sentences:  Since then, jurisdictional issues regarding water rights in the Rio Grande Valley have continued to cause tension between farmers on both sides of the border, according to Mexican political scientist Armand Peschard-Sverdrup.Mexico, United States and Canada signed the North American Free Trade Agreement (NAFTA) in 1994 with the goal of eliminating barriers to trade and investment.Since then, the United States and Mexico have tightened their economic ties.