Andrew M. Walker

Agent based simulation of a small catchment water management in northern Thailand. Description of the CATCHSCAPE model

Abstract Due to mounting human pressure, stakeholders in northern Thailand are facing crucial natural resources management (NRM) issues. Among others, the impact of upstream irrigation management on downstream agricultural viability is a growing source of conflict, which often has both biophysical and social origins. As multiple rural stakeholders are involved, appropriate solutions should only emerge from negotiation. CATCHSCAPE is a Multi-Agent System (MAS) that enables us to simulate the whole catchment features as well as farmer’s individual decisions. The biophysical modules simulate the hydrological system with its distributed water balance, irrigation scheme management and crop and vegetation dynamics. The social dynamics are described as a set of resource management processes (water, land, cash, labour force). Water management is described according to the actual different levels of control (individual, scheme and catchment). Moreover, the model’s architecture is presented in a way that emphasises the transparency of the rules and methods implemented. Finally, one simulated scenario is described along with its main results, according to different viewpoints (economy, landscape, water management).

Flexibility in a Metal–Organic Framework Material Controlled by Weak Dispersion Forces: The Bistability of MIL‐53(Al)

Breathtaking MOFs: DFT calculations reveal that the exceptional, thermally induced density change of the metal-organic framework MIL53(Al) is controlled by a competition between shortand long-range interactions and entropic factors. As shown in the picture (C green, Al cyan, O red, H white), dispersive interactions between the phenyl rings are responsible for stabilizing a narrow-pore form at low temperature. At 325-375 K, vibrational entropy causes the structure to expand markedly, permitting large volumes of light gases to be adsorbed.

The rural constitution and the everyday politics of elections in Northern Thailand

Abstract The Thai coup of 19 September 2006 derived ideological legitimacy from the view that the Thaksin governments electoral mandate was illegitimate because it had been “bought” from an unsophisticated and easily manipulated electorate. There is nothing new about this argument, nor its use in justifying military interference. Political commentators have asserted regularly that the Thai populace lacks the basic characteristics essential for a modern democratic citizenry. Accounts of the deficiencies of rural voters often focus on their parochialism, their lack of political sophistication, the vulnerability to vote buying and the influence of electoral canvassers (hua khanaen). This article challenges this negative portrayal of rural electoral culture. Drawing on ethnographic field work in northern Thailand, it is argued that the everyday politics of elections is informed by a range of different electoral values that shape judgements about legitimate, and illegitimate, political power in electoral contexts. These local values can be usefully thought of as comprising a “rural constitution.”

Elastic anisotropy of D″ predicted from global models of mantle flow

In order to test the hypothesis that seismic anisotropy in the lowermost mantle is caused by the development of a post-perovskite lattice preferred orientation, and that anisotropy can thus be used as a probe of the dynamics of the mantles lower boundary layer, an integrated model of texture generation in D″ is developed. This is used to predict the elastic anisotropy of the lowermost mantle as probed by global anisotropic tomographic inversions. The model combines the current 3D mantle flow field with simulations of the deformation of post-perovskite polycrystalline aggregates. Different descriptions of single crystal plasticity can lead to model results which are anti-correlated to each other. In models where post-perovskite deformation is accommodated by dislocations moving on (010) or (100), patterns of anisotropy are approximately correlated with the results of tomographic inversions. On the other hand, in models where dislocations move on (001) patterns of anisotropy are nearly anti-correlated with tomographic inversions. If all the seismic anisotropy in D″ extracted from global anisotropic inversions is due to the presence of a lattice preferred orientation in post-perovskite in the lowermost mantle, and if the results of the tomographic inversions are not strongly biased by the sampling geometries, these results suggest that, in contrast to ideas based on the 1D anisotropic signal, deformation of post-perovskite in the lowermost mantle may be accommodated by dislocations moving on (010) or (100). Alternatively, a significant portion of the anisotropic signal may be caused by mechanisms other than the alignment of post-perovskite crystals.

A Methodology for Eliciting and Modelling Stakeholders' Representations with Agent Based Modelling.

In the frame of using models to promote dialogue among stakeholders, we are interested in modelling representations as a mean to share viewpoints and opinions. A first trend for modelling representations uses socio-cognitive theories as frameworks for modelling. This article proposes a different approach combining Knowledge Engineering elicitation techniques and Agent Based Modelling. The use of elicitation techniques provides a methodological framework to capture representations directly from stakeholders’ discourse. Agent Based Modelling serves as a modelling structure which enables managing representations’ heterogeneity and conflicting opinions. The proposed methodology is detailed and applied to farmers’ representations of runoff and erosion in south of France. To achieve the construction of a running model of the interviewees’ shared representation, it was necessary to implement theoretical data supplementing the elicited information. We argue that companion modelling approach can help sharing and discussing those non-elicited information with stakeholders.

