To avoid unnecessary crop failures it would be valuable to have a reliable threshold value of the root zone water content for wheat at planting (θrp) below which the risk is too high to plant. An attempt has been made to estimate this value for annual wheat on the Glen/Shorrocks crop ecotope. A simple procedure, based on measured values, was developed for predicting runoff, deep percolation and evaporation (and thus water storage) during the fallow season. This made it possible to estimateθrpvalues for each season. A simple stress-index yield prediction model, in whichθrpwas one of the inputs, was developed and calibrated against measured yields from this crop ecotope over five years. The calibrated model was then used withactualweather data (1922–1987) to predict yields for 66 seasons. Results showed that forθrpvalues of 300 mm or less, 320 mm and 340 mm or more, the chances of attaining a minimum target yield of 1 t ha−1are<50%, 62% and 74% or greater, respectively. Based on these results, 320 mm is considered to be a suitable threshold value ofθrpfor wheat on the Glen/Shorrocks crop ecotope with an effective soil depth of 1 800 mm. This value corresponds with 130 mm of potentially extractable water.