A group of 30 agencies that supply water to homes and businesses throughout the western United States has pledged to rip up lots of decorative grass to help keep water in the over-tapped Colorado River. The agreement signed Tuesday by water agencies in Southern California, Phoenix and Salt Lake City and elsewhere illustrates an accelerating shift in the American West away from well-manicured grass that has long been a totem of suburban life, having taken root alongside streets, around fountains and between office park walkways.