It was launched in 2003 and has become one of the largest programmes for payments of watershed services in the world (Stanton et al. The design of PSAH has been improving over time to increase its cost-effectiveness, but there are concerns about its conservation impact (see Box 3.8). Trading or swapping irrigation water is common practice in Mexico (OECD, 2003). Water trading increased in the 1990s, partly due to the recognition by the 1992 National Water Law of the possibility of “transferring” water rights (Rubinos-Panta et al.,