Policy and investments based on assumptions of rational economic behaviour are often blind to the deeply ingrained social and cultural dispositions that govern choices. For instance, demand-driven ideologies backing community management assume that users will manage and pay for water infrastructure they need. Public awareness campaigns communicate water-related health risks assuming that information will change behaviour. However, extensive evidence across geographies and cultures have proven otherwise. To understand individuals’ and households’ daily water practices and how they vary across different environmental and institutional contexts, we designed and implemented the water diary method in Kenya and Bangladesh. The diaries captured household water source choices and expenditures every day for a whole year, complemented by interdisciplinary analysis of climate, infrastructure, and policy. With global and national monitoring efforts being largely based on aggregate snapshots generated through infrequent surveys, we argue how such granular behavioural dynamics can better inform policy and practice for an equitable water secure future.