Compensation Heat-pulse Method Research Articles

Comparing dual heat pulse methods with Péclet's number as universal switch to measure sap flow across a wide range.

Accurate determination of sap flow over a wide measurement range is important for assessing tree transpiration. However, this is difficult to achieve by using a single heat pulse method. Recent attempts have been made to combine multiple heat pulse methods and have successfully increased the sap flow measurement range. However, relative performance of different dual methods has not yet been addressed, and selection of the numerical threshold used to switch between methods has not been verified among different dual methods. This paper evaluates three different dual methods with respect to measurement range, precision and sources of uncertainty: (method 1) the heat ratio (HR) and compensation heat pulse method; (method 2) the HR and T-max method; and (method 3) the HR and double ratio method. Field experiments showed that methods 1, 2 with three needles and 3 compare well with the benchmark Sapflow+ method, having root mean square deviations of 4.7cmh-1, 3.0cmh-1 and 2.4cmh-1, respectively. The three dual methods are equivalent in accuracy (P>0.05). Moreover, all dual methods can satisfactorily measure reverse, low and medium heat pulse velocities. However, for high velocities (>100cmh-1), the HR+T-max (method 2) performed better than the other methods. Another advantage is that this method has a three- instead of four-needle probe configuration, making it less error prone to probe misalignment and plant wounding. All dual methods in this study use the HR method for calculating low to medium flow and a different method for calculating high flow. The optimal threshold for switching from HR to another method is HR's maximum flow, which can be accurately determined from the Péclet number. This study therefore provides guidance for an optimal selection of methods for quantification of sap flow over a wide measurement range.

Irrigation management with saline groundwater of a date palm cultivar in the hyper-arid United Arab Emirates

The United Arab Emirates has a hyper-arid climate. Irrigation is essential for dates (Phoenix dactylifera L.), an important crop economically and culturally. Groundwater is relied on, yet it is a non-renewable resource at the rate it is being used. Furthermore, as the water-table drops, it is becoming more saline. Law no. 5 has been passed in Abu Dhabi to regulate the use of groundwater and set allocation limits for agriculture. For assessing the allocation of irrigation water to date farms under Law 5, we carried out measurements of tree water-use by the compensation heat-pulse method, complemented by measurements of the changing soil-water dynamics using time domain reflectometry and bulk soil electrical conductivity. Over four years we measured the hourly pattern of Lulu date-palm water use, ETc, at two levels of irrigation-water salinity: Treatment S1 at 5 dS m−1, and S3 at 15 dS m−1. The mid-summer ETc for the S1 Lulu trees is up to 190 L d−1, on average, whereas for the S3 trees ETc is lower at 130 L d−1 (68% of S1) because of the salt. Measurements of canopy radiation interception using a ‘light stick’ showed the S1 trees intercepted 26% of the incident radiation, whereas the S3 trees only intercepted 20% (ratio S3/S1 = 76%). The date yield of the S1 trees was 68 kg tree−1, but was 46 kg tree−1 for the S3 trees (ratio 68%). Current practice is to irrigate trees with 275 L d−1, irrespective of salinity. Our recommendation for Law 5 is to tailor irrigation to the seasonal demand in the reference evapotranspiration of ETo, and allow for a 25% factor-of-safety and a 25% salt leaching fraction. For S1 date palms this would mean an annual average of 210 L d−1, and for S3 just 137 L d−1. This represents savings of 25–50% from current practice.

SEASONAL CHANGES IN PEAR TRANSPIRATION

Flood and sprinkler irrigation systems in Australia have largely been replaced with micro-irrigation. Understanding crop water requirements in these orchards is necessary to avoid over- and under-irrigation so that yield and water are optimized. The objective of this study was to investigate seasonal changes in pear transpiration (T) and link this to reference crop evapotranspiration (ETo) and intercepted photosynthetically active radiation (fPAR). The sap flow technique was used to determine T in a commercial high-density 'Williams' Bon Chretien' pear (Pyrus communis L.) orchard. Measurements were taken at 30-minute intervals using the compensation heat-pulse method and commenced prior to full bloom. ETo was calculated from local weather data measured at the orchard site. fPAR was estimated from an array of 24 PAR sensors positioned on the orchard floor. Daily T increased after full bloom and stabilised at approximately 3 mm/day in late spring and early summer when irrigation was deliberately withheld. Daily T increased to a maximum of approximately 5 mm/ day when irrigation was applied during the period of rapid fruit growth and ETo was high. After harvest, daily T steadily declined. A strong relationship between daily T and fPAR-weighted ETo was found for the period of full irrigation prior to harvest (T = 1.1 fPAR ETo; R2 = 0.76). Daily T during the withholding irrigation and post harvest periods was generally less than daily T based on the relationship established under full irrigation prior to harvest.

