When discussing sustainability it is impor tant to ask: How long should we expect things to
last? How do we defi ne the life of the project? Here is a hint: some dams in Spain and in North Africa
built by the Romans are still in use. That is a long time—2,000 years. There are also irrigation projects
in the PRC that have run continuously for 2,000 years. There is a physical life, clearly; physical life is
how long the thing lasts. But there is an economic life, which says, at some point, the operation and
maintenance costs may over take the value the project is supposed to provide.