Fig. 1. The low (150- to 300-mm annual) precipitation zone of east-central Washington and north-central Oregon covers 1.56 million cropland hectares and is, by far, the largest contiguous cropping zone in the western United States. The Inland Pacific Northwest intermediate (300- to 450-mm) and high (450- to 600-mm) precipitation cropping zones are mostly to the east of the low-precipitation zone.