A method for controlling operation of a self-propelled irrigation system compensates for differences in expected wind drift and evaporative losses for time periods over the course of an irrigation cycle. Speed modifier values are determined for selected time periods to modify the base speed for the pivot arm assembly based on the corresponding expected wind drift and evaporative losses, with offsetting speed modifier values to modify the speed of the pivot arm assembly for other time periods, so that the period of time for completing an irrigation cycle remains unchanged. The pivot arm assembly is then controlled to move at the corresponding modified speed for each time period.
