The Imperial Irrigation District, which holds some of the oldest and largest rights to Colorado River water, on Monday tentatively agreed to a one-time contribution of up to 250,000 acre-feet of surplus water if needed to stave off shortages in Lake Mead. But they tacked on several last-minute conditions aimed at easing farmers’ fears of permanently losing water, and to force federal and state officials to guarantee funding for clean-up of the Salton Sea.