Water production is simply a function of precipitation and evapotranspiration (ET). It is essential to quantify ET and/or its components in regional scale for water resources assessments or functional forest plans. Several models that combine empirical equations, field measurements and remote sensing data have been tested and proposed to achieve this. One of the major parameters in ET estimatons is the leaf area index (LA I). LAI is a key parameter to identify the water consumption of forest stands and thus ET. It is however quite variable in temporal and spatial scales.