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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)

	ABSTRACT

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
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

	INTRODUCTION

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
IN SEMIARID AND ARID REGIONS an adequate supply of irrigation water is a major limiting factor to agricultural production. In such regions, irrigation is essential to maintain crop production over extended periods without rainfall. To optimize irrigation, and thereby reduce water use and waste without reducing cultivated areas or crop yield, an accurate and sustainable irrigation management strategy is required.

Traditionally, there are three basic approaches to determine in situ crop irrigation needs. The micrometeorological approach uses weather data (e.g., daily sunshine, temperature, rainfall) and specific crop (root depth, leaf area, drought tolerance) and soil factors (water retention, hydraulic conductivity) to determine a theoretical soil water balance; irrigation is then applied whenever the soil water deficit falls below a predefined threshold (Allen et al., 1998). Similarly, a soil-hydraulic approach has been developed that uses a direct measure of changes in either the soil's volumetric water content () (neutron scattering, TDR, gypsum blocks) or the soil's matric potential (tensiometers) to schedule irrigation. In both cases, understanding the impact and dynamics of the root-zone soil moisture status is a prerequisite to determining the threshold value of soil water deficit that each crop can tolerate before water stress begins to affect crop growth and productivity.

The difficulty with these first two approaches, in the case of single trees in an orchard, is how to measure the effective soil water status. The depth distribution of active roots and the effective size of the rooting zone are a priori unknown. Their influence on plant water status will depend on many factors, including soil type, growing season, irrigation technology, and crop morphology. To use these soil water balance methods properly, we need to know the depth and distance from tree from which these waters are drawn and, more importantly, we need to install enough sensors to represent that soil water uptake.

An alternative basis to irrigation scheduling relies on plant physiology. Water-stress level is determined by direct measurement of leaf-water potential, leaf temperature, changes in stem diameter, or sap flow rates. Optimum irrigation management seeks to identify a given threshold for plant water status and apply just enough irrigation to minimize the detrimental impacts of water stress.

For the last decade, Naor et al. (1995) have used the changes in stem diameter and stem water status to monitor irrigation. The TDR technology has the potential for an accurate and reliable measurement of changes in stem moisture content. Being relatively free of background interferences and automatable, the TDR appears to be a suitable candidate for routine monitoring of water content of tree stems (_stem) and to follow diurnal changes in the water status of a single tree. During times of high evaporative demand, the tree stem serves as a water storage reservoir. If plant water stress results in a drop in _stem, and if the TDR is capable of measuring that drop_, then we may have the ideal tool for irrigation management which can integrate changes in both soil and plant water content.

	BACKGROUND

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
_stem by Gravimetric Sampling, -probe, and TDR
Soil water stress can induce significant changes in the water content of tree stems. _stem, ranging approximately from 0.10 to 0.40 L L^-1 have been reported by Reynolds (1965), Waring et al. (1979), and Schill et al. (1996). Several techniques (e.g., coring and gravimetric analysis, -probe, and TDR) have been used in the past to monitor _stem changes over periods of days, weeks, and up to a year. The relative water content (RWC) of the tree stem is defined as follows:

[1]

where W_f and W_d are fresh and dry weights (g) of the stem wood sample, V_f (mL) is the wood volume, and r_w and r_s are the density (g mL^-1) of the water and the wood solids, respectively. For routine measurement of RWC, r_s is assumed to be constant, 1.53 g mL^-1, following Jackson et al. (1995). Gibbs (1930)(1958) was one of the first to show that stems of many tree species can undergo substantial seasonal variations in _stem. Reynolds (1965) showed that water storage in the sapwood of large Douglas firs [Pseudotsuga menziesii (Mirbel) Franco] could make a significant contribution to daily transpiration under periods of high evaporative demand. Waring and Running (1978) calculated that an equivalent of about 22 mm of water, the same amount as a medium rainstorm, is stored in the sapwood of old-growth Douglas fir trees. The annual variation in _stem was found to be 0.48 L L^-1 with abrupt changes in _stem occurring over periods of just few weeks. A drop in _stem from 0.76 to 0.56 and a concomitant recovery back to the same value was found to be in good accord with changes in potential evapotranspiration. Waring et al. (1979) reported for Scots pine (Pinus sylvestris L.) trees, that 64% of the stem sapwood cross section was available to conduct transpiration water; a 27% reduction in _stem was noticed over a period of 2 wk. Jackson et al. (1995) reported a 0.17 L L^-1 decrease in relative wood moisture content in a maple (Acer ssp.) tree over the summer months, with values of _stem dropping from 0.56 down to 0.39 L L^-1. Clark and Gibbs (1957) measured _stem in yellow birch (Betula alleghaniesis Britton) trunks and reported a decrease in _stem of 0.20 (0.47–0.27 L L^-1) over the growing season, followed by a 6-wk period during which _stem increased by about 0.12 (0.27–0.39). The above measurements were all done by gravimetric analysis that is destructive and nonrepeatable.

