Interpreting Spatial Yield Variability of Irrigated Spring Wheat

Higher wheat yields exhibit more uniformity and greater spatial correlation. A need exists to evaluate yield variability under high yield conditions attained by irrigation. A two-year field study was conducted with this objective under sprinkler and flood irrigation using estimated evapotranspiration for irrigation scheduling. Transects in two 9-ha fields, one sprinkler and one flood irrigated, were systematically sampled by transects for yield. Both in 1985 and 1986, average yields among transects were significantly different. Only in the sprinkled field were the transect yields the same between years. A significant, negative linear relationship was determined between mean yield and the coefficient of variation of transect yield. All transect yields exhibited serial autocorrelation and yields tended to be higher with smaller, significantly correlated separation distances. All transects had statistically acceptable autoregressive integrated moving average (ARIMA) models. The highest grain yields were more uniform (reduced coefficient of variation) but less serially correlated in terms of separation distance with significant correlation. The latter was unexpected. Soil variability properties may need to be included in models for improved yield prediction involving spatial nonuniformity.