From pixels to policy: evaluating the behavioural impact and cost effectiveness of picture nudges in water conservation at university residences

Our study examines the impact of visual nudges on encouraging water-saving behavior in response to the water shortage in Nelson Mandela Bay. We conducted a randomized experiment in two female student residences at Nelson Mandela University, assigning one as the treatment site with nudge imagery and the other as a control. Water usage was tracked in both sites through a smart water meter system, and the data were analyzed with a difference-in-difference approach. The results indicate that the nudge messages led to a 33% reduction in water usage at the treated residence, amounting to approximately 8,037 liters saved daily. A cost-benefit analysis based on municipal tariffs shows that the experiment's benefits exceeded its costs, even at the lowest tariff rate. This suggests that visual nudges may be an effective tool for local water authorities to address water conservation challenges. A sensitivity analysis further confirms the sustained influence of the visual nudge in promoting voluntary water conservation over time. Based on these findings, we recommend that water authorities implement this approach in high-consumption buildings to foster a conservation-oriented culture.