Technical Abstract: Information about the ratio of transpiration (T) to total evapotranspiration (T/ET) is related to critical global change concerns, including shrub encroachment, non-native species invasion and soil erosion. In this study, a new approach was used 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. For a study period during the North American monsoon season, estimates of T/ET over three years (2004-2006) at grass- and shrub-dominated sites at the USDA Walnut Gulch Experimental Watershed were used to address the hypotheses that T/ET is sensitive to changes in woody plant cover and is correlated with total precipitation, precipitation patterns and total ET. For this study period (August to October), we found a strong, multi-year relation between root zone soil moisture (to 15 cm depth) and TD at both sites. Estimates of TD and ED were summed over the study period for years 2004, 2005 and 2006 to estimate totals over the study period, TS and ES respectively. Results showed that the shrub-dominated site had higher ES than the grass-dominated site for similar precipitation patterns over the study period. TS and ES were related to the number of larger storms during the study period, and this relation was different for the two sites. For this study period, TS was related strongly to ETS, with a slope of 0.90 for the grass-dominated site and 0.84 for the shrub-dominated site for the three 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 rainfall. The uncertainty of the new method due to sampling, instrument and algorithm errors was estimated to be about 3-7 mm or about 4% of TS over the study period.