Abstracts

Baker, J.M. and Nieber, J.L. 1989, 'An Analysis of the Steady State Heat Balance Method for Measuring Sap Flow in Plants', Agricultural and Forest Meteorology, vol. 48, pp. 93-109.

An axisymmetric finite element model fo heat flow was used to evaluate some of the assumptions inherent in the steady-state heat balance method for measuring sap flow in herbaceous plants. Results indicate that the gauge slightly overestimates conduction up and down the stem when sap flow is nearly zero, causing a corresponding underestimate of the sheath conductance and the radial outward heat flux. As sap flow rates increase, the temperature distribution in the stem and gauges is altered to print that the one dimensional Fourier equations are no longer applicable and the individual heat fluxes in the system are poorly estimated. However, the errors are largely reasonably accurate. The model indicates that stem vascular anatomy affects the accuracy of the method, predicting that, in general, the method should be more accurate with dicots than with monocots.

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Baker, J.M. and van Bavel, C.H.M. 1987, 'Measurement of Mass Flow of Water in the Stems of Herbaceous Plants', Plant, Cell and Environment, vol. 10, pp. 777-782.

Heat balance methods of stem flow measurement offer the opportunity to measure directly the mass flow rate of water in plants. We have tested one such approach; the constant power heat balance method of Sakuratani (1981). The results supported his statement of an approximate accuracy of 10% when measuring the transpiration rate of herbaceous plants. The response to sudden changes in the stem flow increases, to the extent that, at flow rates typical of daytime conditions the system is capable of accurately tracking changes in the stem flow within 5 min or less. We describe a new gauge design that is relatively rugged, simple to use with an appropriate digital datalogger and suitable for field use over prolonged periods of time. It does not injure or penetrate the stem, is amenable to continuous and direct recording of the mass flow rate of water in the stem and requires no calibration. A further refinement, which should improve both the accuracy and the dynamic response of the system, is proposed.

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Dugas, W.A. 1990, 'Comparative Measurement of Stem Flow and Transpiration in Cotton', Theoretical and Applied Climatology, vol. 42, pp. 215-221.

Whole plant transpiration (T) measurements have many applications, but appropriate methods have remained somewhat elusive. A new method using a constant power heat balance gauge, wherein the xylem mass flow tate is calculated from a balance of heat into and out of a stem, has been shown to provide accurate stem flow measurements. To evaluate the applicalbility of this promissing method to field experiments, cotton (Gossypium hirsutum L. "GP3774") stem flow measurements were compared with T measured from a weighing lysimeter. Initially to confirm method accuracy, stem flow values were compared in the glasshouse with T values detirmined by mass measurements of a potted plant. The root mean square error (RMSE) between the daylight losses from both (n=16) was 8.6% of the mean measured T values. In the field, hourly stem flow lysimeter T values were also similar, but there was a large variation in stem flow values amoung the different plants. To account for differences in plant size between the plants with guages and all lysimeter plants, stem flow values were adjusted using a sterm area ratio factor, which adjusted values, on the average for the season by 25%. Before adjustment, daylight stem flow totals were consistenlty greater than lysimeter T values. After adjustment, the means differed by only 9%, and the RMSE was reduced from 129 to 69g plant^-1 d^-1. The coefficient of variation of daylight stem flow totals increased throughout the season. In the glasshouse, method accuracy was comparable (errors<±10%) to what has been previoulsy determinded. In the field, determining method accuracy was confounded by plant-to-plant variablity and, possibly, by errors, unique to the gauge design used in this study, at high flow rates. Thus, this method can provide accurate flow measurements from individual herbaceous plants and is a valuable technique for many applications.

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Granier, A. 1985, 'Une Nouvelle Méthode pour la Mesure du Flux de sève Brute dans le Tronc des Arbres (A new method of sap flow measurement in tree stems)', Annals of Forest Science, vol. 42, no. 2, pp. 193-200.

