Buffelgrass savanna is becoming widespread in aridland ecosystems around the world following invasion or deliberate land conversion for cattle forage. There is still a gap of information regarding functional and ecohydrological aspects such as carbon, water, and greenhouse gas exchanges in these highly productive novel ecosystems where buffelgrass is an exotic species. We measured net ecosystem CO₂ exchange (NEE), ecosystem respiration (R_eco), gross primary production (GPP), and evapotranspiration (ET) with eddy covariance techniques over a buffelgrass savanna established for cattle grazing, approximately 30 yr ago within the Sonoran Desert. The savanna was a net carbon sink (NEE -230 g C/m²/yr) during both a year with above average and one with below-average precipitation (NEE -84 g C/m²/yr). Water loss through evapotranspiration (ET) was similar to total annual rainfall input. Up to 62% of the annual fixed carbon and 75% of ET occurred during the summer monsoon season, when 72–86% of annual rainfall occurred and buffelgrass was active. ET from summer months explained 73% of variation in NEE, with an average ET of 50 mm H₂O/month needed to turn the ecosystem into a net carbon sink during this season. Other seasons in the year, when buffelgrass was dormant, contributed with up to 48% of annual fixed carbon but with higher water use efficiency (-NEE/ET). Wediscuss the importance of the seasonal variability in R_eco, GPP, and ET processes and the phenology of native plant species for the net carbon uptake through the year for this managed novel ecosystem.