Analysis of soil moisture variations in an irrigated orchard root zone

Soil moisture and suction head in an irrigated orchard were continuously mo nitored by time domain reflectometry (TDR) probes and gypsum blocks, respec tively, during and between successive irrigation events. On each side of th e 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 ins talled vertically 40 cm below the soil surface (denoted as deep). The varia tion in moisture content measured by the TDR probes between successive irri gation events was qualitatively divided into four stages: the first was dur ing water application; the second initiated when irrigation stopped and the moisture content in the layer sharply decreased, mainly due to free draina ge. The succeeding moderate soil-moisture decrease, caused by the simultane ous diminishing free drainage and root uptake, was the third stage. During the fourth stage, moisture depletion from the layer was solely by root upta ke. The slopes of moisture content variation with time throughout this stag e enabled the monitoring of water availability to the plant. The range of m oisture 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 h ydraulic properties contributed to the unevenness of the moisture distribut ion in the soil profile. However, as soon as moisture content within a laye r reached field capacity, namely the free drainage had stopped, irrigation uniformity had a negligible effect on water flux to the plant roots. The me asured 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 int erface can replenish the moisture being depleted to supply, under non-stres sed 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 incr easing root suction. A decrease in water availability to the plant appears for longer irrigation intervals as a break in the constant soil-moisture de pletion rate during stage 4. This break is better correlated to a threshold K-r value than to threshold values of moisture content or suction. Therefo re, it is suggested that moisture content or suction used to measure water availability or to control irrigation first be alibrated by K-r(theta) or K -r(psi) curves, respectively.