A calibration-free evapotranspiration estimation (CREMAP) method is employed to basin-scale hydrologic problems. CREMAP works on a monthly basis and yields ET estimates within 5-10% of water-balance derived ones. The method is based on simplifying assumptions of the land-surface energy balance, makes use of the complementary relationship of evaporation and requires only basic remote sensing (MODIS daytime surface temperatures), radiation (global radiation), and atmospheric (air temperature and humidity) data.