A simple water balance model was developed for rain-grown, lowland rice on the heavy clay soils of the sub-coastal plain of the Adelaide River in northern Australia. The rice farming system is based on short duration (100 days) photoperiod-insensitive varieties. The seed bed is prepared in the flooded state with a tractor-driven rotary tiller. Aircraft are used to broadcast seed, fertilizer, and pesticides into the flooded fields. The crop is sown during mid-January to early February when rainfall and tillage criteria are satisfied. The model is based mainly on field measurements of evapotranspiration. Estimated dates of soil saturation and depths of ponded water at Coastal Plains Research Station agreed remarkably well with observed values in five wet seasons having widely different rainfall patterns. The model was used to estimate the success frequency of rain-grown, lowland rice at Darwin, Koolpinyah, and Humpty Doo with 93, 48, and 24 years of rainfall records respectively. A computer program was written to obtain daily estimates of soil water storage and depth of ponded water for each wet season from September 1 to May 31. In the program provision was made for maximum field pondage to be successively 6, 8, 10, and 12 inches. Rainfall was regarded as adequate providing there were (a) at least 14 days pondage ≥3 inches between December 31 and February 18 for wet tillage before sowing, (b) at least 80 days between sowing date and the last date at which ponded water was present on the field, (c) not more than 10 consecutive zero pondage days between 50 and 80 days after sowing, corresponding to the stage of panicle initiation through flowering. A 23-year comparison showed that on the average, the water availability was similar at the three stations, although differences sometimes occurred within individual seasons. The results indicate that the expectation of crop failure due to inadequate rainfall at Darwin is about 1 year in 30. In addition there is an expectation of major yield reduction of about 1 year in 10. The seasons of crop failure and low yield tended to occur in groups after relatively long runs of seasons of adequate rainfall. Designing for a maximum field pondage greater than 10 inches did not reduce the estimated number of crop failures at any station. The results are discussed in relation to practical farming operations.