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Abstract. Limited water resources and insufficiently developed infrastructure provide challenges to sustainable production of cotton in Western Kansas. This study aimed to (i) assess the actual seasonal evapotranspiration (ETa) and (ii) determine the grass-reference crop coefficients (Kc) for cotton under different irrigated and rainfed conditions. Four irrigation technologies (LEPA, LESA, MDI1, and MDI2) and rainfed treatments were evaluated under two crop densities (123,553 and 160,618 plants/ha) in a split plot design with three replications using cotton variety PHY 205 W3FE. The soil water balance analysis showed that, of all irrigation technologies, cotton under MDI1 had the highest ETc of 490.3 mm, while the rainfed treatments had the lowest ETc of 287.1 mm. In terms of the impact of crop density, high-density plots registered the higher ETc of 447.1 mm compared to 441.3 mm of low-density plots. Irrigated cotton crop coefficients were estimated at 0.35, 0.92 to 1.04, and 0.39 to 0.48 for initial, mid, and late season stages, respectively. Under rainfed conditions, the crop coefficients were 0.18, 0.46 to 0.48, and 0.10 to 0.28 for the respective growth stages. These results indicate that the irrigated initial Kc is similar to the FAO initial Kc, while mid and late season Kc are lower than the FAO values by 15% and 4%, respectively. This study shows that the development of a site-specific local cotton Kc is important for better understanding of irrigation management strategies and crop water use in Western Kansas.