Diurnal changes in _stem of 0.08 L L^-1 were found in irrigated apple (Malus domestica Borkh.) trees (Brough et al., 1986) and in nonirrigated pines (Pinus contorta Douglas ex Loudon) by means of -probes. Edwards and Jarvis (1983) also reported a large drop in _stem at different radial depths in the xylem of Sitka spruce [Picea sitchensis (Bong.) Carrèire] trees (at 1 and 4 m above ground), as measured by attenuation and gravimetric methods. More recently, Wullschleger et al. (1996) used TDR nondestructively to measure large seasonal increases in _stem in black gum (Nyssa sylvatica Marshall var. sylvatica, 0.125 L L^-1) and red maple (Acer rubrum L., 0.14 L L^-1) trees. Short-term (diurnal) and long-term (seasonal) changes in _stem by TDR were also obtained in a wide variety of tree species (Constantz and Murphy, 1990). Four days after a flooding irrigation event, _stem of an English walnut tree (Juglans regia L.) increased from 0.42 to 0.47 L L^-1. Using the same technique in natural groves of aspen (Populus ssp.), pinion (Pinus edulis Engelm.), cottonwood (Populus deltoides Bartram ex Marshall), and ponderosa (Pinus ponderosa Douglas ex P. Lawson & Lawson), Constantz and Murphy (1990) found absolute values of _stem between 0.20 and 0.70 L L^-1, with an annual change in moisture content between 15 to 70% depending on tree species, as well as soil and atmospheric conditions. When greenhouse arborescent palm [Sabal palmetto (Walter) Schultes & Schultes f.] plants were irrigated, water was withdrawn from the stem during periods of high transpiration to supply between 20 to 40% of the total transpirational demand. Water was then replenished during the nighttime. Absolute values of _stem as measured by TDR were in good correlation with gravimetric measurements of _stem as determined by weight loss (Holbrook and Sinclair, 1992). It should be stressed that when daily estimates of extracted water (determined by TDR) differed from total loss by weighing, there was no consistent bias.

Long-term changes in _stem, as measured by TDR for two consecutive years (a dry year following a wet year), were reported by Wullschleger et al. (1996) for red maple, white oak (Quercus alba L.), chestnut oak (Q. prinus L.), and black gum. Annual changes of _stem were between 0.10 to 0.20 ± 0.04 L L^-1, depending on soil and climate conditions. Agreement with gravimetric analysis of excised stem segments was good. The above experimental evidence shows that TDR is capable of measuring _stem (at different time scales) of different levels of tree water status. Most of the data discussed above are related to non-irrigated, natural trees with large time intervals between measurements. The uniqueness of our study involves more detailed and more frequent measurements of _stem, [by means of short (70 mm) TDR probes, in orchard trees growing under a wide range of induced water-stress conditions] accompanied by simultaneously measured soil water content.

