Axial Conductivities for Individual Roots: Development and Validation for Two Desert Succulents

A morphologically explicit numerical model for analysing water uptake by individual roots was developed based on a conductor network, with specific conductors representing axial or radial conductivities for discrete root segments. Hydraulic conductivity (L p ; m s -1 MPa -1 ) was measured for roots of Agave deserti Engelm. and Opuntia ficus-indica (L.) Miller by applying a partial vacuum to the proximal ends of excised roots in solution. L p was also measured for 40- to 80-mm segments along a root, followed by measurements of axial conductivity and calculation of radial conductivity (...)

Model for Refraction of Water Waves

A simple explicit numerical model suitable for a personal computer is discussed that provides for the refraction and shoaling of linear and nonlinear water waves over irregular bathymetry, including wave‐current interaction. Finite‐differenced forms of the conservation of wave action and the irrotationality of the wave number equations are used in the model. Applications of the model to a planar shoreline, where Snell's law is valid, show that the model results are independent of the step size. Two other bathymetries are examined: one, an idealized coastline with a rip channel; and the other, a section of the New Jersey coastline. The model output is displayed in terms of wave height and direction vectors.

Estuarine response to a freshwater pulse

In September 1984, a controlled reservoir discharge increased the runoff of the Eastmain River (James Bay, Canada) by a factor of 50 over a six-day period. Changes in circulation and salinity resulting from the freshwater pulse were followed over a 15-day period. Results of a one-dimensional finite-difference explicit scheme numerical model with variable discharge are compared with observations. Model input included actual discharge values, observed wind speed and direction, offshore salinity and predicted tidal heights at the river mouth. Both field data and model results show that salinity values in the lower estuary fell to zero within 36 h of the increase in discharge observed 35 km upstream of the river mouth. Semi-diurnal tidal current amplitude lessened as the mean flow increased. Upon return to normal discharge levels at 35 km, the circulation in the estuary reverted to its normal range within one day, whereas salinity adjustment took place over a four-day period.

Salinity intrusion in the Eastmain River estuary following a major reduction of freshwater input

On July 19, 1980, 80% of the runoff of Eastmain River, a subarctic shallow estuary, was diverted into the La Grande River for hydroelectric development. Consequent to the diversion, the estuary, which had been mostly salt‐free, was subjected to a gradual saltwater intrusion along a 10‐km section in its lower reaches. The adjustment of the salinity regime to a new quasi‐steady state took place over a period of about 40 days. The circulation field reached a new quasi‐equilibrium within 8 days. The 2‐month interval following the river diversion, termed the transition period, was analyzed in terms of both time and space modifications of the circulation and salinity fields. A one‐dimensional finite difference explicit scheme numerical model was applied to the estuary as a complement to observational data. Good agreement was obtained between the model results and the low‐frequency current meter observations of salinity and longitudinal velocity. These results showed that the salinity intrusion was primarily governed by tidal dispersion.