Evaluating the potential of a novel dual heat-pulse sensor to measure volumetric water use in grapevines under a range of flow conditions.

The aim of this study was to validate a novel, dual sap-flow sensor that combines two heat-pulse techniques in a single set of sensor probes to measure volumetric water use over the full range of sap flows found in grapevines. The heat ratio method (HRM), which works well at measuring low and reverse flows, was combined with the compensation heat-pulse method (CHPM) that captures moderate to high flows. Sap-flow measurements were performed on Vitis vinifera L. (cvv. Thompson seedless, Chardonnay and Cabernet Sauvignon) grapevines growing in a greenhouse and in three different vineyards, one of which contained a field weighing lysimeter. The combined heat-pulse techniques closely tracked diurnal grapevine water use determined through lysimetry in two growing seasons, and this was true even at very high flow rates (>6L vine-1h-1 for Thompson seedless vines in the weighing lysimeter). Measurements made with the HRM technique under low flow conditions were highly correlated (R2 ~ 0.90) with those calculated using the compensated average gradient method that is used to resolve low flow with the CHPM method. Volumetric water use determined with the dual heat-pulse sensors was highly correlated with hourly lysimeter water use in both years (R2=0.92 and 0.94 in 2008 and 2009 respectively), but the nature of the relationship was inconsistent among replicate sensors. Similar results were obtained when comparing grapevine water use determined from sap-flow sensors to miniaturised weighing lysimetry of 2-year-old potted vines and to meteorological estimates for field-grown vines in two additional vineyards. The robust nature of all of the correlations demonstrates that the dual heat-pulse sensors can be used to effectively track relative changes in plant water use, but variability of flow around stems makes it difficult to accurately convert to sap-flow volumes.

Thermographic measurement of canopy temperature is a useful tool for predicting water deficit effects on fruit weight in citrus trees

Alternative and more practical methods for plant water stress detection than stem water potential (ψs) and stomatal conductance (gs) are needed when regulated deficit irrigation (RDI) strategies are applied. The aim of this experiment was to compare sap flow and canopy temperature (Tc) measurements with more classical methods like ψs or gs to predict the effect of deficit irrigation on fresh fruit weight in citrus trees. The experiment was performed during the summer of 2011 in a “Clementina de Nules” orchard undergoing RDI. Sap flow was determined by means of the compensation heat pulse method in well-watered and RDI trees. Tc was measured continuously with infrared thermometers (IRTs) mounted over the canopies and also weekly with an infrared hand-operated thermographic camera taking frontal images of the sunlit side of tree crowns. Concurrently, ψs and gs were also measured on all trees. Results showed that the evolution of the relative transpiration obtained with the sap flow sensors was in agreement with the plant water stress experienced. The values of Tc obtained with the fixed IRTs, normalized by air temperature (Tc−Ta) were in general poorly related with ψs and gs. However, when Tc was obtained from thermal imaging, there was a good correlation with ψs in days of relatively high water stress (i.e. when ψs differences among treatments were >1.0MPa). The average fruit weight at harvest was significantly correlated with all the stress indicators, and the best correlation was that of thermographic Tc followed by ψs and gs. Overall, results showed that in citrus trees Tc measurement obtained from thermal imaging is a good tool to predict the effect of water deficit on fresh fruit weight.

Modelling canopy conductance and transpiration of fruit trees in Mediterranean areas: A simplified approach

