The influence of the soil water balance on the water fluxes in the soil-plant-atmosphere continuum. Field studies in a vineyard at Santarem, Portugal

Soil water status was measured in the root zone of single plants of cultiva r Trincadeira Preta using TDR-probes and tensiometers in summer 1996 (see a lso Silvestre er al. 1998). Changes of soil water potential were described by differences in water tension (dT=psi(1)-psi(2)) over a 30 minutes period (Gruler, 1998). At the onset of transpiration, dT decreased and during nig ht dT increased, indicating periods of drying and wetting. Higher amplitude s of the oscillation were observed at lower soil water content. A decrease in dT in the morning began at the same time with the onset of sap flow. The increase in dT during the afternoon showed, at lower soil water content, a time lag in comparison to start of decrease of transpiration. Short-term c hanges of solar radiation were immediately detected by changes of sapflow v elocity and dT. The course of the soil water potential is discussed in rela tion to root water uptake and transpiration.