Information about the ratio of transpiration (T) to total evapotranspiration (T/ET) is related to critical global change concerns, including shrub encroachment and non-native species invasion. In this study, a new approach was developed to partition measurements of ET into daily evaporation (ED) and daily transpiration (TD) in a semiarid watershed based on the low-cost addition of an infrared thermometer and soil moisture sensors to existing eddy covariance and Bowen ratio systems. The difference between the mid-afternoon and pre-dawn soil surface temperature (Δt) was used to identify days when EDapproached a seasonal minimum (EDmin) and thus, TD ≈ ETD − EDmin. For other days, an empirical approach was used to partition ETD into ED and TD based on volumetric soil moisture. The method was tested using Bowen ratio estimates of ET and continuous measurements of surface temperature with an infrared thermometer (IRT) at a grassland and shrubland site within the Walnut Gulch Experimental Watershed in southeast Arizona USA in years 2004–2006. Validation was based on a second dataset of Bowen ratio, IRT and shrub sap-flow measurements in 2003. Results showed that reasonable estimates of TD were obtained for a multi-year period with ease of operation and minimal cost. Estimates of TD and ED were summed over the study period when plants were actively transpiring for years 2004, 2005 and 2006 to estimate totals over the study period, TS and ES, respectively. Preliminary analysis suggests that the accuracy of TSestimates was 7% of the total measured sum and the precision of TS estimates was about 4%. For this study period, TS was related strongly to ETS, with a slope of 0.79 for the grass-dominated site and 0.64 for the shrub-dominated site for the 3 years. Thus, for these sites during the study period in these years, the TS/ETS was higher for the grass-dominated site than for the shrub-dominated site, and did not vary systematically with variation in amounts and timing of precipitation. The Δt-based partitioning method has potential for international application in other well-instrumented ecosystems but will need to be tested for application when evaporation is limited by energy rather than water.