In a significant step toward construction of the Bay Area’s first major new reservoir in nearly two decades, Silicon Valley’s largest water provider has begun negotiations to buy more than 12,000 acres of rural ranch land — an area nearly half the size of San Francisco. The Santa Clara Valley Water District is hoping to build an $800 million reservoir in southern Santa Clara County near Pacheco Pass, along with a dam up to 300 feet high. The reservoir would hold 130,000 acre-feet of water — enough to meet the water needs of 650,000 people for a year.