Calibration and validation of hydrodynamic, salt and heat transport models for Ria de Aveiro lagoon (Portugal)

Ria deAveiro is a very complex shallow water coastal lagoon located on the northwest of Portugal. Important issues would be left unanswered without a good understanding of hydrodynamic and transport processes occurring in the lagoon. Calibration and validation of hydrodynamic, salt and heat transport models for Ria de Aveiro lagoon are presented. The calibration of the hydrodynamic model was performed adjusting the bottom friction coefficient, through the comparison between measured and predicted time series of sea surface elevation for 22 stations. Harmonic analysis was performed in order to evaluate the model's accuracy. To validate the hydrodynamic model measured and predicted SSE values were compared for 11 stations, as well as main flow direction velocities for 10 stations. The salt and heat transport models were calibrated comparing measured and predicted time series of salinity and water temperature for 7 stations, and the RMS of the difference between the series was determined. These models were validated comparing the model results with an independent field data set. The hydrodynamic and the salt and heat transport models for Ria de Aveiro were successfully calibrated and validated. They reproduce accurately the barotropic flows and can therefore adequately represent the salt and heat transport and the heat transfer processes occurring in Ria deAveiro.

Diffusion coefficients in dense and supercritical fluids

Os coeficientes de difusão (D 12) são propriedades fundamentais na investigação e na indústria, mas a falta de dados experimentais e a inexistência de equações que os estimem com precisão e confiança em fases comprimidas ou condensadas constituem limitações importantes. Os objetivos principais deste trabalho compreendem: i) a compilação de uma grande base de dados para valores de D 12 de sistemas gasosos, líquidos e supercríticos; ii) o desenvolvimento e validação de novos modelos de coeficientes de difusão a diluição infinita, aplicáveis em amplas gamas de temperatura e densidade, para sistemas contendo componentes muito distintos em termos de polaridade, tamanho e simetria; iii) a montagem e teste de uma instalação experimental para medir coeficientes de difusão em líquidos e fluidos supercríticos. Relativamente à modelação, uma nova expressão para coeficientes de difusão a diluição infinita de esferas rígidas foi desenvolvida e validada usando dados de dinâmica molecular (desvio relativo absoluto médio, AARD = 4.44%) Foram também estudados os coeficientes de difusão binários de sistemas reais. Para tal, foi compilada uma extensa base de dados de difusividades de sistemas reais em gases e solventes densos (622 sistemas binários num total de 9407 pontos experimentais e 358 moléculas) e a mesma foi usada na validação dos novos modelos desenvolvidos nesta tese. Um conjunto de novos modelos foi proposto para o cálculo de coeficientes de difusão a diluição infinita usando diferentes abordagens: i) dois modelos de base molecular com um parâmetro específico para cada sistema, aplicáveis em sistemas gasosos, líquidos e supercríticos, em que natureza do solvente se encontra limitada a apolar ou fracamente polar (AARDs globais na gama 4.26-4.40%); ii) dois modelos de base molecular biparamétricos, aplicáveis em todos os estados físicos, para qualquer tipo de soluto diluído em qualquer solvente (apolar, fracamente polar e polar). Ambos os modelos dão origem a erros globais entre 2.74% e 3.65%; iii) uma correlação com um parâmetro, específica para coeficientes de difusão em dióxido de carbono supercrítico (SC-CO2) e água líquida (AARD = 3.56%); iv) nove correlações empíricas e semi-empíricas que envolvem dois parâmetros, dependentes apenas da temperatura e/ou densidade do solvente e/ou viscosidade do solvente. Estes últimos modelos são muito simples e exibem excelentes resultados (AARDs entre 2.78% e 4.44%) em sistemas líquidos e supercríticos; e v) duas equações preditivas para difusividades de solutos em SC-CO2, em que os erros globais de ambas são inferiores a 6.80%. No global, deve realçar-se o facto de os novos modelos abrangerem a grande variedade de sistemas e moléculas geralmente encontrados. Os resultados obtidos são consistentemente melhores do que os obtidos com os modelos e abordagens encontrados na literatura. No caso das correlações com um ou dois parâmetros, mostrou-se que estes mesmos parâmetros podem ser ajustados usando um conjunto muito pequeno de dados, e posteriormente serem utilizados na previsão de valores de D 12 longe do conjunto original de pontos. Uma nova instalação experimental para medir coeficientes de difusão binários por técnicas cromatográficas foi montada e testada. O equipamento, o procedimento experimental e os cálculos analíticos necessários à obtenção dos valores de D 12 pelo método de abertura do pico cromatográfico, foram avaliados através da medição de difusividades de tolueno e acetona em SC-CO2. Seguidamente, foram medidos coeficientes de difusão de eucaliptol em SC-CO2 nas gamas de 202 – 252 bar e 313.15 – 333.15 K. Os resultados experimentais foram analisados através de correlações e modelos preditivos para D12.

