Evapotranspiration (ET) describes the sum of plant transpiration and evaporation into the atmosphere. This is a critical component of the regional water cycle. Yet historical measurements of ET do not exist in Indiana. Therefore, models are needed to develop ET estimates. It is anticipated that using these models, the development of Indiana ET climatology can be completed. ET gages were installed at two Purdue Agricultural Centers (PAC) in the 2008 growing season and nine PAC locations in the 2009 and 2010 growing seasons over a “reference” vegetative surface. Comparisons were made between these reference ET measurements and different ET models. The comparisons were divided into two separate categories. The first compared ET measurements with simulation models (RefET) when all measured weather inputs were provided from the study site. These weather inputs include temperature, solar radiation, wind, and dew point temperatures. The second set of comparisons focused on model inputs that were observed at nearby airports. These locations were determined by the proximity of the airport to the ET gage location. Airport data from six locations were used in this study. These airports include Valparaiso (PPAC), Evansville (SWPAC), Purdue University (ACRE and TPAC), Cincinnati (SEPAC), Fort Wayne (NEPAC) and Muncie (DPAC). This airport data is independent of the ET measurements which were observed at PAC locations throughout the state. After this a correlation analysis was undertaken comparing the measured RefET and RefET models. In both cases, 15 different ET models were run to calculate RefET and compared with the ETgage measurements. The results suggest the FAO 56 Penman-Monteith and the Full ASCE Penman-Monteith model performed the best with the measurements. Depending on crop type, the typical RefET rates for a growing season across Indiana are approximately 75 mm per month or about 45 mm of actual ET loss per month.