FACTORS UNDERLYING GRAIN YIELD SPATIAL VARIABILITY IN THREE IRRIGATED WHEAT FIELDS
Three irrigated wheat fields in western Yolo County, California, were studied to determine the magnitude and 
causes of spatial variability in grain yield. Point samples of soil properties were taken on a regular 61 m grid in each 
field. False color infrared aerial photographs were taken of each field under bare soil conditions and twice during the 
cropping season. Yield monitor data were collected at harvest. Two of the three fields showed substantial spatial 
variability in grain yield. Correlation analysis and CART (classification and regression trees) were used to identify 
factors underlying grain yield spatial variability. Primary causes of variability were soil texture, weed level, and soil 
phosphate level. Results of the CART analysis were used to subdivide the two fields with high spatial variability into 
management zones.