Analysis of soil moisture variations in an irrigated orchard root zone

Soil moisture and suction head in an irrigated orchard were continuously monitored by time domain reflectometry (TDR) probes and gypsum blocks, respectively, during and between successive irrigation events. On each side of the trees in the plot, two 30-cm long probes were installed vertically 10 cm below the soil surface (denoted as shallow) and another two probes were installed vertically 40 cm below the soil surface (denoted as deep). The variation in moisture content measured by the TDR probes between successive irrigation events was qualitatively divided into four stages: the first was during water application; the second initiated when irrigation stopped and the moisture content in the layer sharply decreased, mainly due to free drainage. The succeeding moderate soil-moisture decrease, caused by the simultaneous diminishing free drainage and root uptake, was the third stage. During the fourth stage, moisture depletion from the layer was solely by root uptake. The slopes of moisture content variation with time throughout this stage enabled the monitoring of water availability to the plant. The range of moisture content variations and moisture depletion rates between subsequent irrigation events was higher in the shallow (10-40 cm) than in the deeper (40-70 cm) layer. Irrigation nonuniformity and spatial variability of soil hydraulic properties contributed to the unevenness of the moisture distribution in the soil profile. However, as soon as moisture content within a layer reached field capacity, namely the free drainage had stopped, irrigation uniformity had a negligible effect on water flux to the plant roots. The measured data indicate that soil moisture is fully available to the plant as long as the momentary moisture flux from the soil bulk to the soil-root interface can replenish the moisture being depleted to supply, under non-stressed conditions, the atmospheric water demand. This flux is dominated by the local momentary value of the soil's bulk hydraulic conductivity, K_r, and it stays constant for a certain range of K_r values, controlled by the increasing root suction. A decrease in water availability to the plant appears for longer irrigation intervals as a break in the constant soil-moisture depletion rate during stage 4. This break is better correlated to a threshold K_r value than to threshold values of moisture content or suction. Therefore, it is suggested that moisture content or suction used to measure water availability or to control irrigation first be alibrated by K_r(θ) or K_r(ψ) curves, respectively.