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- Breuer, L., et al.
(författare)
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Assessing the impact of land use change on hydrology by ensemble modeling (LUCHEM). I : Model intercomparison with current land use
- 2009
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 32:2, s. 129-146
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces the project on 'Assessing the impact of land use change on hydrology by ensemble modeling (LUCHEM)' that aims at investigating the envelope of predictions on changes in hydrological fluxes due to land use change. As part of a series of four papers, this paper outlines the motivation and setup of LUCHEM, and presents a model intercomparison for the present-day simulation results. Such an intercomparison provides a valuable basis to investigate the effects of different model structures on model predictions and paves the ground for the analysis of the performance of multi-model ensembles and the reliability of the scenario predictions in companion papers. in this study, we applied a set of 10 lumped, semi-lumped and fully distributed hydrological models that have been previously used in land use change studies to the low mountainous Dill catchment. Germany. Substantial differences in model performance were observed with Nash-Sutcliffe efficiencies ranging from 0.53 to 0.92. Differences in model performance were attributed to (1) model input data, (2) model calibration and (3) the physical basis of the models. The models were applied with two sets of input data: an original and a homogenized data set. This homogenization of precipitation, temperature and leaf area index was performed to reduce the variation between the models. Homogenization improved the comparability of model simulations and resulted in a reduced average bias, although some variation in model data input remained. The effect of the physical differences between models on the long-term water balance was mainly attributed to differences in how models represent evapotranspiration. Semi-lumped and lumped conceptual models slightly outperformed the fully distributed and physically based models. This was attributed to the automatic model calibration typically used for this type of models. Overall, however, we conclude that there was no superior model if several measures of model performance are considered and that all models are suitable to participate in further multi-model ensemble set-ups and land use change scenario investigations.
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- Huisman, J. A., et al.
(författare)
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Assessing the impact of land use change on hydrology by ensemble modeling (LUCHEM) III : Scenario analysis
- 2009
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 32:2, s. 159-170
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Tidskriftsartikel (refereegranskat)abstract
- An ensemble of 10 hydrological models was applied to the same set of land use change scenarios. There was general agreement about the direction of changes in the mean annual discharge and 90% discharge percentile predicted by the ensemble members, although a considerable range in the magnitude of predictions for the scenarios and catchments under consideration was obvious. Differences in the magnitude of the increase were attributed to the different mean annual actual evapotranspiration rates for each land use type. The ensemble of model runs was further analyzed with deterministic and probabilistic ensemble methods. The deterministic ensemble method based on a trimmed mean resulted in a single somewhat more reliable scenario prediction. The probabilistic reliability ensemble averaging (REA) method allowed a quantification of the model structure uncertainty in the scenario predictions. It was concluded that the use of a model ensemble has greatly increased our confidence in the reliability of the model predictions.
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- Viney, N. R., et al.
(författare)
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Assessing the impact of land use change on hydrology by ensemble modelling (LUCHEM) II : Ensemble combinations and predictions
- 2009
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 32:2, s. 147-158
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports on a project to compare predictions from a range of catchment models applied to a mesoscale river basin in central Germany and to assess various ensemble predictions of catchment streamflow. The models encompass a large range in inherent complexity and input requirements. In approximate order of decreasing complexity, they are DHSVM, MIKE-SHE, TOPLATS, WASIM-ETH, SWAT, PRMS, SLURP, HBV, LASCAM and IHACRES. The models are calibrated twice using different sets of input data. The two predictions from each model are then combined by simple averaging to produce a single-model ensemble. The 10 resulting single-model ensembles are combined in various ways to produce multi-model ensemble predictions. Both the single-model ensembles and the multi-model ensembles are shown to give predictions that are generally superior to those of their respective constituent models, both during a 7-year calibration period and a 9-year validation period. This occurs despite a considerable disparity in performance of the individual models. Even the weakest of models is shown to contribute useful information to the ensembles they are part of. The best model combination methods are a trimmed mean (constructed using the central four or six predictions each day) and a weighted mean ensemble (with weights calculated from calibration performance) that places relatively large weights on the better performing models. Conditional ensembles. in which separate model weights are used in different system states (e.g. summer and winter, high and low flows) generally yield little improvement over the weighted mean ensemble. However a conditional ensemble that discriminates between rising and receding flows shows moderate improvement. An analysis of ensemble predictions shows that the best ensembles are not necessarily those containing the best individual models. Conversely, it appears that some models that predict well individually do not necessarily combine well with other models in multi-model ensembles. The reasons behind these observations may relate to the effects of the weighting schemes, non-stationarity of the climate series and possible cross-correlations between models.
