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<br />.-l <br />00 <br />to <br />C' <br />o <br /> <br />C' is an excellent correlation between the model and the field conditions after <br /> <br />the more soluble sodium salts have been leached as predictl!d by the model. <br /> <br />The model indicates that a shift in chemistry will occur when the gypsum has <br /> <br />been leached from the soil. The field conditions indicate that, after more <br /> <br />than 60 years of irrigation, the gypsum has not been leach,.d from the soil. <br /> <br />Reductions in ground-water will further delay the shift in chemistry. <br /> <br />Weather Data <br /> <br />Weather data for the Grand Valley are available from at least 7 sources <br /> <br />throughout the valley. The oldest records are continuous since the mid-1890s. <br /> <br />Table 6 is a summary of climatological sources and period of record. <br /> <br />Topographic Mapping <br /> <br />Ortho or topographic mapping covering the irrigated a:rea of the valley <br /> <br />was completed in 1975, with a horizontal scale of one inch equaling 200 feet <br /> <br />and contour interval of two feet. The finished product consists of 488 <br /> <br />reproducible orthophotomaps, each covering an area of approximately one mile <br /> <br />in the east-west direction and approximately one-half mile in the north-south <br /> <br />direction. <br /> <br />The purpose of the mapping was to provide a reliable base to design <br /> <br />system improvement features and to record field sizes and ownerships. <br /> <br />Color Infrared Aerial Photos <br /> <br />In 1976 two aerial photography contracts were initiated. The flight dates <br /> <br />were planned so that comparisons could be made to identify high water tables, <br /> <br />seepage areas, natural drainage patterns and vegetative growth along water- <br /> <br />20 <br />