Showing papers by "International Institute for Applied Systems Analysis published in 1989"

Some numerical experiments with variable-storage quasi-Newton algorithms

Abstract: This paper describes some numerical experiments with variable-storage quasi-Newton methods for the optimization of some large-scale models (coming from fluid mechanics and molecular biology). In addition to assessing these kinds of methods in real-life situations, we compare an algorithm of A. Buckley with a proposal by J. Nocedal. The latter seems generally superior, provided that careful attention is given to some nontrivial implementation aspects, which concern the general question of properly initializing a quasi-Newton matrix. In this context, we find it appropriate to use a diagonal matrix, generated by an update of the identity matrix, so as to fit the Rayleigh ellipsoid of the local Hessian in the direction of the change in the gradient. Also, a variational derivation of some rank one and rank two updates in Hilbert spaces is given.

Simulation of the long-term soil response to acid deposition in various buffer ranges

Abstract: A soil acidification model has been developed to estimate long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. Its major outputs include base saturation, pH and the molar Al/BC ratio, where BC stands for divalent base cations. Apart from net uptake and net immobilization of N, the processes accounted for are restricted to geochemical interactions, including weathering of carbonates, silicates and Al oxides and hydroxides, cation exchange and CO2 equilibriums. First, the model's behavior in the different buffer ranges between pH 7 and pH 3 is evaluated by analyzing the response of an initially calcareous soil of 50 cm depth to a constant high acid load (5000 molc ha−1 yr−1) over a period of 500 yr. In calcareous soils weathering is fast and the pH remains high (near 7) until the carbonates are exhausted. Results indicate a time lag of about 100 yr for each percent CaCO3 before the pH starts to drop. In non-calcareous soils the response in the range between pH 7 and 4 mainly depends on the initial amount of exchangeable base cations. A decrease in base saturation by H/BC exchange and Al/BC exchange following dissolution of Al3+ leads to a strong increase in the Al/BC ratio near pH 4. A further decrease in pH to values near 3.0 does occur when the A1 oxides and/or hydroxides are exhausted. The analyses show that this could occur in acid soils within several decades. The buffer mechanisms in the various pH ranges are discussed in relation to Ulrich's concept of buffer ranges. Secondly, the impact of various deposition scenarios on non-calcareous soils is analyzed for a time period of 100 yr. The results indicate that the time lag between reductions in deposition and a decrease in the Al/BC ratio is short. However, substantial reductions up to a final deposition level of 1000 molc ha−1 yr−1 are needed to get Al/BC ratios below a critical value of 1.0.

Optimal trajectories associated with a solution of the contingent Hamilton-Jacobi equation

Abstract: In this paper we study the existence of optimal trajectories associated with a generalized solution to the Hamilton-Jacobi-Bellman equation arising in optimal control. In general, we cannot expect such solutions to be differentiable. But, in a way analogous to the use of distributions in PDE, we replace the usual derivatives with “contingent epiderivatives” and the Hamilton-Jacobi equation by two “contingent Hamilton-Jacobi inequalities.” We show that the value function of an optimal control problem verifies these “contingent inequalities.” Our approach allows the following three results: (a) The upper semicontinuous solutions to contingent inequalities are monotone along the trajectories of the dynamical system. (b) With every continuous solutionV of the contingent inequalities, we can associate an optimal trajectory along whichV is constant. (c) For such solutions, we can construct optimal trajectories through the corresponding optimal feedback. They are also “viscosity solutions” of a Hamilton-Jacobi equation. Finally, we prove a relationship between superdifferentials of solutions introduced by Crandallet al. [10] and the Pontryagin principle and discuss the link of viscosity solutions with Clarke's approach to the Hamilton-Jacobi equation.