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- Vautard, R., et al.
(författare)
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Skill and uncertainty of a regional air quality model ensemble
- 2009
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Ingår i: Atmospheric Environment. - : Elsevier BV. - 1352-2310. ; 43:31, s. 4822-4832
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Tidskriftsartikel (refereegranskat)abstract
- Recently several regional air quality projects were carried out to support the negotiation under the Clean Air For Europe (CAFE) programme by predicting the impact of emission control policies with an ensemble of models. Within these projects, CITYDELTA and EURODELTA, the fate of air quality at the scale of European cities or that of the European continent was studied using several models. In this article we focus on the results of EURODELTA. The predictive skill of the ensemble of models is described for ozone, nitrogen dioxide and secondary inorganic compounds, and the uncertainty in air quality modelling is examined through the model ensemble spread of concentrations. For ozone daily maxima the ensemble spread origin differs from one region to another. In the neighbourhood of cities or in mountainous areas the spread of predicted values does not span the range of observed data, due to poorly resolved emissions or complex-terrain meteorology. By contrast in Atlantic and North Sea coastal areas the spread of predicted values is found to be larger than the observations. This is attributed to large differences in the boundary conditions used in the different models. For NO2 daily averages the ensemble spread is generally too small compared with observations. This is because models miss highest values occurring in stagnant meteorology in stable boundary layers near cities. For secondary particulate matter compounds the simulated concentration spread is more balanced, observations falling nearly equiprobably within the ensemble, and the spread originates both from meteorology and aerosol chemistry and thermodynamics. © 2008.
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| 9. |
- Graff, Richard A., et al.
(författare)
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Property Rights and Corporate Finance
- 2005
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We examine a central result in corporate finance - the Modigliani-Miller capital structure irrelevance proposition - from a Coasian property rights perspective. Building upon the work of Coase, Demsetz and Cheung, we develop an enabling methodology to study the impact of positive Coasian transaction costs. When the Modigliani-Miller assumption of default-free debt is relaxed in the analysis of corporate leverage, either long-lived transaction costs related to property rights must be explicitly assumed away, or long-lived transaction costs related to property rights must be incorporated into the analysis.
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- Graff, Richard A., 2000, et al.
(författare)
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Property Rights, Risk and Leverage
- 2005
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Risk matters when corporate debt has a positive probability of default. Lenders have traditionally used covenants to protect their property rights because the financing and operating decisions of firms can reduce the value of the firm`s outstanding debt. We examine the use of captive finance subsidiaries and special purposed entities (SPEs) to partition default risk within the firm. A more complex arrangement of property rights within the firm allows the parent firm to retain operating flexibility while offering lenders better protection. We conclude that capital structure is a relevant decision variable for corporate managers because firms are able to obtain leveraged finance at a lower cost when risk is partitioned using separate legal structures within the firm.
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| 11. |
- Menon, Mahesh, et al.
(författare)
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Temporal difference modeling of the blood-oxygen level dependent response during aversive conditioning in humans : effects of dopaminergic modulation.
- 2007
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Ingår i: Biological Psychiatry. - 0006-3223 .- 1873-2402. ; 62:7, s. 765-772
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: The prediction error (PE) hypothesized by the temporal difference model has been shown to correlate with the phasic activity of dopamine neurons during reward learning and the blood-oxygen level dependent (BOLD) response during reward and aversive conditioning tasks. We hypothesized that dopamine would modulate the PE related signal in aversive conditioning and that haloperidol would reduce PE related activity, while an acute dose of amphetamine would increase PE related activity in the ventral striatum.METHODS: Healthy participants took an acute dose of amphetamine, haloperidol, or placebo. We used functional magnetic resonance imaging (fMRI) to measure the BOLD signal while they carried out an aversive conditioning task, using cutaneous electrical stimulation as the unconditioned stimulus (US) and yellow and blue circles as conditioned stimulus (CS+ and CS-, respectively).RESULTS: Prediction error related BOLD activity was seen only in the ventral striatum in the placebo subjects. The subjects given amphetamine showed a wider network of PE related BOLD activity, including the ventral striatum, globus pallidus, putamen, insula, anterior cingulate, and substantia nigra/ventral tegmental area. Haloperidol subjects did not show PE related activity in any of these regions.CONCLUSIONS: Our results provide the first demonstration that the modulation of dopamine transmission affects both the physiological correlates and PE related BOLD activity during aversive learning.
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| 14. |
- Spangberg, K, et al.
(författare)
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SveDem, how do you do, Sweden?
- 2005
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Ingår i: INTERNATIONAL PSYCHOGERIATRICS. - 1041-6102. ; 17, s. 337-338
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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