The irrigation rate for lemons (and citrus in general) growing in Negev, Israel, ranges between 600 to 900 mm yr^-1 depending on tree size, soil type, and rainfall. On the other hand, direct measurements of actual tree water use, as determined from heat-pulse measurements of stem sap flow (Cohen, 1991), prove that the total annual tree transpiration ranged between 350 and 400 mm yr^-1, for the coastal area. These values should be corrected by 10 to 15% to account for the higher evaporation demand in the semiarid Negev. It appears that current irrigation practices are over watering the trees. Drainage, surface evaporation, and salt leaching consume the excess irrigation water. Closing the gap between plant needs and practical irrigation rates may lead to improvements in water use efficiency. The objectives of the study were (i) to evaluate the performance of TDR in measuring _stem of lemon trees and (ii) to test the practicality of _stem measurement as an indicator to monitor irrigation.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Experimental Design
The experiment was performed in a commercial lemon orchard at Sde Nitzan, in the northwestern Negev region of Israel. The regional climate is Mediterranean with hot, dry summer and warm, rainy winter (November–March). The annual precipitation varies between 150 and 350 mm yr^-1 with an average of 280 mm yr^-1. Evaporation rate of a Class A pan stationed 1 km away from the research site is 55 ± 10 mm wk^–1 during the summer months. The soil is a fine sand regosol, containing carbonates (Xeric Torripsament) of 40 to 60 g kg^-1 clay, 50 to 70 g kg^-1 silt, 700 g kg^-1 fine sand, and 170 to 210 g kg^-1 of coarse sand. The orchard trees were planted in 1996 at a spacing of 3 by 6 m. The lemon variety is Villafranca on Volkameriana (Citrus volkameriana Ten. & Pasq.) rootstock. Irrigation was supplied via two drip lines spaced 1.0 m on either side of the tree row. Each drip line has pressure compensated emitters of 1.6 L h^-1 that are spaced at 0.75-m intervals. The farmer used a "calendar" irrigation management: weekly irrigation rates were 60 mm in May, 68 mm in June, 75 mm in July, and 82 mm in August and September. The monthly irrigation amount was applied in nine, equal volume, irrigation events. Within the commercial orchard, an experimental area was established using lemon trees of a similar size arranged in a randomized block design with five replicates for each irrigation treatment and five trees per replicate. The present study used measurements from just two of the replicates. Leaf temperature, and leaf water potential (LWP) were measured to ensure that the applied irrigation regimes succeeded in imposing the target levels of water stress.

Four different water-stress levels were established within the experimental block by manipulating the irrigation system.

1. Full—The farmer's normal irrigation rate, aimed at minimum water stress and maximum crop yield. The full irrigation treatment was considered as control and applied 60 to 80 mm of water each month during the summer period.
   
2. 50%—This treatment applied half the full irrigation. The water was applied only to one side of the tree's root zone.
   
3. Split—This treatment applied half the full irrigation. However, the water was applied to alternate sides of the root zone at 3-wk intervals.
   
4. Dry—This treatment was not irrigated for the duration of the study.
   

The four irrigation treatments were conducted for 75 d between 24 June and 7 Sept. 2001. All treatments were irrigated twice each week with treated sewage water ( = 0.9 dS m^-1). Irrigation was applied during the middle of the night to reduce water loss due to surface evaporation.

Techniques
Because the dielectric constant of water ( 80) is larger than that of other soil constituents (_air = 1, _solids = 2–5), any change in the bulk dielectric of a composite material containing water, soil, and air, predominantly reflects a change in water content (Topp et al., 1980). An empirical relationship (calibration equations) is used to convert TDR measurements of _a into values. In this study, we used two calibration equations [Topp et al. (1980) for the soil and Wullschleger et al. (1996) for the tree stem]. Wullschleger et al. (1996) produced a single calibration curve for four different tree species (red maple, white oak, chestnut oak, and black gum) that were in good match with the Constantz and Murphy (1990) calibration data. The combined data were fitted to the following second order quadratic equation

[2]

Similar calibration equations were obtained by Green and Nadler (unpublished data, 1999) from kiln dry wood blocks that, after saturation with water under vacuum, were equilibrated at different pressures on a standard soil pressure plate. The gradually drying blocks were then weighed and was determined by TDR in the moist wood after each drying stage.

• _stem —Three TDR probes were installed into the stem of each tree. Three millimeter holes were drilled, through a metal leader, in the stem 0.3 and 0.5 m above the soil level and two of the TDR probes were installed horizontally (namely, rods in plane in right angle to stem) and the third was installed vertically (namely, rods in plane parallel to stem) into the trunk to span the whole tree diameter. The stem TDR probes were 70 mm in length and made from three rods of 3-mm diam stainless steel at a 20-mm spacing and were installed some 50 d before measurements commenced, to minimize the risk of any wound recovery effects (Wullschleger et al., 1996). A total of 24 TDR probes were installed in all eight replicates.
  
• _soil—Four TDR probes were installed into the root-zone soil surrounding each tree. The soil TDR probes were 200 mm in length and made from three rods of 3-mm diam stainless steel, at a 50-mm spacing. In most cases, the soil TDR probes were installed vertically into the ground, at depths of 0.1 to 0.3, 0.3 to 0.5, 0.5 to 0.7, and 0.7 to 0.9 m below the soil surface. However, for the split irrigation treatment, to represent both alternating sides and still maintain the same number of soil measurements, the soil TDR probes were installed at depths of 0.3- to 0.5- and 0.5- to 0.7-m layers, located on both sides of the root zone.
  