Improving current approaches to quantify the transpiration of fruit trees is needed for water allocation purposes and to enhance the precision of water applications under full and deficit irrigation. Given that transpiration of tree crops is mainly modulated by canopy conductance (Gc) and vapour pressure deficits, we developed a functional model of tree transpiration by quantifying an average daily Gc based on radiation use efficiency and CO2 assimilation. For model calibration, an extensive experimental dataset of tree transpiration (Ep) was collected in many of the main temperate fruit tree species, namely, apricot, apple, citrus, olive, peach, pistachio, and walnut, all under non-limiting water conditions, in different orchards in Spain and California (USA). In all species, Ep was assessed by measuring sap flux with the Compensation Heat Pulse method for several months, and a transpiration coefficient (Kt) was calculated as the ratio of measured Ep to the reference evapotranspiration. For three deciduous species (apricot, peach and walnut) Kt showed maximum values close to 1, a value which stayed more or less constant throughout the summer in peach and walnut. The maximum Kt values were measured in pistachio (1.14) while they only reached 0.35 in olive and citrus trees. In the latter two species, Kt varied seasonally between 0.2 and 0.6 depending on the weather. The average Gc in July was high for apple, walnut, peach and pistachio (range 0.240–0.365molm−2s−1) and low for olive and orange (range 0.074–0.100molm−2s−1). The calibrated model outputs were compared against measured Ep data, suggesting the satisfactory performance of a functional model for Ep calculation that should improve the precision of current empirical approaches followed to compute fruit tree water requirements.

Can heat-pulse sap flow measurements be used as continuous water stress indicators of citrus trees?

Transpiration of well-watered and regulated deficit irrigated (RDI) citrus trees was determined by sap flow (SF) measurements using the compensation heat-pulse method. Its potential for detection of plant water stress was evaluated in comparison with measurements of midday stem water potential (wstem). The study was carried out during 2 years in two commercial groves of Clementina de Nules (CN) and Navel Lane Late (NLL). SF measurements were taken in two trees per treatment instrumented with two identical gauges per tree in NLL and two different types of gauges (type 1 shorter than type 2) in CN. The absolute SF values underestimated the tree water use. Averaged over the entire period of water restrictions, a reduction of about 50 % in water application in the RDI trees of both species decreased tree transpiration compared to the control trees only by a 15 %. Both the nocturnal-to- diurnal SF ratio and the relative transpiration were in good agreement with differences in wstem. Overall, results sug- gest that SF measurements should be preferentially used in relative terms. Sap flow sensors are useful for detecting plant water stress, but they also highlight some of the problems for accurately measuring transpiration.

Trunk sap flow characteristics during two growth stages of apple tree and its relationships with affecting factors in an arid region of northwest China

a b s t r a c t Sap flow is an important parameter for orchard irrigation scheduling. According to seasonal variation of trunk diameter and leaf area index (LAI), the whole growth stage of apple tree was divided into stage I (rapid leaf and trunk growing) and stage II (rapid fruit enlarging and maturing). Trunk sap flow (SF) of apple tree irrigated with 100% evapotranspiration (ET) was measured using the compensation heat-pulse method, and meteorological variables, soil water content (� ), LAI, maximum daily stem shrinkage (MDS) and leaf water potential (ϕl) were monitored during the whole stages from 2008 to 2010 in an apple orchard of arid region of northwest China. The relationships between SF and affecting factors were also analyzed. The results show that LAI increased rapidly at stage I, and SF was mainly determined by vapor pressure deficit (VPD), LAI and reference evapotranspiration (ET0). However, LAI was relatively stable at stage II, and SF was affected by VPD, ET0 and � . Under the condition of border irrigation with 100% ET, tree SF could be calculated using VPD, ET0 and LAI at stage I, and VPD, ET0 andat stage II. Moreover, SF had significantly negative linear relationship with ϕl and exponential relationship with MDS. Therefore, it is beneficial for more accurate modeling of truck sap flow and orchard water management under border irrigation by analyzing major factors affecting sap flow during two stages of apple tree. © 2011 Elsevier B.V. All rights reserved.

Spatial sap flow and xylem anatomical characteristics in olive trees under different irrigation regimes

The compensation heat pulse (CHP) method is widely used to estimate sap flow and transpiration in conducting organs of woody plants. Previous studies have reported a natural azimuthal variability in sap flow, which could have practical implications in locating the CHP probes and integrating their output. Sap flow of several olive trees (Olea europaea L. cv. 'Arbequina') previously grown under different irrigation treatments were monitored by the CHP method, and their xylem anatomical characteristics were analyzed from wood samples taken at the same location in which the probes were installed. A significant azimuthal variability in the sap flow was found in a well-irrigated olive tree monitored by eight CHP probes. The azimuthal variability was well related to crown architecture, but poorly to azimuthal differences in the xylem anatomical characteristics. Well-irrigated and deficit-irrigated olive trees showed similar xylem anatomical characteristics, but they differed in xylem growth and in the ratio of nocturnal-to-diurnal sap flow (N/D index). The results of this work indicate that transpiration cannot be accurately estimated by the CHP method in olive trees if a small number of sensors are employed and that the N/D index could be used as a sensitive water status indicator.

