Precipitation is greatly affected by elevation. Thus, PRISM modeling software has been used to create the spatial data layers for precipitation.

Annual values allow classification of regions into arid, semi-arid, and humid areas and provide a starting point for defining species suitability zones. Seasonal (winter, spring, summer, autumn) precipitation provides further information - allowing areas to be categorized as providing adequate or inadequate seasonal precipitation. Monthly values are preferred, however, as a compromise between coarse seasonal and annual values and more difficult to obtain and use weekly or daily data models.

Typically, highly productive cultivated forages (ryegrasses, orchardgrass, tall fesuce, alfalfa, white clover, red clover) need approximately 18-25 inches of annual precipitation. Below 18 inches, semi-arid tolerant types (wheatgrasses, stipa, etc.) are utilized.

Combining soil texture and depth information with precipitation allows calculation of available soil moisture. Calculated evapotranspiration (with --- and ---) futher improves suitability estimates. ET calculations indicate approximately ---" of water is used per ---."