A 4.9-m coaxial cable (RG58U) was manually used to connect each of the TDR probes to a cable tester (Tektronix 1502B, Beaverton, OR, USA). The cable tester was used to collect the TDR traces. The trace analysis procedure measured the apparent length, l_a, of the TDR trace by manual identification of the probe's end-point reflection. The apparent dielectric is calculated by the following: = l_a/l, where l (mm) is the actual length of the TDR probes (70 mm for the stem probes and 200 mm for the soil probes). During the course of the field measurements, we also measured the air temperature, T_air [°C], to correct the instrument measurements for the temperature effects on the dielectric properties of coaxial cables. Cables and equipment were shaded so that they were in air temperature. The probe's apparent length (l_a, mm) was calculated by subtracting a predetermined (laboratory) cable length from the measurement of the distance to the end-point reflection, further corrected for any temperature effects. Two probeless cables, having the same cable length as the TDR probes cables, were used to obtain an empirical temperature-cable length relations reflecting the influence of air temperature on the apparent dielectric length of the coaxial cable (l_{a cable}). In addition, we measured l_a and the temperature of a block of lemon wood (WB) that had been soaked in water for four days while applying vacuum, and then wrapped in PVC and aluminum to minimize water loss. The wooden block was left under the tree and subjected to approximately the same environment as experienced by the stem TDR probes. We observed that l_{a, probe} of the WB had a steady range indicating negligible water loss and yielded an appropriate temperature correction factor.

Because of the critical effect of temperature on l_{a cable} and l_{a probe}, an additional set of field measurements was conducted at the end of the season (DOY 319) when similarity between the irrigation treatments could be assumed. In this case, two alterations were made to our measurement procedure compared with the previous field measurements.

1. For each probe, the start-point was also measured. This additional measurement enabled us to estimate l_{a probe} by two independent methods, and thereby checked our empirical temperature correction factor. Figure 1 shows a comparison between routine measure of the probe's end point, corrected for temperature effects of l_{a cable}, and direct calculation of l_a determined by subtracting measured l_{a cable} from the probe's end point.
   
2. In the insulated WB, we monitored the temperature of the air, T_air, and the temperature of the wooden block, T_WB. These additional measurements enabled us to estimate the measurement error associated with these two temperatures (Fig. 2) . We found the l_{a probe}-T_WB relations were irregular and mostly positive, which is counter to the theoretical expectations, while the l_{a probe}–T_air relations were quite stable (±2mm), and close to the measurement resolution of the cable tester.
   

View larger version (21K): [in this window] [in a new window]   Fig. 1. A comparison between two methods for evaluating temperature effect on the coaxial cable dielectric length (la probe): 1) Measuring only the probe's end-point reflection and subsequently correcting for temperature-induced shifts of the probe's length (abscissa) compared to 2) the measurement that subtracts the directly-measured probe beginning from measured end-point reflection (ordinate). The 1:1 line is shown.

View larger version (23K): [in this window] [in a new window]   Fig. 2. Directly measured la probe installed in the wood block as a function of Tair (A) and TWB (B). The corresponding values of stem are within the TDR resolution for a wide range of both air and wood block temperature.

Potential Sources of Experimental Error
The natural variability between different trees species in terms of their water-stress-induced changes in stem moisture content is large. Thus, it would be inappropriate to project the likely irrigation response of lemon trees on the basis of literature values from other species. Reliable and continuous measurements of _stem are difficult to make, and there are several sources of error that can affect the accuracy of a TDR measurement, as described below.