Estimation of water use of mature coconut (Cocos nucifera L.) cultivars (CRIC 60 and CRIC 65) grown in the low country intermediate zone using the compensation heat pulse method (CHPM)

The compensation heat pulse method (CHPM) was used to determine the diurnal variation and daily total water use of two coconut varieties (i.e. CRIC 60 and CRIC 65) growing on two contrasting land suitability classes (i.e. S2, Kurunegala series, and S4, Kuliyapitiya series) in the low- country intermediate zone (IL1) of Sri Lanka. The average daily total sap sow (F d ) of CRIC 60 and CRIC 65 on S2 were 120 and 90 L d -1 while on S4 it was 92 and 79 L d -1 respectively.

Evaluation of sap flow methods to determine water use by cultivated palms

Sustainable management of water resources allocated to palm crops requires reliable tools for measuring palm water use. Thermometric sap flow methods developed for woody dicot plants hold great potential for use in palms, but there have been few investigations to determine whether such methods can be used successfully in ‘woody’ monocots. Here, we evaluate two sap flow methods for measuring whole-plant water use by potted cocos palms ( Syagrus romanzoffiana): the Heat Ratio Method (HRM) and the Compensation Heat Pulse Method (CHPM). Measurements of whole-plant water use from the HRM and the CHPM were compared to gravimetric measurements acquired from an electronic balance. Of the two methods, the HRM gave the most accurate results and it most precisely described patterns of transpiration with respect to environmental conditions and leaf morphology. Estimates of total daily water use from the HRM and the balance were highly correlated ( R 2 = 0.92; P < 0.0001) and very near to a 1:1 relationship—an excellent result given the potential for error associated with each method. As expected, the CHPM was seriously limited at low flow rates, but it agreed well with the HRM at higher flow rates (heat pulse velocity >4 cm h −1). Anatomical investigations revealed that vascular bundles in measured palm fronds were evenly distributed and the distance between bundles was comparable in scale to the diameter of probes of HRM sensors, and most likely an order of magnitude smaller than the probable zone of thermal influence for HRM measurements. This contention was supported by results of mathematical modeling suggesting that HRM heat pulse velocities in palm-like sapwood remain largely unaffected by increases in heterogeneity caused by larger xylem vessels and wider interstitial tissues. Although wounding models for heat pulse sap flow methods have not historically catered for the effects of discrete vascular bundles, they are sufficiently approximate given our empirical validations, and we conclude that palm sapwood is thermally homogenous enough for the HRM and the CHPM to be used without modification. Overall, we present strong evidence that heat pulse methods such as the HRM and the CHPM can be used successfully in woody monocots. Finally, we note that the CHPM is likely to perform better in cases of very high flow rates, while the HRM provides more comprehensive monitoring of the usual range of flow rates including at night, early morning and during water deficit.

New approach for measuring low sap velocities in trees

The compensation heat-pulse method (CHPM) for measuring sap velocity in tree stems shows a poor performance for low velocities. Here we present a new procedure, that extends the validity of the technique down to zero and even for small negative values of the corrected heat-pulse velocity v c . A simulation model of heat transport for the sensor inserted in the tree showed that v c is linearly related to the average temperature difference (Δ T a ) between the 2 needles from negative v c up to 30 cm h −1. The relation was confirmed for olives and pistachio trees in the field with no apparent variation for very long periods (months). The procedure, named “calibrated average gradient” (CAG), consists in averaging the Δ T a measurements after the pulse, then obtaining empirically the function v c = f ( Δ T a ) in its linear domain, when v c is still measurable with the traditional CHPM. The calibrated function can then be used to calculate v c below the minimum velocity measurable by the CHPM as determined by the measuring system resolution. The v c = f ( Δ T a ) function is sensor-specific and depends only on sensor characteristics and thermal properties; its empirical fit allows not only estimating v c for low transpiration conditions, but may also serve to check possible variations in sensor performance or changes in thermal properties of xylem tissue caused by wounding, advocating the need to re-install sensors in a new position.

