N.S. Conceição, M.I. Ferreira, C.A. Pacheco, M. Fabião, L. Boteta, J.C. Silvestre
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 Tactual/Tmaximum), T being estimated from Kcb×Ks×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 Tmeasured/TNDVI, and its relationship with a water stress indicator, predawn leaf water potential (Ψp), in order to contribute to the discussion on the meaning of TNDVI. 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 Tmeasured/(ETo×KcbNDVI). In fact, a good agreement was found between calculated Ks and Ψp (Ks = 1.3×e(3.1× Ψp)). Using this last equation or equivalent, it would be possible to map actual T from Ψp and Kcb (estimated from NDVI). The equation suggests that, for this range of Ψp (-0.2>Ψp>-0.7 MPa), NDVI does not incorporate the effect of water stress on T.
Conceição, N.S., Ferreira, M.I., Pacheco, C.A., Fabião, M., Boteta, L. and Silvestre, J.C. (2011). TRANSPIRATION FROM A VINEYARD IN SOUTH PORTUGAL - STRESS COEFFICIENTS, NDVI AND LEAF WATER POTENTIAL . Acta Hortic. 922, 277-284
DOI: 10.17660/ActaHortic.2011.922.36
remote sensing, crop coefficient, normalized difference vegetation index, deficit irrigation, Vitis vinifera L., water stress, eddy covariance

Acta Horticulturae