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This thesis is a synthesis that organizes the conceptual and philosophical foundations for the use of hydrological models in planning watershed conservation and expansion of green infrastructure. With the aim of creating an integrated map of ideas, the work not only connects theoretical and practical foundations but also offers a structured guide for future investigations and applications in the field. The initial chapter presents the epistemological bases, addressing the justifications and limitations of hydrological models. The second chapter develops an ontological approach with Systems Dynamics, exploring how model architecture defines hydrological responses and identifying points for strategic intervention. In the third chapter, the thesis reviews the evolution of hydrological paradigms, culminating in the Theory of Connectivity, which proposes an unification of surface and subsurface flow processes. The final chapter explores the role of Ecological Economics in watershed management, using the PLANS model in Payment for Ecosystem Services (PES) schemes to prioritize conservation areas at the operational scale of the farms. By synthesizing these concepts, the thesis establishes a structured body of knowledge that facilitates the advancement of new research, enabling the scientific community to articulate sustainable and adaptive solutions for watershed management with greater clarity and objectivity.

Nature-based solutions (NBS), especially associated with payment schemes for environmental services (PES), are an emerging issue in water resource planning. Thus, this research aimed to contribute to improving the understanding of how NBS can be developed in the long-term planning of water systems that supply cities. We sought to answer (1) how to identify the best strategy to expand a set of NBS in a given water system and (2) how to estimate the cost and benefit ratio of developing a set of NBS. The research was successful in its objectives because a metamodel was developed that integrates simulation models with optimization by dynamic programming. These models were conceived, as well as their parameters, based on what case studies, experimental studies and observational studies found in the literature suggest. The objective function of the optimization was to identify the expansion path that minimizes, over the planning horizon, the total cost of the water system, which was considered the sum of the scarcity cost, treatment cost and cost of NBS expansion. The benefit of the NBS expansion was conceived as the reduction in the total cost compared to the inaction scenario. The best expansion strategy was understood by the one that temporarily optimizes the advantages of each NBS in relation to the pressures presented by the future scenario. The application of the research problem to the Rio dos Sinos watershed suggested that terraced grasslands would consist of the NBS most suitable for improving water availability, while reforestation would be more suitable for improving water quality. Even so, the results pointed out that the large-scale expansion of NBS over the water systems responsible for supplying big cities would be economically unfeasible when assessing the direct costs and benefits. These results made it evident that the PES and similar programs observed in practice in Brazil operate in areas at least ten times smaller than those necessary to produce noticeable impacts on the water availability of large water systems, which generally occupy hundreds of square kilometers.

This study explores the hypothetical implementation of a stormwater wetland as a Best Management Practice (BMP) to mitigate urbanization impacts on stream waters within a 106-acre catchment of the Arroio do Salso creek watershed in Porto Alegre, RS. This watershed is part of the larger Lake Guaíba watershed, which is the sole water source for Porto Alegre and is regulated under the Class 2 water quality standard of CONAMA’s Resolution 357/2005. Additionally, the Arroio do Salso creek watershed is the focus of urban planning efforts aimed at waterways recovery, situating this research within the broader context of environmental planning at both regional and municipal levels. The study assessed the effectiveness of the stormwater wetland technology in improving water quality and regulating runoff flow across three scenarios: pre-development, post-development, and future. These assessments were conducted using a quali-quantitative hydrological model simulation that included non-point source pollution and utilized the EPA SWMM 5.0 and CLIMABR applications over a period of ten years of synthetic rainfall. The analysis of exceedance curves for various water quality components demonstrated significant improvements in water quality in the post-development and future scenarios, with system treatment removal efficiencies between 80 to 90% at least 50% of the time. However, compliance with water quality standards was only achievable in the future scenario, within a range of 30% to 35% of the time, due to enhanced infrastructure measures such as separate sewer coupling and combined sewer control. Regarding runoff flow regimes, the stormwater wetland notably regularized moderate discharges in both the post-development and future scenarios, maintaining available flows between 1 to 100 liters per second for extended durations compared to other conditions. Despite relying heavily on model parameters transposed from the literature, the findings suggest that stormwater wetlands represent a viable option within a suite of strategies for environmental recovery programs aimed at improving urban waterways.