Water use and response of a dry-farmed olive orchard recently converted to irrigation

Experimental results obtained in Southern Portugal from a dry-farmed mature olive tree orchard recently converted to drip irrigation are described. Water use and response to two irrigation management practices by olive trees was monitored with sap flow compensation heat pulse sensors, ‘Watermark’ granular matrix block sensors and a capacitance probe. The 80-plus-year-old mature olive tree orchard planted on a 12 m by 12 m spacing layout was converted in 2005 from dry-farming to drip irrigation and subjected to two water treatments: trees irrigated daily to supply for crop water demand and trees irrigated before-flowering, during pit-hardening and before crop-harvesting. Sap flow sensors were implanted in sample trees at three different positions around the trunk and measurements were taken at 30 min intervals during 4 months, from April to mid-August of 2005. Tree transpiration rates were estimated as average of sap flow rates. When trees were fully irrigated, the observed differences in daily sap flow rate amplitude were explained by the natural trees difference in canopy cover, plant height and conductance of water vapour sites. However, when deficit irrigation was prescribed and, when the trees stopped being irrigated, they gradually lost their ability to adequately respond to the evaporative demands of the day, showing smaller variations in amplitudes sap flow. After irrigation ceased in May 15, transpiration rate gradually decreased from its maximum of 7 l h−1, when trees were fully irrigated and soil water content was near to field capacity, to values of less than 3 l h−1 by July 3 as the soil water content gradually acted as the transpiration limiting factor. Transpiration rates recovered after irrigation was re-introduced on July 4. Although low in the non-irrigation period, transpiration rates never dropped to zero and stayed between 37 and 50 l d−1 from May 27 to June 9, as trees were able to extract soil water in the absence of irrigation. Olive trees maintained transpiration to levels as high as 50 l d−1 suggesting that long after irrigation is suppressed, a considerable amount of water held in the soil is made available to the trees. Differences in evapotranspiration and transpiration rates during the same period also indicated that olive trees, making use of the extensive root system developed in the 12 m by 12 m tree spacing, were able to extract soil water and maintain transpiration levels as high as 50 l d−1, while soil water balance indicated tree evapotranspiration rates close to zero. This particular ability of dry-farmed olive trees to remove water held in the soil under adverse conditions of very low soil moisture and uncertainties associated with the real volume of soil effectively explored by the root system, make profile probe sensors, regardless of their accuracy, unsuitable for control of water uptake and management of dry-farmed olive orchards recently converted to irrigation. Likewise, watermark sensors, able to capture the variations of high soil water, failed to provide meaningfully values below 0.28 cm3 cm−1. Both sensors should be used with care in managing irrigation of olive tree orchards and, rather than relying on soil moisture status, monitoring tree water-use and response with the compensation heat-pulse method seems a more appropriate approach.

Calibration of sap flow estimated by the compensation heat pulse method in olive, plum and orange trees: relationships with xylem anatomy

The compensation heat pulse method is widely used to estimate sap flow in conducting organs of woody plants. Being an invasive technique, calibration is crucial to derive correction factors for accurately estimating the sap flow value from the measured heat pulse velocity. We compared the results of excision and perfusion calibration experiments made with mature olive (Olea europaea L. 'Manzanilla de Sevilla'), plum (Prunus domestica L. 'Songal') and orange (Citrus sinensis (L.) Osbeck. 'Cadenero') trees. The calibration experiments were designed according to current knowledge on the application of the technique and the analysis of measured heat pulse velocities. Data on xylem characteristics were obtained from the experimental trees and related to the results of the calibration experiments. The most accurate sap flow values were obtained by assuming a wound width of 2.0 mm for olive and 2.4 mm for plum and orange. Although the three possible methods of integrating the sap velocity profiles produced similar results for all three species, the best results were obtained by calculating sap flow as the weighted sum of the product of sap velocity and the associated sapwood area across the four sensors of the heat-pulse-velocity probes. Anatomical observations showed that the xylem of the studied species can be considered thermally homogeneous. Vessel lumen diameter in orange trees was about twice that in the olive and plum, but vessel density was less than half. Total vessel lumen area per transverse section of xylem tissue was greater in plum than in the other species. These and other anatomical and hydraulic differences may account for the different calibration results obtained for each species.

