Technical Abstract: Precipitation controls many ecological processes in semiarid environments. Co-occurring plant functional types promote the efficient use of available resources and influence biophysical processes that control biogeochemical cycling and energy exchange. Unraveling components of the hydrological cycle is critical for understanding these processes. The relative contributions of transpiration (T) and soil evaporation (E) to total ET was determined in a semiarid woodland ecosystem dominated by mesquite trees (Prosopis velutina), perennial C4 grasses (Sporobolus wrightii) and annual herbs. We used 'Keeling plots' (isotope mixing relationships) of water vapor to partition ET. We collected water vapor from a vertical profile in the turbulent vegetation boundary layer and water from plants and soils before, during and after the monsoon rainy period on 2002. The contribution of plant transpiration to total evapotranspiration (T/ET) was responsive to inputs of precipitation; T/ET varied from 1.00 to 0.95 during dry periods before and after the monsoon season, and declined to a minimum of 0.40 immediately after precipitation events during the peak of the rainy season. ET fluxes from eddy covariance measurements were combined with partitioning estimates to investigate seasonal dynamics of component fluxes as function of environmental forcing. An independent assessment of the understory T contribution based on samples collected within the profile of the understory layer (0.1 ' 2 m height) and ET fluxes from eddy covariance measurements taken below the tree canopy, indicate that this compartment represented about 20% (~1 mm/d) of the total ET flux during the peak-growing season when the understory vegetation was fully developed. Further, soil evaporation comprised a significant fraction of ET only when soil volumetric water content in the top 5 cm exceeded a threshold value of 0.1 cm3/cm3. As evidenced by the dynamic contribution of soil, trees and understory vegetation to ET in response to different biotic and abiotic controls, our data illustrate the utility of Keeling plots of water vapor for describing ecosystem-level water dynamics of semiarid environments. This approach is especially robust when combined with temporally and spatially compatible measurements of ET flux.