Temperature-Dependence of the Cable Length (l_{a cable}-T relations)
For short TDR probes, it is necessary to correct the cable's apparent length for temperature-induced changes in l_{a cable} that originate from the way l_{a probe} is measured. Before the start of the field measurements, l_{a cable} of each probe was accurately determined in the laboratory (25°C), and used to locate the beginning point of the probe's trace. In the field, to save time, only the reflection point at the probe's end point was determined. The probe's apparent length (l_{a probe}) was then obtained by subtracting the known l_{a cable} values. Because there is no specific information on the l_{a cable}-T relations for our cables (RG58U) we had to rely on experimental data to develop an empirical correction factor. Routinely, the relationship between l_a and T_air was obtained in the field by measuring the apparent length of two 4.9-m shorted cables (i.e., cables with no TDR probes) over a range of different air temperatures. The results are shown in Fig. 3 , along with similar results obtained from an earlier study (Green and Nadler, Hort Research, NZ, unpublished data) in which 5000 pairs of l_a–T_air values were recorded. There is a good correspondence between these three independent data sets; the relationship is good enough to explain the influence of air temperature on an RG58U cable of 4.9-m length. From our error analysis, we calculated that a 1°C increases in air temperature reduces l_a of a 4.9-m-long coaxial cable by about 0.4 to 0.6 mm, depending on T_air. Converting this temperature response into we calculate a 1°C temperature change results in an error in equal to 0.0015 to 0.0022 and 0.00041 to 0.00064 L L^-1 in the 70- and 200-mm probes, respectively. The temperature effect on l_{a cable} is approximately of the same magnitude as the daily changes in stem water content, _stem (see below). Thus, it is vital to apply this correction if we are to accurately observe the short-term changes in _stem.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Changes in the apparent dielectric length of a 4.9 m coaxial cable (la cable) as a function of air temperature determined from five experimental data sets and the equation of best fit to those data sets (See text).

Stem Wound Response to Probe Installation
One out of the four species tested by Wullschleger et al. (1996) showed a significant installation effect on the measurement of _stem that lasted up to 10 wk following probe installation. The range of _stem measurements in the other three species fell within the spatial or experimental error. No further installation effects were reported for the following two years. For this reason we chose to wait for a period of 50 d after installation before taking the first TDR measurements of _stem. Thereafter, we have assumed the wound response to be small.

	RESULTS

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Field Measurements
TDR Technology
Time domain reflectometry measured in the stem of the lemon trees and in the root-zone soil surrounding each tree. TDR measures the apparent dielectric, _a, of the soil-water-air mixture. Measurements were performed 72 h after the last irrigation, except on DOYs 198 and 247, when they were made 24 h after irrigation. During most field days the TDR probes were measured at three times, 0530 to 7:00, 0900 to 1030, and 1200 to 1330 h, (Fig. 3). Figure 4 shows the time course of _soil and _stem for the four irrigation treatments, as measured before and during the stress imposing period. Most of the values fell in the range 0.08 to 0.16 with extreme values between 0.05 and 0.18. As expected for a very sandy soil, the range of _soil is quite narrow and the extreme values of the different treatments are quite similar.

View larger version (52K): [in this window] [in a new window]   Fig. 4. Seasonal pattern of moisture content in the tree stem (stem) and the root-zone soil (soil) L L-1 for the full (A, E), 50% (B, F), split (C, G), and dry (D, H) irrigation treatments, respectively. (The notations H and V indicate horizontal or vertical installation of TDR probes. DOY refers to the Day of Year. Exceptionally in G, depths of probes are 0.3–0.5 and 0.5–0.7 m, on both sides of the tree.)

In each of the irrigated treatments, _soil clearly showed periods when increased following a series of irrigation events. In contrast, _soil of the dry treatment showed a fairly steady decrease over the whole of the water stress period. In absolute terms there is, as expected, more water stored in the root zone of the fully irrigated trees compared with the dry trees; intermediate levels of soil moisture were recorded in the root zone of trees in both the 50% and split irrigation treatments.

The TDR measurements of soil moisture at upper 1 m suggest that some surplus irrigation was applied after about DOY = 220 since _soil slowly increased for all irrigated treatments soon after that day (Fig. 4E, F, and G). There were also periods when water was accumulating in the different soil layers. The length of these periods and magnitude of _soil generally declined in the order full > 50% > split. It is also clear from Fig. 4E–H that the root uptake activity is greatest from the surface roots. The largest decreases in _soil are recorded by the TDR probes at depths of 0.1 to 0.3 m. Root uptake activity by the deeper roots, at depths of 0.7 to 0.9 m, is smaller than in the layers above. Green et al. (1997) have also shown a similar pattern of preferential water uptake by the surface roots of irrigated apples trees. _stem values gradually decreased during the water stress stage (DOY 175–250) for all irrigation treatments (Fig. 4A–4D). Given the large spatial variability within a single tree, we could find no systematic differences between the horizontal and vertical orientation of the stem TDR probes, with respect to the absolute values of _stem, the long term (seasonal) changes in the magnitude of _stem, and the short term (diurnal) fluctuations in _stem. The diurnal fluctuations are presumably related to the daily cycle of stem shrinkage and swelling of the tree stem. The most outstanding feature of the measurements of _stem is the wide scatter between measurements within a single tree. This scatter suggests that the spatial variability within the stem is at least as large as the seasonal range of _stem that was recorded by a single TDR probe. In addition, the daily cycle in _stem was observed to be up to 30% of the full seasonal fluctuation in _stem, as recorded during the whole water stress period.