The method described in this paper is based on a thermal sensor compsed of two probes radially inserted in the sapwood of the trunk. One of those probes is heated at a constant energy and the other considered as a temperature reference. A simple equation enables us to calculate the sapflow as a function of the difference of the temperature between the two elements. A calibration has been made on pieces of trunk of different species. Owing to its sensitivity and its low cost, this system may fit for the quantitative measurement of forests transpiration.

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Granier, A., Biron, P., et al. 1996, 'Comparisons of Xylem Sap Flow and Water Vapour Flux at the Stand Level and Derivation of Canopy Conductance for Scots Pine', Theoretical and Applied Climatology, vol. 53, pp. 115-122.

Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus syvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaighn of the international programme REKLIP. Sap flow was measured every 30min using both radial constant heating (Granier, 1985) and two types of Cermak sap flow meters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8cm; max 17.5cm). Available energy was high during the observation period (5.5 to 6.9mm.day^-1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7mm.day^-1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33mm h^-1, ie, 230W m^-2. Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90min behind vapour flux. After taking into account this time lag in the sap flow data sed, a good agreement was found between both methods; sap flow=0.745* vapour flux, r²=0.86. The difference between the two estimates was due to the understory transpiration. Canopy conductance (g_c) wass calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4m above the canopy. Variations of g_c were well correlated (r²=0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitave expression for g_c=f(R,vpd) wa very similar to that previously found with maratime pine (Pinus pinaster) in the forest of Les Landers, South Western France.

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Granier, A., Huc, R., et al. 1996, 'Transpiration of Natural Rain Forest and its Dependence on Climatic Factors', Agricultural and Forest Meteorology, vol. 78, pp. 19-29.

Sap flow was measured on several species from the tropical rain forest in French Guiana during two successive years over the dry season. On bright days, sap flow densities (i.e. sap flow per unit of sapwood area) exhibited high variations from one species to another. Higher rates (3 to 4kg dm^-2h^-1) were observed on late stage forest species like Dicorynia guianensis, Eperua falcata or E. grandifolia, and lower rates on Vouacapoua americana and Carapa procera (1.0 to 1.5kg dm^-2h^-1). Calculated stand sap flow (F) was closely dependant on air vapour pressure deficit and less correlated to global radiation. A simple model of canopy conductance variations and hence of stand transpiration was derived from these measurements. Sap flow was linearly related to Penman Evapotranspiration (PET), the ratio F/PET being close to 0.75 under dry canopy conditions, as previously reported by Shuttleworth et al. (1984) in Central Amazon.

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Grime, V.L., Morison, J.I.L., et al. 1995, 'Including the Heat Storages Term in Sap Flow Measurements with Stem Heat Balance Method', Agricultural and Forest Meteorology, vol. 74, pp. 1-25.

The importance of the change in stem temperature and therefore the heat storage term in the stem heat balance method of measuring sap flow is estimated. Results from a range of measurements on a model stem, potted sunflower plants in a glasshouse, and Guiera snegalensis shrubsin the Sahel, Niger, are presented. A novel analysis of the heat balance in zero flow conditions allows the accurate determination of the gauge radial conductance and the stem segment heat capacity, both of which are required for accurate sap flow measurement with good dynamic resolution in low flow conditions. In high sap flow conditions the change in heat storage consitutes only a small component of the balance, and can be neglected, especially for small stems. The improved accuracy and dynamic resolution for stems of any size if heat storage is included allowed the measurement of low night-time flows during rehydratioin, and of redistribution of water between stems of G. snegalensis bushes in the field following rain.

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Ham, J.M., Heilman, J.L., et al. 1990, 'Determination of Soil Water Evaporation and Transpiration from Energy Balance and Stem Flow Measurements', Agricultural and Forest Meteorology, vol. 52, pp. 287-301.

