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I I 1 I I I I I I . I. I I I I. I I I I 1844 necessary to develop a climatic zone characterization based on the regression analyses to enable a reasonable estimate of crop suitability and potential yield. These climatic zone data also formed the basis for the estimate of crop water requirements which was determined by additional engineering analyses (Agricultural Engineering Task A). Based on the regression analyses and projected crop suitability, an elevation increment of 400 feet was selected to use in developing climatic zone characterizations. The average annual characteristics of the 10 climatic zones are summarized in Table 1.1. Project area climatic conditions are the major limiting factor to crop suitability and yield. These 10 climatic zones thus form the basis for the determination of crop suitability and yield without restrictions from existing natural resource conditions. The base crop yields for each suitable climatic zone are then modified based on limiting the natural resources (water quality and soil physical/chemical character- istics) under which they are grown. 1.3.2 Irrigation Water Quality Existing irrigation water quality was evaluated based on reported water quality data for the potential irrigation water sources as reported by the U.S. Geological survey (USGS). Irrigation water quality was generally evaluated in regard to three potential limiting effects: 1) salinity, 2) soil permeability, and 3) toxic ions. Irrigation water salinity is sufficiently high in La Plata and Mancos River waters to potentially impact the production of salt 1-6