Figure 5A shows the stem moisture recorded over the whole season, normalized to the dawn value recorded on the first day (DOY = 154) for each of the four irrigation treatments. In other words, we have selected the first day of measurement as our reference level. Triplet data points shown in Fig. 5 represent the dawn, morning, and noon values of recorded during each of our weekly visits, respectively. Our TDR measurements showed that the _stem values in the full irrigation treatment varied over a narrow range, rising by 0.02 L L^-1 above the reference value before DOY = 205, and subsequently decreasing by 0.03 below the reference level thereafter. Over the same time period _stem values in the 50% treatment decreased steadily, by about 0.07, while the stem moisture content of the split and dry treatments decreased by about 0.12 L L^-1.

View larger version (19K): [in this window] [in a new window]   Fig. 5. Changes in the average moisture content of the tree stem (stem) and the root-zone soil (soil), normalized to the corresponding values measured on the first day of the experiment.

The TDR results indicate a good correspondence between soil water status and stem moisture status. Dawn and noon LWP values both before and during the water stress period reflect the effect of irrigation management on the tree's water status (Fig. 6) . High leaf water potential during the day is consistent with the tree transpirational demands. High leaf water potential at dawn is symptomatic of a tree that has not completely recovered its water status and is likely to be exhibiting some degree of water stress. As expected, the values of LWP recorded between DOY 156 and177, when the trees were not under stress, were much lower than those values during the water stress period. The dawn measurements of LWP revealed an increasing gap between the full irrigation treatment, the 50%, and split treatments. Midday LWPs of the full treatment consistently ranged between 1.9 and 2.2 MPa, whereas LWP of the 50% and the split treatments increased gradually by about 0.05 to 0.4 MPa. A complete recovery in LWP values was observed in all treatments some 10 d after the return to full irrigation.

View larger version (25K): [in this window] [in a new window]   Fig. 6. Leaf water potential (LWP) as function of time (DOY) as measured at dawn (broken lines) and noon (solid lines) for the four irrigation treatments. Also shown are prestress (DOY < 181) and post-stress (DOY > 250) levels.

	DISCUSSION

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Errors in Measurement

1. Operator error. A technical error obtained when measuring by the same TDR probe because of poor probe installation or air gaps around the TDR rods. This error was quantified by examining the measurement variability of _soil determined in the dry irrigation treatment. In this case, the standard deviation (SD) of the measurements from the eight soil TDR probes, as measured between DOY 191 and 247, ranged between 0.0023 and 0.0071 with an average SD of 0.0038 (L L^-1, Table 1). Soil measurements of _soil were consistent over this period but factually we have no evidence that the variability in _soil did not change between DOYs 191 and 247, for the different soil depths. We assume that the values of the dry treatment were constant because (i) there was no irrigation or rainfall, (ii) 3 wk were allowed for drainage in a sandy soil, and (iii) the temperature was fairly steady.
   

View this table: [in this window] [in a new window]   Table 1. Experimental error of measured by short (70 mm), long (200 mm), and literature reported TDR probes in stem and soil under four water stress levels.

2. Experimental error in determining the temperature effect on l_{a cable}. A systematic error in the measurements arising from the temperature influence on the coaxial cable dielectric length, l_a. The experimental error associated with changes in T_air and their effect on the TDR measurement of l_{a cable} was estimated from data from two probeless, 4.9-m cables (Fig. 3). Assuming the physical length of these cables remains constant, any variation in l_{a cable} essentially represents the temperature response of the dielectric properties of the coaxial cable itself. From 47 pairs of T_air – l_a measurements, the temperature variation equates to an error of between 0.75 x 10^-3 and 0.88 x 10^-3 L L^-1 for the 200-mm TDR probes, and is equivalent to 0.01 and 0.011 L L^-1 for the 70-mm probes. This estimate accounts for several potential sources of error including nonuniformity of the cables, errors in the real length determination, some operator bias, and the accuracy of the air temperature measurement (±1°C). A wide range of coaxial cables exist and no reliable correction factors can be assumed since the dielectric constant of a polymeric material can vary with density, degree of crystallinity, and other details of a particular sample. We have no real way of measuring the actual temperature environment of the whole cable. Therefore, since there was no specific information available for the L_{a cable} - T relations in our cable (RG59U), we used the experimental data to determine an empirical relationship.

