Farmers use sprinklers not only to nourish crops but also to protect them from frost. This is the most efficient form of irrigation. No one knows exactly how many acres are still under rill irrigation. Estimates range from 40 percent to 60 percent of the 400,000 acres irrigated in the Bureau of Reclamation's Yakima Project. Assuming 50 percent, that means about 200,000 acres still have rill irrigation.
Experts disagree just how harmful rill irrigation is.
Some say it can be wildly inefficient, requiring more water to be drawn from the river than the other two methods. Lower river levels hurt fish in a variety of ways, including high water temperatures. But for the past decade, officials have been shepherding farmers toward alternatives to rill irrigation. The state Department of Ecology has said it wants most rill irrigation done away with and the river clean by 2017. One reason is water quality.