The Remote Sensing Group of the Optical Sciences Center at the University of Arizona has a history of collecting ground-based atmospheric data in support of calibration/validation and for atmospheric correction. This work has included the determination of columnar water vapor based on measurements of direct solar irradiance. In the past, the conversion of these data to transmittance and then column water vapor has relied upon a modified Langley approach and two-parameter band model of absorption developed in the 1980s at the University of Arizona. More recently, the RSG has used the well-known MODTRAN code for its prediction of at-sensor radiance and atmospheric correction. This work examines the use of the same MODTRAN code for the retrieval of column water vapor to simplify the overall processing approach of the RSG, as well as providing consistency between the measurements and the predicted at-sensor radiance. The water vapor retrieval using MODTRAN follows the same basic approach as the previous method except that the water vapor absorption parameters are obtained from MODTRAN. A sensitivity analysis is performed to examine the influence of the MODTRAN input parameters on the retrieval. The results of the new method are compared with results from GPS-derived column water vapor. These preliminary results show that the MODTRAN-based values have an accuracy of 10% and agreement with the GPS-derived results is better than 10%.