3. Experimental error due to radial variability of stem morphological. The stem's natural variability yields a wide range in _stem values and makes difficult the task of obtaining an effective TDR measurement of tree water status. To estimate the "natural variability," we have calculated the average value from the 24 stem probes and the 32 soil probes in each of the dawn, morning, and noon measurements (Table 2). Compared with the comparatively small experimental error expected from a single TDR measurement, the natural variability expressed by all eight replicates was medium (0.017 L L^-1) in the root-zone soil and quite high (SD = 0.14 L L^-1) in the tree stem. The SD in our TDR measurements due to natural variability in is higher by a factor of 4.5 for the 200-mm-long soil probes, and 37 times for the 70-mm stem probes. We attribute this variability to two major sources. There is a real spatial variability in the water content of both the soil and the stem. But there are also difficulties in interpreting some of the stem's TDR traces. In some instances, because the probes are so short, the probe's end-point reflections were not very sharp. This occasionally caused some doubtful observations of reflection point. Installing more TDR probes into a tree stem would not help to correct this problem, but it would allow us to select just those probes that produced signals that were easier to interpret. The experimental error (natural variation) associated with the TDR measurement of _stem in the lemon trees was found to be similar to the radial variation of _stem reported in previous dielectric studies (Constantz and Murphy, 1990, Schill et al., 1996). In the stems of our lemon trees, the radial variation in _stem was found to be about 0.085 L L^-1. Specific to grapefruit, Cohen (1991) found for six trees that the averaged transpiration rate had a high SD (40–95 L d^-1) when the soil dried out by 0.28 L L^-1. A 2-fold difference was found in yield (130–260 kg yr^-1 tree^-1) and a 2.5-fold difference in transpiration rate (40 x 98 L d^-1) between 22 different trees. Thus, citrus trees exhibit a measurable response to water-stress, in terms of changes in transpiration and productivity. The corresponding stress response could be reflected by changes in stem moisture content. Our results show that TDR can be used successfully to measure such changes in tree water status.

View larger version (39K): [in this window] [in a new window]   Fig. 7. Changes in the electrical conductivity of the soil pore solution (w) as a function of time (DOY), for the 4 irrigation treatments (See text) and at four different depths in the root-zone soil.

View larger version (20K): [in this window] [in a new window]   Fig. 8. The seasonal changes in the amount of water stored in the root-zone soil for the four treatments, assuming its volume to be 1.8 m3.

A Speculation
In addition to a gradual decrease in _stem over the summer months, our TDR measurements indicated an unexpected positive daily cycle for _stem with values increasing from dawn through to midday (Fig. 9) . This increase appears to be most pronounced during the DOYs 190 to 250. The diurnal changes in _stem, as determined by a single measurement, are still well within the range of variability exhibited within a single tree stem. We consider these daily fluctuations real, and repeatable, although we have no mechanism to explain its occurrence. It is widely accepted that the stem wood often shrinks during the day and stem water potential (SWP) often drops (i.e., becomes more negative) as a result (Naor et al., 1995) but there have been previous reports of the opposite behavior occurring. Positive changes in _stem, between 0.025 to 0.06 L L^-1, were observed between the hours of 10:00 and 15:00 in Scots pine trees (Waring et al., 1979) and increases of about 10% in _stem have been observed between 1100 and 1600 h in 20-yr-old lodgepole pine trees (Edwards and Jarvis, 1983).

View larger version (29K): [in this window] [in a new window]   Fig. 9. Averaged (n = 6) values of the daily difference in stem moisture status (stem– stem, dawn) by irrigation treatment and as a function of time.

1. The most questionable part of the work was the need to apply a temperature correction to the TDR measurements. We can show this correction is appropriate since the magnitude of the daily fluctuations in the wooden block were small. In the tree stem, fluctuations in are not identical for all treatments, although the same correction has been applied. At least two of the _stem measurements showed a decline over the day and, in some cases, the value of _stem did not change much over the day. During autumn period (DOY = 319, data not shown), only a few of the stem TDR probes showed this "positive" daily cycle, to a much smaller extent than in the summer months (maximal daily increase of _stem was only about 0.015 L L^-1 while almost half the probes showed the "expected negative" cycle). The daily amplitude of the _stem fluctuations was reduced later in the season for the same daily temperature range (7–29.3°C).
   
