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DIVISION S-1—SOIL PHYSICS

Evaluation of TDR Use to Monitor Water Content in Stem of Lemon Trees and Soil and Their Response to Water Stress

^a Soil and Water Institute, Agricultural Research Organization, Ministry of Agriculture, State of Israel, POB 6 Bet Dagan, Israel, 50250
^b Gilat Research Center, Mobile Post Negev, 85280, Agricultural Research Organization, Ministry of Agriculture, State of Israel
^c Environmental Group, HortResearch, Private bag 11-030 Palmerston North, New Zealand

^* Corresponding author (vwnad{at}volcani.agri.gov.il)

The purpose of the study was to compare the response of TDR-determined stem (_stem) and soil (_soil) water content to different irrigation managements. _stem (L L^-1) was measured with three-rods TDR probes (70 mm) installed vertically or horizontally into predrilled holes in the trunk of 5-yr-old lemon [Citrus limon (L.) Burman f.] trees in a semiarid region (Israel). Four irrigation treatments were established to deliver 100% ("full"), half of this amount on one side of the tree ("50%"), same amount as in 50% but applied to alternate sides (3-wk intervals) of the tree ("split"), and 0% ("dry") of the orchard's normal irrigation volume (typically 150–200 m³ wk^-1 ha^-1). Treated sewage water (_w = 0.9 dS m^-1) was used to irrigate the trees for 75 d (end of June and mid September). Changes in _soil, and _stem were monitored at weekly intervals. Leaf water potential and temperature measurements were used to verify the achieved water stress levels. _stem of the full treatment fluctuated by about 0.02 to 0.03 L L^-1 above and below the "prestress" reference level (beginning of the season). In contrast, _stem of the 50% treatment declined (by about 0.07 L L^-1) steadily over the season. For the split and dry treatments, _stem decreased by about 0.12 L L^-1 relative to the reference level. _soil and _stem values suggest that some surplus irrigation was applied to the full and 50% treatments. It was found that water stress was reflected in TDR-measured _stem changes but that these changes were too small for routine irrigation control.

Abbreviations: L L^-1, volumetric water content (e.g., _{soil, stem}) • dS m^-1, electrical conductivity • _a dS m^-1, bulk soil • l_a, apparent length of a transmission line ( = ‘dielectric length’) • l_{a, cable} or l_{a, probe}, apparent length of the coaxial cable or TDR probe • , dielectric constant • WB, wood block • TDR, time domain reflectometry • LWP, leaf water potential

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