This study investigates basin‐scale hydrologic implications of the replacement of forest‐dominated land cover by rubber plantations in Montane Mainland Southeast Asia. The paper presents a new method for estimating the water demand of rubber and consequently water losses to the atmosphere through rubber evapotranspiration (ET). In this paper we argue that rubber ET is energy‐limited during the wet season, but during the dry season water consumption is mostly governed by environmental variables that directly affect rubber phenology, namely, vapour pressure deficit, temperature and photoperiodicity. The proposed ET model is introduced into a hillslope‐based hydrologic model to predict the basin‐scale hydrologic consequences of rubber replacing native vegetation. Simulations suggest greater annual catchment water losses through ET from rubber dominated landscapes compared to traditional vegetation cover. This additional water use reduces discharge from the basin, or its storage. Copyright © 2010 Wiley, Ltd.