Irrigation management practices in the Texas High Plains (THP) might be improved if we could ascertain the proportion of rainfall utilized by the crop in any given rainfall event. For instance, the primary source of irrigation water in the THP is pumped from the Ogallala Aquifer (OA), and is enriched in 18O compared to rainfall captured water. Given this expected difference, it should be possible to differentiate the proportion of water from each water source in soil or plant extracted water. To this end, cotton was grown using two irrigation management practices: subsurface drip and center pivot. The water used for irrigation was pumped from the Ogallala aquifer, and rainfall was gathered in a modified tipping bucket rain gauge with a 500 ml beaker placed to collect the rain with 100 ml mineral oil to prevent evaporation. Additionally, plant and soil samples were collected at hourly intervals following each precipitation event. Water was then extracted from the soil and plant samples using cryogenic vacuum distillation, and analyzed for 18O/16O ratios using the DLT-100 Liquid-Water Isotope Analyzer from Los Gatos Research Inc. The difference in isotope concentrations in the extracts was used to compare rainwater use across the different irrigation management practices. Results might suggest changes to the way in which we apply irrigation water that would improve root growth and distribution to enhance the capture of rainfall.