Mexican officials are scrambling to come up with a plan to increase the amount of water the country sends to the United States because of growing concern that President Donald Trump could drag a dispute over an 81-year-old water treaty into trade negotiations, according to three sources familiar with the matter. Under a 1944 treaty that outlines water sharing between the two countries through a network of interconnected dams and reservoirs, Mexico must send 1.75 million acre-feet of water to the U.S. from the Rio Grande every five years.