The United States and Mexico have agreed to renew and expand a far-reaching conservation agreement that governs how they manage the overused Colorado River, which supplies water to millions of people and to farms in both nations, U.S. water district officials said. The agreement to be signed Wednesday calls for the U.S. to invest $31.5 million in conservation improvements in Mexico’s water infrastructure to reduce losses to leaks and other problems, according to officials of U.S. water districts who have seen summaries of the agreement.