The research was designed to understand the factors that have sustained the use of a seven-hundred-year-old irrigation system (muang fai) in northern Thailand, despite the challenge of a new technology: pumping irrigation water from underground. There were two main objectives. The first was to assess whether the muang fai system is sustainable. The second was to determine the contribution of the muang fai to the groundwater sustainability in the study region. This second objective is related to the United Nation’s Sustainable Development Goal 6 (SDG6), water security. The results show that membership of the muang fai is changing slowly and the system is inherently stable, even given various pressures that tend to work against it, such as the increasing average farm size. Also, although the muang fai conserves water relative to other forms of irrigation, there is adequate recharge in the majority of the region to render this contribution to sustainability of the watershed relatively small.