Water management in delta floodplains worldwide faces many challenges due to the changing climate and increasing human intervention in the hydrological regimes of rivers. Irrigation and drainage systems are necessary components of a water management strategy that aims to support human habitation and agricultural production, but which need effective coordination in order to adapt to exogenous impacts. However, management of such systems often fails for a variety of reasons. In the floodplain of the Vietnamese Mekong Delta, irrigation and drainage systems under dike protection confront ineffective implementation of water management policy, posing a challenge for adaptation to exogenous impacts on the hydrological regime. Over the past two decades, farmers have increasingly cultivated annual triple‐rice crops in high‐dike compartments, ignoring government regulations that call for flood retention. This study analyses interviews with farmers to identify their motivation for not implementing the triennial cropping off‐season advised by the local government (3–3–2 cycle). Our findings show that farmers have avoided implementing the 3–3–2 cycle because of various disadvantages that the system presents for them. Local officials, in turn, have accepted farmers' disregard of the rule. Lessons learnt from this study are considered to explore measures to effectively adapt to future hydrological changes. © 2017 The Authors. Irrigation and Drainage published by John Wiley & Sons Ltd on behalf of International Commission for Irrigation and Drainage