The Smartphone App for Scheduling Irrigation in Cotton uses weather data to estimate daily crop water use. It maintains a daily soil water balance by subtracting daily crop water use from available soil moisture and adding rain or irrigation. The App estimates the amount of rain received at registered fields from national weather data sets, but performs the best when users update the rain data using information from a local rain gage. The App has been evaluated against a variety of other irrigation scheduling methods and has performed well. It consistently results in yield gains and significant improvements in irrigation water use efficiency when compared to traditional scheduling methods.