2. The increase in _stem by midday could be related to the effect of long cables on the TDR pulse frequencies distribution. According to Logsdon (2000), long coaxial cables (e.g., >5 m) can reduce the higher frequency components of the TDR trace. This can influence the frequency dependent part of that is associated with the bound water. Logsdon (2000) has reported a strong effect of temperature on l_{a cable} when measuring high values in soils having a high _a value (surface area > 77 m² g^-1), and using a 30-m-long cable. In contrast, she also reported that the value determined for a sandy soil having a low _a (namely, a low surface area) was negligibly affected by temperature and only influenced by the longest cable length tested (viz., 51.8 m in her study, compared to 4.9 m used in the present study). So we can probably rule out the contribution of any high-frequency TDR effects since they are likely to make a negligible contribution to the diurnal pattern of stem moisture content. In any case, if there were a temperature effect, than a large part of it (>50%) would be explained by our simple empirical calibration.
   
3. Yet another explanation for the observed increase in stem moisture content between dawn and midday could be the special aspect of the TDR principle of operation; how TDR differentiates between bound and free water molecules. It is possible for the TDR measurement of stem moisture to indicate an increase in _stem even when the absolute number of H₂O molecules in the sampled volume remains constant. This increase would be due to H₂O molecules that change their status from bound to free water as may happen when H₂O molecules, bound or absorbed to sugars, proteins, other organic molecules, or cell membranes, are relieved of their bonding. On summer mornings, the water transpiration demand may change the relative stability or fraction of bound water residing in the tree stem.
   
4. Assuming any air gap between the TDR rods and the stem material, which may have formed during drilling and probe installation, was not completely filled, we have to accept the possibility that some water will be retained in the gap. This water will be less strongly bound than any water that is present in the sapwood surrounding the probes. It is therefore suggested that the morning after water flow has filled this gap, an increasing demand for transpirational water by the leaves around 1000 h could help to remove some of this the water. (Currently, an additional experiment is being designed to test this last suggestion.)
   

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
_stem can easily be measured by TDR. Unfortunately, the TDR measurements of stem moisture content do not always have the desired accuracy and time response required to manage effectively an orchard irrigation system. Minor improvements may be achieved by the use of heat insulated cables, applying fully automatic and more frequent sampling, and by installing a larger number of TDR probes and selecting only the well behaving ones.

Among the major sources contributing to the uncertainty in our measurements of _stem is the tree's natural variability both around and within a single tree stem. This variability will be related to the tree's morphology and so it may vary widely among different tree species. Temperature effects on the measurement of are important, but can be corrected. With due care, the operator errors are also likely to be small, particularly if the waveform analysis procedure is automated. Lastly, we note that the cost of a TDR device means the method is still very much in the domain of the researcher and may be prohibitive for small farmers. With a price of 12 000 $US in Israel and a minimum period of 10 yr of reliable service, the annual price of a cable tester reduces to about 1200, $US which may be economical to use on some crops or to use by a collective of local growers.

The use of TDR to monitor stem moisture may be more successful in trees that have a higher water content (e.g., avocado, Persea ssp.) and/or a larger xylem-conducting cross section. It may also be interesting to see if the same technology could detect the electrical conductivity of any electrolytes that are flowing in the tree stem. In the future, such measurements may have potential in developing simple electronic tools that can monitor both water and nutrient use efficiency.

Given the unavoidable and unpredictable nature of stem sapwood variability, we do not advocate the use of several probes from different trees to get a measure of the average stem water content. Rather, we suggest the farmer or irrigation consultant focus on the daily and seasonal changes in that could be determined via a single TDR probe. The amplitude of the daily fluctuations in _stem will be related to the transpirational demand as well as a measure of the root-zone moisture status. For the time being, it remains a research problem to unravel the complexities of these hydraulic linkages.

	ACKNOWLEDGMENTS

 
The authors express their deepest thanks to Mr. Netzer Shamir, Sde Nitzan Mobile Post Negev, 85470, in whose orchard the measurements were conducted, for a close cooperation, and to Mario Ryppa (Gilat Research Center) for constructing the TDR probes.

Received for publication February 7, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION BACKGROUND MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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