SCDAS — Decision support system for group decision making: Decision theoretic framework

Abstract: This paper presents the methodological framework for the Group Decision Support System named SCDAS. The system supports a group of decision makers working together on selecting the best alternative from a given, finite set of alternatives. The framework utilizes aspiration-led and quasisatisficing paradigms for eliciting user's preference, and the achievement function for ranking alternatives. Possible implementation of the system within the framework of a computerized teleconferencing system is discussed. Also, previous experience in applying the SCDAS system is presented.

Climate-induced water availability changes in Europe.

Abstract: Water availability changes in Europe are discussed in this paper, as seen in a perspective of climatic change. Such changes will be assessed in terms of the shifts in the watering of terrestrial ecosystems, as well as the shifts in the availability in terrestrial freshwater systems. A changing climate will have a major impact on the availability of water. Both wetter and drier conditions will have major implications for societal activities and land use patterns (e.g., agriculture, urban activities, waste water disposal, etc.). Some future research and monitoring activities are proposed by the authors to assess linkages in hydrological shifts to changes in land use patterns.

On the Stabilizing Effect of Predators and Competitors on Ecological Communities

Abstract: Ecological communities can lose their permanence if a predator or a competitor is removed: the remaining species no longer coexist. This well-known phenomenon is analysed for some low dimensional examples of Lotka-Volterra type, with special attention paid to the occurrence of heteroclinic cycles.

Putting trained labour power to work: the dilemma of education and employment.

Abstract: The employment problem of the LDCs arises from their age distribution, and especially the age distribution of their educated manpower. The supply of college graduates tends to increase faster than industry's demand for them. On one calculation for Indonesia, college graduates increase annually by 0.64 per thousand labour force in 1980, while demand goes up by 0.40 per thousand labour force. That means that 0.24 (per 1,OOO 1980 labour force) of college graduates will one year later have had to lake jobs that in 1980 were filled by senior high school graduates. This ‘pushdown’ effect applies at each level; 2.34 per thousand labour force who were junior high school graduates will have taken jobs that before were filled by elementary school graduates. The effect is rifling in any one year, but over 10 years he cumulative effect is great: for example, by 1990 virtually all those holding junior high school jobs (as classified in 1980) would have graduated from senior high school, on the model here used. If this...

On the Set-Valued Calculus in Problems of Viability and Control for Dynamic Processes: The Evolution Equation

Abstract: The paper deals with the problem of specifying the assembly of all solutions to a differential inclusion that satisfy a preassigned constraint on the state space variables (the "viability" tube). A "funnel equation" that describes the evolution of the cross-section of this tube (the "attainability domain") is then proposed and a feedback control problem for a system with state constraints is then proved to be solved on the basis of these constructions.

Mathematical Programming Package HYBRID

Abstract: HYBRID is a mathematical programming package which includes all the functions necessary for the solution of multicriteria LP problems and single-criteria linear-quadratic problems. HYBRID is specially useful for dynamic problems since the applied algorithm exploits the structure of a dynamic problem and the user has the advantage of handling a problem as a dynamic one which results in an easy way of formulation of criteria and of interpretation of results. HYBRID is oriented towards an interactive mode of operation in which a sequence of problems is to be solved under varying conditions (e.g., different objective functions, reference points, values of constraints or bounds). Criteria for multiobjective problems may be easily defined and updated with the help of the package. Besides that HYBRID offers many options useful for diagnostic and verification of a problem being solved. HYBRID is available in two versions: one for VAX 6210 (running under Ultrix-32) and one for a PC compatible with PC IBM/AT/XT.