Computer modelling of the energies and vibrational properties of hydroxyl groups in α- and β-Mg2SiO4

The structure, formation energies and infrared (IR) active vibrational modes of hydrous defects in the iron free end members of two of the most important minerals of the Earths mantle, alpha- and beta-Mg2SiO4, are studied by atomic-scale computational modelling in order to identify the hydrogen incorporation mechanism observed in experiment. Two computational methods are used; calculations based on inter-atomic potentials provide information on all defect configuration in the two minerals, and a combined quantum mechanical/molecular mechanics embedded cluster method is used to validate selected results. For forsterite (alpha-Mg2SiO4), the results suggest that IR bands at low frequencies (wavenumbers 3000-3250 cm(-1)) are related to protons populating M1 vacancies. Despite the unfavourable creation of silicon vacancies, calculated medium- and high-frequency IR bands are linked to protons occupying vacant Si sites. For iron-free wadsleyite (beta-Mg2SiO4) IR frequencies for hydrated cation vacancies have been calculated for the first time. The main doublet at 3360-3326 cm(-1) is attributed to two OH groups located in a vacant M3 site. IR bands at higher wavenumber such as the anisotropic doublet at 3615-3580 cm(-1) appear to be linked to OH in vacant Si sites. Low accuracy on the calculated frequencies does not permit a strict and rigorous assignment of each individual IR band observed in hydrous forsterite and wadsleyite. However, it does allow the identification of the most favourable site for protonation and provides a useful approximation to the corresponding IR stretching frequencies for a given hydrogen incorporation mechanisms in these nominally anhydrous silicate structures.

Atomic scale models of dislocation cores in minerals: progress and prospects

Abstract Recent advances in computer simulation at the atomic scale have made it possible to probe the structure and behaviour of the cores of dislocations in minerals. Such simulation offers the possibility to understand and predict the dislocation-mediated properties of minerals such as mechanisms of plastic deformation, pipe diffusion and crystal growth. In this review the three major methods available for the simulation of dislocation cores are described and compared. The methods are: (1) cluster-based models which combine continuum elastic theory of the extended crystal with an atomistic model of the core; (2) dipole models which seek to cancel the long-range elastic displacement caused by the dislocation by arranging for the simulation to contain several dislocations with zero net Burgers vector, thus allowing a fully periodic super-cell calculation; and (3) the Peierls-Nabarro approach which attempts to recast the problem so that it can be solved using only continuum-based methods, but parameterizes the model using results from atomic-scale calculations. The strengths of these methods are compared and illustrated by some of the recent studies of dislocations in mantle silicate minerals. Some of the unresolved problems in the field are discussed.

Evidence from numerical modelling for 3D spreading of [001] screw dislocations in Mg2SiO4 forsterite

Computer simulations have previously been used to derive the atomic scale properties of the cores of screw dislocations in Mg2SiO4 forsterite by direct calculation using parameterized potentials and via the Peierls–Nabarro model using density functional theory. We show that, for the [001] screw dislocation, the parameterized potentials reproduce key features of generalized stacking fault energies when compared to the density functional theory results, but that the predicted structure of the dislocation core differs between direct simulation and the Peierls–Nabarro model. The [001] screw dislocation is shown to exhibit a low-energy non-planar core. It is suggested that for this dislocation to move its core may need to change structure and form a high-energy planar structure similar to that derived from the Peierls–Nabarro model. This could lead to dislocation motion via an unlocking–locking mechanism and explain the common experimental observation of long straight screw dislocation segments in deformed olivine.

Defects and dislocations in MgO: Atomic scale models of impurity segregation and fast pipe diffusion

Dislocations are known to influence the formation and migration of point defects in crystalline materials. We use a recently developed method for the simulation of the cores of dislocations in ionic materials to study the energy associated with the formation of point defects close to the core of a ½ {10} edge dislocation in MgO. These are then compared with the energies for the same point defects in otherwise perfect MgO. It is found that all of the defect species are bound to the dislocation core, with binding energies of between 1.5 and 2.0 eV. Vacancies are found to be most stable when they remove under-coordinated ions at the tip of the extra half plane, while the impurities are most stable within the dilatational stress field below the glide plane. By mapping the distribution of energies for point defects around the dislocation line we reveal the coupling between the effective point defect size and the stress field associated with the dislocation. We also examine the energy barrier to diffusion of vacancies along the dislocation line and find that vacancy migration along the dislocation line will be substantially enhanced compared to migration through the dislocation-free crystal structure. Activation energies are 0.85–0.92 of the barrier in the perfect crystal, demonstrating the importance of pipe diffusion along extended defects for low temperature mobility in ionic materials.

The origin of the compressibility anomaly in amorphous silica: a molecular dynamics study

We propose an explanation for the anomalous compressibility maximum in amorphous silica based on rigidity arguments. The model considers the fact that a network structure will be rigidly compressed in the high-pressure limit, and rigidly taut in the negative pressure limit, but flexible and hence softer at intermediate pressures. We validate the plausibility of this explanation by the analysis of molecular dynamics simulations. In fact this model is quite general, and will apply to any network solid, crystalline or amorphous; there are experimental indications that support this prediction. In contrast to other ideas concerning the compressibility maximum in amorphous silica, the model presented here does not invoke the existence of polyamorphic phase transitions in the glass phase.