Understanding and modelling hydrological and soil erosion processes in burnt forest catchments

Forest fires implications in overland flow and soil erosion have been researched for several years. Therefore, is widely known that fires enhance hydrological and geomorphological activity worldwide as also in Mediterranean areas. Soil burn severity has been widely used to describe the impacts of fire on soils, and has being recognized as a decisive factor controlling post-fire erosion rates. However, there is no unique definition of the term and the relationship between soil burn severity and post-fire hydrological and erosion response has not yet been fully established. Few studies have assessed post-fire erosion over multiple years, and the authors are aware of none which assess runoff. Small amount of studies concerning pre-fire management practices were also found. In the case of soil erosion models, the Revised Universal Soil Loss Equation (RUSLE) and the revised Morgan–Morgan–Finney (MMF) are well-known models, but not much information is available as regards their suitability in predicting post-fire soil erosion in forest soils. The lack of information is even more pronounced as regards post-fire rehabilitation treatments. The aim of the thesis was to perform an extensive research under the post fire hydrologic and erosive response subject. By understanding the effect of burn severity in ecosystems and its implications regarding post fire hydrological and erosive responses worldwide. Test the effect of different pre-fire land management practices (unplowed, downslope plowed and contour plowed) and time-since-fire, in the post fire hydrological and erosive response, between the two most common land uses in Portugal (pine and eucalypt). Assess the performance of two widely-known erosion models (RUSLE and Revised MMF), to predict soil erosion rates during first year following two wildfires of distinctive burn severity. Furthermore, to apply these two models considering different post-fire rehabilitation treatments in an area severely affected by fire. Improve model estimations of post-fire runoff and erosion rates in two different land uses (pine and eucalypt) using the revised MMF. To assess these improvements by comparing estimations and measurements of runoff and erosion, in two recently burned sites, as also with their post fire rehabilitation treatments. Model modifications involved: (1) focusing on intra-annual changes in parameters to incorporate seasonal differences in runoff and erosion; and (2) inclusion of soil water repellency in runoff predictions. Additionally, validate these improvements with the application of the model to other pine and eucalypt sites in Central Portugal. The review and meta-analysis showed that fire occurrence had a significant effect on the hydrological and erosive response. However, this effect was only significantly higher with increasing soil burn severity for inter-rill erosion, and not for runoff. This study furthermore highlighted the incoherencies between existing burn severity classifications, and proposed an unambiguous classification. In the case of the erosion plots with natural rainfall, land use factor affected annual runoff while land management affected both annual runoff and erosion amounts significantly. Time-since-fire had an important effect in erosion amounts among unplowed sites, while for eucalypt sites time affected both annual runoff and erosion amounts. At all studied sites runoff coefficients increase over the four years of monitoring. In the other hand, sediment concentration in the runoff, recorded a decrease during the same period. Reasons for divergence from the classic post-fire recovery model were also explored. Short fire recurrence intervals and forest management practices are viewed as the main reasons for the observed severe and continuing soil degradation. The revised MMF model presented reasonable accuracy in the predictions while the RUSLE clearly overestimated the observed erosion rates. After improvements: the revised model was able to predict first-year post-fire plot-scale runoff and erosion rates for both forest types, these predictions were improved both by the seasonal changes in the model parameters; and by considering the effect of soil water repellency on the runoff, individual seasonal predictions were considered accurate, and the inclusion of the soil water repellency in the model also improved the model at this base. The revised MMF model proved capable of providing a simple set of criteria for management decisions about runoff and erosion mitigation measures in burned areas. The erosion predictions at the validation sites attested both to the robustness of the model and of the calibration parameters, suggesting a potential wider application.