AbstractIn Southeast Asia, peatland is widely distributed and has accumulated a massive amount of soil carbon, coexisting with peat swamp forest (PSF). The peatland, however, has been rapidly degraded by deforestation, fires, and drainage for the last two decades. Such disturbances change hydrological conditions, typically groundwater level (GWL), and accelerate oxidative peat decomposition. Evapotranspiration (ET) is a major determinant ofGWL, whereas information on theETofPSFis limited. Therefore, we measuredETusing the eddy covariance technique for 4–6 years between 2002 and 2009, including El Niño and La Niña events, at three sites in Central Kalimantan, Indonesia. The sites were different in disturbance degree: aPSFwith little drainage (UF), a heavily drainedPSF(DF), and a drained burnt ex‐PSF(DB);GWLwas significantly lowered atDF, especially in the dry season. TheETshowed a clear seasonal variation with a peak in the mid‐dry season and a large decrease in the late dry season, mainly following seasonal variation in net radiation (Rn). TheRndrastically decreased with dense smoke from peat fires in the late dry season. AnnualETforced to close energy balance for 4 years was 1636 ± 53, 1553 ± 117, and 1374 ± 75 mm yr−1(mean ± 1 standard deviation), respectively, atUF,DF, andDB. The undrainedPSF(UF) had high and rather stable annualET, independently of El Niño and La Niña events, in comparison with other tropical rainforests. The minimum monthly‐meanGWLexplained 80% of interannual variation inETfor the forest sites (UFandDF); the positive relationship betweenETandGWLindicates that drainage by a canal decreasedETatDFthrough loweringGWL. In addition,ETwas decreased by 16% atDBin comparison withUFchiefly because of vegetation loss through fires.