Application of Two Soil Acidification Models to Historical Soil Chemistry Data from Sweden

Abstract: Soil chemistry data from southern Sweden, first sampled in 1949 and resampled in 1984, are used to test two models of the RAINS (Regional Acidification INformation and Simulation) model for soil acidification. Both versions were developed for predicting the acidification of forest soils on a large regional scale. The first one is part of the present RAINS model, designed at IIASA as a tool for evaluating policies for controlling the acidification of Europe’s environment. The second model, SMART (Simulation Model for Acidification’s Regional Trends), comprises a more detailed description of the cation exchange processes and will replace the first in the overall RAINS system. The soil chemistry data from southern Sweden offer a unique opportunity to test these models, since they provide a detailed description of the changes of the pool of base cations as well as the soil pH at different depths in the soil profile in 1949 and 1984. Results show that the changes in the amount of exchangeable base cations are predicted reasonably well by both models. The order of magnitude of the pH changes is predicted in the shallow profiles of the most acid soils, but they are clearly underestimated when deeper soil layers are included. The findings suggest that even rather simple soil acidification models can describe the changes of base cation reservoirs in soils. Describing soil pH, an “intensive” variable, is clearly more difficult. Since these models are driven by acid deposition and reproduce the observed base cation depletion reasonably well, the deposition of acidifying compounds is found to be the main source of soil acidification at these sites.

Modeling Time Patterns of Forest Soil Acidification for Various Deposition Scenarios

Abstract: A soil acidification model has been developed to estimate long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. Major model outputs include base saturation, pH and the Al/BC ratio, where BC stands for divalent base cations. The processes accounted for are restricted to geochemical interactions, including weathering of carbonates, silicates and aluminum hydroxides, cation exchange and CO2 equilibria. With the exception of silicate weathering, all processes are described by equilibrium reactions. Model parameters mainly refer to equilibrium constants for the various processes, while model variables include the amounts of base cations and aluminum in carbonates, hydroxides, the exchange complex and in soil solution. The model behavior in the different buffer ranges between pH 7 and pH 3 has been evaluated by analyzing the response of an initially calcareous soil to a constant high acid load (5000 molc ha−1 yr−1) over a period of 500 years. In calcareous soils, weathering is fast and the pH remains high (near 7) until the carbonates are exhausted. In non-calcareous soils the response in the range between pH 7 and pH 4 mainly depends on the initial amount of exchangeable base cations. Although the model cannot be strictly verified for its ultimate use of long-term predictions, it can be used as a tool to obtain a conceptual understanding of long-term soil responses for the broad concern of resource managers and policy makers. Therefore it will be linked to the Regional Acidification Information and Simulation model (RAINS) to analyze the impact of various emission scenarios on a European scale.

HYBRID A Mathematical Programming Package

Abstract: HYBRID 3.1. is a mathematical programming package which includes all the functions necessary for the solution of linear programming problems, both static and dynamic (in fact also for problems with structure more general then the classical formulation of dynamic linear problems). HYBRID 3.1. may be used for both single- and multi-criteria problems. The package may be also used for solving single-criteria linear-quadratic problems. Since HYBRID is designed for real-life problems, it offers many options useful for diagnostics and verification of a model for a problem being solved.

Expanding territories: transport systems past and future

Abstract: Transport and communication systems are the elements which integrate human activities in space and time. They record our past and will be the important determinants of our future. As a rule, those forms of infrastructure which are used for the movement of people, goods and information diffuse slowly and span many decades from their first introduction to obsolescence. Some of them are almost immortal, even though they often provide different services from those originally intended. Obsolete canals were reused to build railways a century ago in England. Ancient Roman roads have often been buried beneath modern highways. Old harbours are being converted into modern commercial and residential areas.

Broad Scale Effects of Sulfur Deposition: A Response Surface Analysis of a Complex Model

Abstract: Prediction of the broad-scale environmental effects of acid deposition requires the extrapolation of site specific measurements to larger geographical regions. This paper uses Monte Carlo simulation of the RAINS Lake Model to investigate changes in parameter sensitivity and prediction uncertainty for geographic regions which differ in critical soil characteristics and levels of sulfur deposition. Statistical analysis of the results show that: (1) sensitivities and uncertainties shift with regional changes in soil characteristics; (2) simple response surface models can sometimes describe the pattern of predicted sulfur deposition effects; and (3) the degree of fit of the response surface model (R2) indicates the relative homogeneity of response to acid deposition.