Frequent measurements of soil water evaporation (E) and transpiration (T) are needed to quantify energy and water balances of sparse crops. Field experiments were conducted in Lubbock, TX to examine the feasiblity of partioning evapotranspiration (ET) from a cotton crop (Gossypium hirsutum L.) during periods of partial cover. The Bowen ratio energy balance method and heat balance stem flow measurements were used to make near-instantaneous measurements of ET and T, respectively. Transpiration on a unit land area basis was determined by normalizing stem flow measurements by leaf area or plant density. Soil water evaporation was computed as the diffference between ET and T. The accuracy of the method was evaluated by comparing calculated values of E with measured values obtained from soil microlysimeters. Measurements over an 8-day period following an irrigation indicated that daily values of claculated E were within 0.5mm of measured values in six out of seven comparisons when stem flow msasurements were normalized on a leaf area basis. On average, daily calculated E was within =-11% of measured values. Calculated and measured cumulative E agreed to within 0.6mm at the end of the evaluation period. Computing T by normalizing stem flow on a plant density basis resulted in oversetimates of T and underestimates of E. Error analysis indicates that the precision of the E estimate decreases rapidly as evaporation becomes a smaller fraction of ET, and is influenced equally by the resolution of the stem flow and leaf area masurements. This study demonstrates that high frequency, independent measurements of soil and canopy evaporation can be obtained by measurement of ET and stem flow.

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Heilman, J.L., Brittin, C.L., et al. 1989, 'Water Use by Shrubs as Affected by Energy Exchange with Building Walls', Agricultural and Forest Meteorology, vol. 48, pp. 345-357.

Landscape plants in urban areas are routinely grown next to buildings which are sources of sensible heat and radiation. An experimental study was conducted to explore how building walls affect water use by adjacent landscape plants.Instantaneous rates of sap flow were measured using heat-balance, stem-flow, gauges attached to wax leaf ligustrum shrubs growing adjacent to all four walls of a building shell. For comparison, sap flow was also measured on shrubs grown away from the influence of the building. Peak flow in plants adjacent to each wall accurred when direct beam irradiance on the wall and wall tempereature were at their maxima. Peak flow was highest in plants adjacent to east and west walls, and lowest in plants adjacent to the north wall. Longwave radiation emitted by the walls appeared to be a major factor affecting flow while reflected radiation from walls was of secondary importance because of the low albedo of the walls. Cumulative flow was greatest in the shrubs grown away from the influence of the building, probably due to the absence of any shading by walls during the day, and to wind speeds that were higher than those adjacent to the building.

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Heilman, J.L. and Ham, J.M. 1990, 'Measurement of Mass Flow Rate of Sap in Ligustrum japonicum', HortScience, vol. 25, no. 4, pp. 465-467.

The heat balance method of measuring mass flow of sap was tested on wax leaf ligustrum (Ligustrum japonicum Thumb) to evaluate its usefulness for measuring water use in shrubs, Sap flow measurements were compared with gravimetric estimates of transpiration in growth chamber and field environments. Sap flow measurements in both environments were within 10% of transpiration, which compared favourably with results reported for herbaceous plants by other researchers. Sizable differences in sap flow, due mainly to differences in leaf area, were found among, five plants tested in the field. When flow was expressed on a unit leaf-area basis, differences among plants were greatly reduced. Measurements under partly cloudy skies with fluctuating plants ere greatly reduced. Measurements under partly cloudy skies with fluctuating irradiance showed that changes in sap flow matched those occuring in irradiance.

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Köstner, B., Biron, P., et al. 1996, 'Estimates of Water Vapor Flux and Canopy Conductance of Scots Pine at the Tree Level Utilizing Different Xylem Sap Flow Methods', Theoretical and Applied Climatology, vol. 53, pp. 105-113.

