The first-ever official shortage on the Colorado River has intensified a debate over how to provide water for 40 million people across the Southwest and irrigate fields of thirsty crops like wheat, cotton and alfalfa. Few voices outside government are more influential than that of the Walton family, billionaire heirs to the Walmart Inc. fortune, who have long advocated water markets as a key part to solving the region’s woes. But some environmental groups say the Waltons drown out other, nonmarket approaches.