A SIMPLE SINGLE-LAYER MODEL TO ESTIMATE TRANSPIRATION FROM VEGETATIONUSING MULTISPECTRAL AND METEOROLOGICAL DATA

A methodology is developed here to model evapotranspiration (lambda E- c) from the canopy layer over large areas by combining satellite and g round measurements of biophysical and meteorological variables. The mo del developed here follows the energy balance approach, where lambda E -c, is estimated as a residual when the net radiation (Rn), sensible h eat flux (H) and ground Bur (G) are known. Multi-spectral measurements from the NOAA Advanced Very High Resolution Radiometer (AVHRR) were u sed along with routine meteorological measurements made on the ground to estimate components of the energy balance. The upwelling long wave radiation, and H from the canopy layer were modelled using the canopy temperature, obtained from a linear relation between the Normalized Di fference Vegetation Index (NDVI) and surface temperature. This method separates flux measurements from the canopy and bare soil without the need for a complex two layer model. From theoretical analysis of canop y reflectance, leaf area, and canopy resistance, a model is developed to scale the transpiration estimates from the full canopy to give an a rea averaged estimate from the mean NDVI of the study area. The model was tested using data collected from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), an d the results show that the modelled values of total surface evapotran spiration from the soil and canopy layers vary from the ground measure ments by less than 9 per cent.