This chapter emphasizes the construction of a decision support tool (DST) in the DSS, elucidating the relationship between tourism and water quality using system dynamics (SD) modeling. We divide the environmental sub-model into three categories: tourism, water quantity, and water quality, also applied several important assumptions in building the models. Our Stock-Flow Diagram (SFD) model provides a detailed analysis of the complex interactions within the tourism industry at Lake Singkarak, highlighting the interconnected relationship between tourists, tourism revenue, and environmental sustainability. The model’s scope is limited to analyzing annual changes, encompassing the period from 2013 to 2022 for existing conditions and projecting future scenarios from 2023 to 2042. The water quantity sub-model comprehensively analyzes rainfall dynamics, inflow and outflow rates, lake volume and elevation changes, and hydroelectric power generation. The water quality sub-model focuses on the pollutants entering the lake through rivers, fish farming activities, and tourism. We found that the current number has not significantly affected the quality of the lake’s water, unlike other anthropogenic activities, such as agriculture. We highlighted that increasing the number of tourists up to threefold is still possible, assuming that all the pre-requisite assumptions are met. Overall, our current models are sufficient to inform sustainable tourism management strategies for the lake. Initiatives such as expanding wastewater treatment facilities and promoting eco-friendly tourism practices can enhance economic benefits while protecting the lake’s ecosystem.