Yields of rainfed winter crops such as wheat can be substantially improved with limited supplemental irrigation. The upper Karkheh River basin in Iran has 15 840 km² of rainfed crops. A GIS method was designed to identify suitable areas for irrigation and a routine was developed to allocate water uses and route the flows downstream. A maximum of 13 % of the rainfed cropland could be irrigated under normal flow, 9 % if environmental flow requirements are considered and 6 % under drought conditions.