During the Hartheim Experiment (HartX) 1992 conducted in the upper Rhine Valley, Germany, three different methods were used to measure sap flow in Scots pine trees via heating of water transported in the xylem; (1) constant heating applied redially in the sap wood ("Granier-system" -G), (2) constant heating of a stem segment ("Cermák-system" -C), and (3) regulated variable heating of a stem segment that locally maintains a constant temperature gradient in the trunk (Cermák / Schulze-system" -CS). While the constant heating methods utilize changes in the induced temperature gradient to quantify sap flux, the CS-system estimates water flow from the variable power requirement to maintain a 2 or 3 degree Kelvin temperature gradient over a short distance between inserted electrodes and referance point. The C- and CS- systems assume that all transported water is ecompassed and equally heated by the electrodes. In this case, flux rate is determined from temperature difference into sap flow density. Estimates of sapwood area are used to calculate the total flux. All three methods assume that the natural fluctuation in temperature of the trunk near the point of insertion of heating and sensing elements is the same as that where reference thermocouples are inserted.

Using all three systems, 24 tree were simutaneously monitored during the HartX campaign. Tree size within the stand ranged between 18 and 61 cm circumference at breast height, while sample trees ranged between 24 and 55 cm circumference. The samllest trees could only be measured by utilizing the G-system. Sap flow rates of individual trees measured at breast height increased rapidly in the morning along with invreases in irradiance and vapor pressure deficit (D), decreased slowly during the course of the afternoon with continued increase in D, and decreased more slowly during the night. Ignoring potential effects introduced by he different methods, maximum flow rates of individual trees ranged between 0.5 and 2.5kg H₂O h^-1 tree^-1 or 0.3 and 0.6mm h^-1 related to projected crown area of trees and daily sums of sap flow for individual trees varied between 4.4 and 24kg H₂O tree^-1 d^-1 or 1.1 and 6.0 mm d^-1. Maximum sap flow rates per sapwood area of trees varied least for the G-system (11 - 17 g cm^-2 h^-1) and was of similar magnitude as the C-(8 - 21 g cm^-2 h^-1) and CS-system (4 - 14 g cm^-2 h^-1). Regressions of total tree conductannce (g_t) derrived from sap flow estimates demonstrated the same linear increase of conductance with increasing irradiance, however decrease of conductance with invreasing D under non-limiting light conditions was different for the three systems with strongest reduction of gt measured with the CS-system followed by the C- and G-system. This led to different estimates of daily sap flow rates especially during the second part of the measurement period. Variation in sap flow rates is explained on the basis of variation in leaf area index of individual trees, heterogeneity in soil conditions, and methodological differences in sap flow measurements. Despite the hightly uniform plantation forest at the scale of hectares, the heterogeneity in tree size and soil depth at the scale of sqare meters still make it difficult to appropriately and efficiently select sample trees and to scale-up water flux from individual trees to the stand level.

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Lascano, R.J., Baumhardt, R.L., et al. 1992, 'Measurement of Water Flow in Young Grapevines using the Stem Heat Balance Method', American Journal of Enology and Viticulture, vol. 43, no. 2, pp. 159-165.

There are numerous published discussions on the water use and water requirements of grapes (Vitis vinifera L.), but data on the actual water use of individual plants in an undisturbed field envrionment are not available. We assessed the appllicablility of the stem heat balance method, using commercially available gauges, to determine the daily and seasonal water use of the three-year-old Chardonnay plants in New Deal, TX. We first evaluated the gauges in the field by sumutaneously measuring the hourly and daily water use of a potted three-year-old Cabernet Sauvignon plant gravimetrically and with a stem flow gauge. Second, we measured, over a period of 100 days between anthesis and harvest, starting on May 17 1990, the daily sap flow of five Chardonnay plants in a vineyard that was irrigated by flooding. Our results show that the stem flow gauges were accurate within 5% to 10% fo the daily value of transpiration as measured gravimetrically. The mean cumulative sap flow for the plants in the field over the entire measurement period was 461±44kg/plant or 124±12mm on an area basis. When the total sap flow for each plant was normalized by its leaf area, the variability among plants was greatly reduced. The mean cumulative evaoptranspiration (ET) over the 100-day period was measured as 528±13mm, implying that soil evaporation was 77% of the ET. We conclude: (1) that the stem heat balance method is capable of accurately measuring the daily water use of grape plants in the field; (2) that for our experimental conditions, the water use by the grape plants was low compared to the total water loss from the field, to the water use of other field crops in the same area, and to the calculated potential ET of 870mm.

