摘要 : Transpiration (T) of a disease-free crop, well-fertilized and well watered can be estimated using reference evapotranspiration (ETo) and basal crop coefficients (Kcb) When plants are under water stress it is necessary to consider ... 展开 Transpiration (T) of a disease-free crop, well-fertilized and well watered can be estimated using reference evapotranspiration (ETo) and basal crop coefficients (Kcb) When plants are under water stress it is necessary to consider a stress coefficient (Ks, defined as T_actual/T_maximum), T being estimated from Kcb x Ks x ETo. Remote sensing has recently been used for Kcb estimations, namely using normalized difference vegetation index (NDVI). Due to the controversy on how NDVI reflects short or long term stress conditions, we compare ground truth observations for T (combination of sap flow and eddy covariance techniques) and estimated T for the crop (using NDVI, TNDVI). Our aim is to analyze the ratio T_measured/TNDVI, and its relationship with a water stress indicator, predawn leaf water potential ( Psi _p), in order to contribute to the discussion on the meaning of T_NDVI. To study this, we used data from experimental work on a vineyard during summer 2008 and 2009, in the South of Portugal (38.050 N, 7.921 W). Average ETo during summer was 7 mm/day. T during summer ranged between 0.5 and 3.0 mm/day. NDVI was calculated from LANDSAT 5 TM imagery and Kcb, estimated using a methodology developed on DEMETER project, ranged from 0.62 to 0.75 for the 7 days of observation. Assuming NDVI equation provides Kcb, therefore not reflecting a component of Ks, this coefficient would be calculated as T_measured/(ETo x KcbNDVI). In fact, a good agreement was found between calculated Ks and Psi _p (Ks=1.3 x e^{(3.1 x Psi p)}). Using this last equation or equivalent, it would be possible to map actual T from Psi _p and Kcb (estimated from NDVI). The equation suggests that, for this range of Psi _p (-0.2> Psi _p>-0.7 MPa), NDVI does not incorporate the effect of water stress on T. 收起