Determining pasture evapotranspiration using active optical sensor derived normalized difference vegetation index

Actual evapotranspiration (ETc) is one of the important parameters that determines the daily and seasonal water requirement by the crop community. It varies with numerous factors including weather, soil moisture availability and other crop related factors such as growing stage, fraction of field coverage and crop vigour. In this study we investigated the relationship between normalized difference vegetation index (NDVI) that is closely related to photosynthetically-active biomass (PAB) and the evapotranspiration of pasture at different soil moisture condition. A portable enclosed chamber was used to measure ETc of a target pasture canopy and consequently the NDVI with a hand held active optical sensor. The portable chamber was calibrated in the laboratory and produced a calibration factor of C=1.02. Field experiments were conducted on the UNE SMART Farm in Tall Fescue pastures (Festuca arundinacea var. Dovey). Under limiting soil moisture condition the relationship between NDVI and ETc showed a negative correlation (R2=0.73) whereas a strong and positive correlation (R2=0.82) were observed in a non-limiting soil moisture condition.