In many countries, hydropower provides a dominant share of the electricity supply. This dependence brings environmental benefits and competitive costs, but it also creates a vulnerability: When droughts occur or rainfall patterns change, the available energy decreases, and system security is put to the test. In this context, planning investments based only on historical averages is no longer sufficient. The practical question is how to decide, years in advance, what to build and what to reinforce when the key resource, water, can behave in very different ways.

