Across the western United States, warm-season grasslands are being invaded by the exotic perennial grass, Eragrostis lehmanniana (Lehmann lovegrass). The objective of this study was to quantify the change in surface water balance, particularly the evaporation from bare soil, associated with E. lehmanniana invasion. Following a protracted drought, the Kendall grassland in the USDA-ARS Walnut Gulch Experimental Watershed in southeast Arizona transitioned from a diverse, native bunchgrass community to one dominated by E. lehmanniana. A network of 20 microlysimeters was deployed to measure daily soil evaporation (ED) in 2005 and again in 2007 (pre- and post-invasion years, respectively). This was supported with continuous measurements of evapotranspiration (ET), precipitation (P), runoff (R), surface soil moisture (θ), and solar irradiance (L) at Kendall from 2002 to the present. An empirical equation was developed to derive ED based on θ measured midday at 5 cm depth and average daily L. This was applied to years 2002–2007 during the vegetation growing season (June through October). Results confirmed that total ET over the growing season (ETS) was a function of season-long infiltration (where IS = PS − RS) for growing seasons over the past decade regardless of vegetation type, where ETS/IS was slightly greater than one in years drier than average and close to one in years with greater than average infiltration. For years of similar precipitation patterns and ETS/IS, the contribution of evaporation E to ET for the growing season (ES/ETS) doubled with the invasion of E. lehmanniana. Variation in ES/ETS ranged from 0.26 to 0.60 for years 2002–2007, where variation was related primarily to inter-annual precipitation patterns in the early season and to distinctive vegetation transformation in the middle season. These results are a first step toward understanding the ecohydrological consequences of E. lehmanniana invasion in semiarid grasslands.