Soil water depletion rates under large grapevines

Abstract

Over two consecutive seasons, daily rates of change in the water content of the top 120 cm of soil were calculated for sample sites located at 50 cm from the butts of 12 Colombard vines on Ramsey rootstock which were part of a controlled deficit irrigation experiment. Records were selected where the rate represented a change due solely to depletion of soil water by grapevine evapotranspiration (ΔSET). Soil water availability was quantified as a logarithmic transformation of the absolute value of the root-weighted measure of soil matric potential. ΔSET was negatively correlated with soil water availability and positively correlated with reference crop evapotranspiration (ETo). At very high values of soil water availability, i.e. soil matric potential between -8 and -16 kPa, and after the canopy was fully developed, ΔSET had a 1:1 relationship with ETo. When root-weighted soil matric potential was greater than -16 kPa, the mean values of the ratio of soil water depletion rate to reference crop evapotranspiration (ΔSET/ETo), varied from 0.1 to 1.2 for 30-day periods between bud burst and leaf fall. Considering the full set of measurements, ΔSET/ETo was highly negatively correlated with soil water availability for all 30-day periods except those less than 60 days from bud burst. In the period 150 to 179 days after bud burst, 70% of the variance in ΔSET/ETo was accounted for by regression on soil water availability for values of root-weighted soil matric potential between -8 and -100 kPa. During this period a fall in root-weighted soil matric potential from -10 to -42 kPa was associated with a decline in the value of ΔSET/ETo from 1.3 to 0.6. At both high and low values of ETo, the ratio of ΔSET/ETo fell linearly with declining soil water availability. The changes in values of midafternoon stem water potential and stomatal conductance associated with this decline in ΔSET/ETo are discussed.