The GOSSYM simulation model is dependent upon accurate weather data. Errors in that data may be introduced by instrument inaccuracies and by differences between the real environment in the field and the environment at the weather station. To estimate the magnitude of such errors on water related components of the simulations, a series of simulations were run with systematic changes in the weather data. The results of the simulations indicate that errors in wind, sun and air temperature induce minor variations in modeled results with Mississippi conditions. Commonly expected errors in solar radiation caused large changes in the date of irrigation after a late June rain. Particular attention must be given to solar radiation sensors to insure best possible accuracy with the GOSSYM model.