Evapotranspiration and soil water dynamics of peach trees under water deficits

Among the factors influencing the decision-making process in irrigation, the threshold level of available soil water content (AWC) is critical for irrigation timing. An experiment was conducted with mature peach trees (Prunus persica L. Bastch) using a large weighing lysimeter to establish the threshold levels of AWC for evapotranspiration (ET), leaf conductance and photosynthesis during a deficit irrigation period. Three approaches were used to determine the AWC in the lysimeter and it was found, via continuous monitoring of soil water content with capacitance sensors, that the trees had not fully explored the soil in the lysimeter. The threshold level for the onset of ET decline was 60 and 53% of AWC when AWC was calculated as the difference between the field capacity and the wilting point of the soil layer explored by roots (90 cm) and as the total water extracted by the tree from the lysimeter, respectively. When the whole soil depth of the lysimeter (160 cm) was considered as a reservoir for water uptake, the threshold level for ET decline started at 94% of AWC; a value so high as to suggest erroneous assumptions. Similar threshold values were found for the reduction of leaf conductance and photosynthesis. Soil water monitoring in the lysimeter gave diurnal patterns of water extraction that lagged behind the transpiration of the lysimeter for about 2 h in the morning and ended earlier in the afternoon. Also, measured peak extraction rates near the emitters were several times higher than those representing tree transpiration from lysimeter weight loss. Soil water extraction patterns in trees outside the lysimeter that were watered with the same deficit irrigation regime differed from those in the lysimeter in that considerable extraction occurred from 90 to 180 cm. The different tree responses to deficit irrigation in and outside the lysimeter demonstrate the importance of considering wetting patterns, soil depth, and root exploration in high frequency irrigation management.