Assessments of the costs and benefits of adaptation measures for water systems are important to inform decisions about prioritising adaptation responses and to improve their cost-effectiveness. Yet, the evidence base estimating the costs and benefits of adapting water systems remains sparse (Box 2.3). In principle, decisions makers considering the timing of adaptation actions will compare the present value of adaptation today with the present value of adaptation at a later stage. The decision to act earlier or later will essentially depend on three factors: i) the difference in adaptation costs over time, ii) the short-term benefits of adaptation, and iii) the long-term effects of early adaptation (Agrawala and Fankhauser, 2008). They include making adjustments to long-lived infrastructure investments thus avoiding the cost and inconvenience of expensive retrofits.