Background: There is increasing recognition of the complexity underlying WASH conditions in developing countries. This article explores the complexity by assessing the vulnerability of a specific area to poor WASH conditions using a qualitative approach. Methods: We present our findings for the district of East Sumba in Indonesia. This area is known as one of the poorest regions in Indonesia with inadequate WASH services, indigenous belief that hinder the practice of WASH-related behaviours, and has a high rate of children malnutrition. All the factors that contribute to poor WASH conditions were discussed through the lens of the Financial, Institutional, Environmental, Technological, and Social (FIETS) framework. We then summarised the factors and visualized the “system” using a mind map which shows how factors are interconnected and helps to find the root causes of poor WASH conditions. Results: There are three main challenges that inhibit the improvement of WASH conditions in this area: inadequate institutional capacity, water scarcity, and poor socio-economic conditions. We found that a village leader is the most important actor who influences the sustainability of WASH services in this area and healthcare workers are influential WASH promoters. This study also shows how culture shapes people’s daily lives and institution performance, and influences the current WASH conditions in East Sumba. The mind map shows there is an overlap and interconnection between FIEST aspects and WASH conditions in the study area. Conclusion: WASH conditions are influenced by many factors and are often interconnected with each other. Understanding this complexity is necessary to improve WASH conditions and sustain adequate WASH services in developing countries. Finally, WASH interventions have to be considerate of the prevailing cultural practices and should involve multidisciplinary stakeholders.

The sustainability of the water supply program in developing countries is influenced by many inter-linked and dynamic factors, suggesting the need to analyse the system behaviour of the water supply program. However, no study analyses factors influencing the sustainability of rural drinking water supply programs holistically, and this study aims to fill that gap. This study utilized a system dynamics approach based on a case study of a community-based rural drinking water supply program (PAMSIMAS in Bahasa) in Magelang Regency, Indonesia. Five sustainability aspects were considered in the model development and simulation: Financial, institutional, environmental, technical, and social aspects. Eight scenario analyses related to those five aspects were conducted. The causal loop diagrams suggest that the overall loop in the system is reinforcing, meaning that the improvement in one aspect will improve the overall condition of the system and deterioration in one aspect will reduce the overall condition of the system. Scenario analysis shows that external fund is critical to support the program financially, especially at the beginning of the project when the piped system is being built and water revenue is still low. Scenario and sensitivity analyses revealed that human factors, i.e., the performance of the water board and response and support from the community, positively influence the sustainability of the water supply program. Additionally, the water board plays a key role in accelerating the pipe network growth. Finally, this paper argues that visualising and simulating the causal relationship and dynamic behaviour of the rural water supply program are critical for water stakeholders to better design and implement the water supply program.