Variability of yield of an irrigated crop and its causes: 1. Statement of the problem and methodology

Crop yield depends on a large number of soil‐, water‐, and atmosphere‐related parameters. Some of these parameters are spatially variable and most of them are subject to uncertainty. Consequently, the yield is also spatially variable and its prediction is uncertain. The aims of this study were to (1) estimate the degree of yield variability as a function of variability of various parameters and (2) attempt to single out those parameters, the variability of which are estimated to have the largest impact upon the variability of yield of an irrigated crop. The first part of the study presents the general methodology developed in order to achieve these aims. It consists of a numerical deterministic model which predicts crop yield relative to its potential yield for a given irrigation regime and quality of water and for given soil and atmospheric conditions. Subsequently, the model is used to derive the values of expectation and variance of the relative irrigated crop yield as functions of these values of the various parameters.