Linking the transpirable soil water content of a vineyard to predawn leaf water potential measurements

International audienceA new expression of the time derivative of predawn leaf water potential was proposed by equalizing two expressions of the grapevine transpiration. This expression was established when transpiration is the only driver of the vineyard water balance. Under Mediterranean climate, this condition is met for long periods of drought i.e. most of the summer time under the hypothesis that there is no water movement (capillary rise) from the deep layers of the soil. The proposed approach showed that changes in predawn leaf water potential (?pd) values are in inverse relation with the Total Transpirable Soil Water (TTSW) which characterizes the maximal water stock of the soil and in direct relation with reference evapotranspiration (ETo) and the basal crop coefficient (kcb) of the vine. The relation between ?pd changes and cumulated ETo is linear with a slope related to the ratio of 'kcb to TTSW'. This ratio can be therefore estimated from field measurements and climatic data. This approach was tested on two year observations performed by Acevedo-Opazo et al. (2010) in vineyards cultivated with cv. Shiraz and Mourvedre, without any irrigation. Analysis of the data obtained in 49 sites per vineyard for several dates in the very dry summers of 2003 (Shiraz) and 2005 (Mourvedre) showed strong and significant adjustments of the model. These results proved the linear relationship between the sum of ETo and ?pd. For each site of the vineyard, the approach demonstrated the possibility to provide site-specific estimates of the ratio of kcb to TTSW. This theoretical and practical development could have applications for water management and soil studies in Mediterranean vineyards