Developments in applied methods for estimating crop water use (evapotranspiration) have made it possible to estimate root zone soil moisture deficits in irrigated fields with reasonable accuracy. A zero soil moisture deficit (SMD=O) implies that the root zone storage capacity (influenced by soil textures and rooting depths) is filled to "field capacity." Day-to-day crop water use produces a water deficit, whereas irrigations and rainfalls (when they occur) reduce the deficit. Irrigation scheduling implies that at some level of water deficit an irrigation system must be started and run until the prevailing deficit is reduced to lower levels if near maximum yield is the production goal. The following material includes an algorithm for irrigation scheduling, source code with documentation (written in Applesoft Basic), and a users guide.