A computer model for use in predicting the impact of various improvement options on command area expansion of a deep tubewell irrigation system has been developed. A field study was conducted on 11 selected deep tubewells located at two sites, namely Dhamrai and Thakurgaon, in Bangladesh to collect necessary physical data to provide irrigation engineering insight and a basis for testing and application of the model. The results of the investigation revealed that the actual discharge of most of the deep tubewells was lower than the design or rated capacity. The duration of pump operation in the majority of the study deep tubewell areas was extremely low. The conveyance losses in all of the study systems were very high. Even though a portion of those losses was reused in the system, particularly in rice growing areas, they caused serious difficulty in delivering water to remote fields, thus, restricting the command area. The overall irrigation efficiencies in all of the wheat growing areas of Thakurgaon were low. Considerable losses and waste of water after it was diverted from the field channels due to lack of leveling and poor water control were mostly responsible for the low efficiencies . Excessive percolation losses due to inadequate puddling of the soils during land preparation and frequent drought conditions resulted from poor distribution of water in some of the rice growing areas of Dhamrai caused relatively low overall irrigation efficiencies. The overall efficiencies of the pumps both driven by diesel engine and electric motor were found to be much lower than the recommended values. The developed model was applied to predict the expected command area under both existing and modified physical conditions. The model predicted command areas were compared with the actual field observed values and necessary adjustments were discussed. The impact of each of the improvement alternatives considered in the study was determined by comparing the predicted command area under the modified conditions with that under the existing physical settings. An economic analysis was performed to determine the cost effectiveness of most of the alternatives. Application of the model in evaluating the farmers' pump operation practices was demonstrated. Use of the model to assist the farmers in planning the command area and delivery schedule was also indicated. Although the model was developed to use in a deep tubewell irrigation system, it can also be used in the system served by a low-lift pump or by a turnout in a large irrigation project.