The prediction of yield variability of an irrigated crop, as influenced by the uncertainty of soil, plant, and boundary conditions parameters, and an investigation through numerical simulation, using the probability theory apparatus, are tested against field data. Among the large number (19) of parameters, six which have a significant impact on yield variability have been singled out. These include two plant parameters with an average relative contribution of 23%, three soil hydraulic parameters with a relative impact of 47%, and one irrigation boundary condition parameter, related to the nonuniformity of water application, with 30% average contribution upon yield variability. An additional set of results is related to the yield coefficient of variation. Its range was simulated to be generally between 0.3 and 1, with an average of approximately 0.6. A similar range, but with a somewhat lower average was measured in the field. The detection of the parameters of large impact, obtained by both simulation and field validation, give an indication of weak links which require further investigation, and for which collection of field data is important.