Overpumping of the Ogallala Aquifer in western Kansas due to expansion of irrigated agriculture is believed to have resulted in reductions in water storage volumes and drops in the water table in several regions. It is widely agreed upon that water demand should be reduced through increases in water use efficiency to make irrigated agriculture in the region more sustainable. The existence of several terms related to the efficiency with which irrigation water is used, however, may be hampering the development of appropriate water conservation policies. In particular, while there has been an emphasis on increasing water application efficiencies of irrigation systems, little empirical testing has been done on the effects of such changes on water use efficiency. The primary purpose of this paper is to determine the impacts of irrigation system type, as well as other variables, on irrigation water use efficiency for a sample of 43 irrigators in western Kansas between 1992 and 1999. The use of panel data permits both cross-sectional and temporal comparisons, with the latter being particularly relevant as there was a large-scale shift in irrigation system types during the study period. The method used to quantify water use efficiency or, more specifically, excess irrigation water used, is Data Envelopment Analysis (DEA). Based upon linear programming techniques, DEA creates a "best practice" production frontier based on the irrigators that produced their level of crop output with the least amount of water. What is implied is that those who are able to produce their output levels using the least amount of water are better water managers. These farms then serve as benchmarks against which the water use inefficiency of all other irrigators, or amount of "excess water" used, can be measured. The relationship between the magnitude of irrigation water excess, or reduction potentials, and irrigation system type and a number of other farm characteristics are then determined. A major finding of the study is that there is only a weak relationship between irrigation system type and the level of excess irrigation water used. In particular, flood irrigation systems are not clearly inefficient and management may play a significant role in the levels of water use efficiency that can be reached. Additional results indicate positive relationships between water excess and age of farmer, negative relationships between water excess and farm size, and differences in water excesses between farms located in different groundwater management districts.