A validation, comparison and error analysis of two heat-pulse methods for measuring sap flow in Eucalyptus marginata saplings

We validated and compared two heat-pulse methods for measuring sap flow in potted Eucalyptus marginata Donn ex. Smith (jarrah) saplings. During daylight hours and under well-watered conditions, rates of sap flow (0.1-0.5 kg h-1) measured by the established compensation heat-pulse method (CHPM) and the newly developed heat-ratio method (HRM) were similar to rates measured with a weighing lysimeter, and most of the time there was no significant difference (P<0.001) between methods. The HRM accurately described sap flow at night when rates of flow were low (< 0.1 kg h-1) or near zero, but the CHPM was unable to measure low rates of sap flow due to its inability to distinguish heat-pulse velocities below a threshold velocity of 0.1 kg h-1 (3-4 cm h-1). The greatest potential for error in the calculation of daily sap flow was associated with the misalignment of temperature sensors, the estimation of sapwood area and the method used to acquire total sap flow from point measurements of sap velocity. A direct comparison of the two heat-pulse methods (applied synchronously) revealed that the HRM had a more convincing mechanism for correcting spacing errors and was more resistant to random fluctuation in measurements than the CHPM. While we view the HRM more favourably than the CHPM in some key areas, both methods are valid and useful, within their constraints, for measuring transpiration in jarrah and other woody species.

Heat-pulse measurements of sap flow in olives for automating irrigation: tests, root flow and diagnostics of water stress

The compensation heat-pulse method for measuring sap flow is tested here in olive trees ( Olea europaea L.). We describe a rigorous three-way examination of the robustness of the technique for this species, and examine the potential of the technique for an automatic control of the irrigation system. Two tests were carried out using heat-pulse gear inserted into the stem of 12-year-old ‘Manzanilla’ olive trees. One test used forced-flow through a stem section, and the other involved measured water uptake by an excised tree. The measured sap flow in these two tests was in agreement with calculations from heat-pulse velocities when using a standard ‘wound correction’ to account for the presence of the probes and the disruption to the sap flow. Thus, this technique for monitoring transpiration can, we feel, be used with confidence in olives. The third experiment was carried out in the field, where we analysed sap flow data from two 29-year-old olive trees — one tree was under regular drip irrigation and the other was from dry-farming conditions. We use measurements of sap flow in the trunk to examine the hydraulic functioning of the tree, and to explore some diagnostics of water stress. Our heat-pulse measurements in the irrigated olive tree exhibited a profile of sap flow that was weighted towards the outer xylem of the tree trunk while the water-stressed trees in the field showed a profile of sap flow weighted towards the centre of the trunk. The loss of hydraulic functioning in the outermost section of the vascular system, as a result of water stress, we consider to be due both to stomatal control and to embolisms in the xylem vessels. The fourth experiment was also carried out in the field, in which sap flow measurements were made at three locations in the trunk as well as in two roots of another 29-year-old olive tree. The soil explored by each root, on opposite sides of the trunk, was differentially wetted by separate irrigation of each side. Our data showed that the surface roots were able to absorb water immediately after wetting, despite a reasonably prolonged period of moderate drought. Root activity quickly shifted to the regions where the soil had been wetted. A root in dry soil exhibited no flow at night, whereas sap flows of about 0.02 l h −1 were measured around midnight in the root drawing water from the wetter soil. Our observations suggest that the hydraulic behaviour of the trunk and surface roots might be used as a diagnostic of the onset, or severity, of water stress. Here there is not the imperative to replicate, for the prime goal is not transpiration estimation. Rather interpretation of the diurnal dynamics is used to infer the onset, or severity of water stress. The compensation heat-pulse seems a suitable technique for automatically controlling the irrigation system of olives, and probably other trees, based either on the estimation of the short-time dynamics of transpiration, or on changes in the hydraulic behaviour of the trees.

An improved heat pulse method to measure low and reverse rates of sap flow in woody plants.

The compensation heat pulse method (CHPM) is of limited value for measuring low rates of sap flow in woody plants. Recent application of the CHPM to woody roots has further illustrated some of the constraints of this technique. Here we present an improved heat pulse method, termed the heat ratio method (HRM), to measure low and reverse rates of sap flow in woody plants. The HRM has several important advantages over the CHPM, including improved measurement range and resolution, protocols to correct for physical and thermal errors in sensor deployment, and a simple linear function to describe wound effects. We describe the theory and methodological protocols of the HRM, provide wound correction coefficients, and validate the reliability and accuracy of the technique against gravimetric measurements of transpiration.