Towards satellite-derived global estimation of monthly evapotranspiration over land surfaces

The NOAA AVHRR provides daily global spectral measurements of reflected and emitted radiation from Earth. These data contain information on land characteristics, which could be utilized to derive evapotranspiration. A global land surface data base has been developed from 9-year time series of multispectral AVHRR data consisting of Channels 1,2,4, and 5. The data are sampled in time and space and mapped to (0.15 degrees)(2) grid on a weekly basis. Processing includes proper calibration, cloud screening, monthly averaging, interpolation and smoothing. It is shown, in the present study, that when evaporation exceeds precipitation the soil moisture availability parameter beta correlates well with an index of vegetation activity NDVI, derived from the visible and near-IR AVHRR measurements. If potential evaporation Ep is estimated, e.g. from Thornthwaite's empirical formula that uses only air temperature as input, then the actual evapotranspiration, E, can be calculated using the NDVI-derived beta as a fraction of Ep. In this study, the NDVI-beta relationship is determined using a previously developed climatology of beta based on hydrologic balance calculations. An example of NDVI-beta relationship for July over the globe is given.