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Lu, P., Biron, P., et al. 1995, 'Water Relations of Adult Norway Spruce (Picea abies (L.) Karst) under Soil Drought in the Vosges Mountains: Water Potential, Stomatal Conductance and Transpiration.', Annals of Forest Science, vol. 52, pp. 117-129.

The effects of soil water depletion on sap flow, twig water potential, stomatal and canopy conductance were analysed in 2 plots of a 30-year-old stand of Norway spruce. One was subjected to an imposed drought; the other was watered by irrigation. Predawn water potential in trees from the dry plot decreased to -1.2 MPa. In the watered plot, a low between-tree variability of sap flux density was observed, with maximum values of 1.2-1.9 dm³.dm^-2.h^-1, corresponding to about 0.5mm.h^-1. Tree transpiration and stomatal conductance showed a strong reduction in association with drought development, during which the predawn water potential decreased from -0.4 to -0.6MPa. Canopy conductance was calculated from the reverse of the Penman-Monteith equation assuming that vapour flux over the stand was equal to the estimated stand sap flow. Effects of climatic factors and drought on canopy conductance variations were take into account in a multi-variable transpiration model.

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Ringersma, J., Mechergui, M., et al. 1996, Transpiration Measurements in Date Palms Using the Granier method, American Society of Agricultural Engineers, Proceedings of the International Conference. 3-6 Nov. 1996 pp. 141-145.

This paper describes the use of the Granier method for determination of the sap flow in date palms. Measurements were taken over two periods, one in the summer and one in the autumn. The aim of the research was to determine whether the Granier method is suitable for determining of the transpiration of date palms. Measurements were conducted at places both high and low on the trunk and both deeply and superficially in the xylem. The results so far have produced knowledge of the sap flow pattern, and a reasonable indication of absolute values for water movement in the whole palm.

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Simpson, D.G. 2000, 'Water use of Interior Douglas-fir', Canadian Journal of Forest Research, vol. 30, pp. 534-547.

Water use of individual Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees was measured in two plots at a forest site in southern British Columbia, Canada. Average daily early summer water use by trees with diameters of 7.5-70cm varied from 1.8 to 166 L. Sap flux density (cm² water/cm² sapwood per hour) was linearly related to shoot xylem pressure potential and was found to invrease with increasing vapour pressure deficit (VPD) and short-wave irradiance (I), reaching maximum rates with VPD>0.6kPa and I>200 W.m^-2. Daily sap flux density varied among trees but was not related to tree diameter, so an average value of 113704L.m^-2 sapwood area was used to estimate average early summer stand trarnspiration for the two plots of 1.08 and 1.5mm.d^-1. A close curvilinear relationship (r²=0.85) was found between stem coss-sectional area increment and sapwood area. The relationshop was only slightly better (r²=0.89) between are increment and early summer individual tree water use. Stand volume growth for 1988-1998 for the two plots was 36-47 m³.ha^-1. Stem volume relative growth rate over this 10-year period is estimated at 0.027 and 0.029 m³.m^-3.a^-1.

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Smith, D.M. and Allen, S.J. 1996, 'Measurement of Sap Flow in Plant Stems', Journal of Experimental Botany, vol. 47, no. 305, pp. 1833-1844.

Transpiration rates for the whole plants, individual branches or tillers can be determined by techniques which measure the rate at which sap ascends stems. All of these methods use heat as a tracer for sap movement, but they are fundamentally different in their operating principles. Two methods commonly employed, the stem heat balance and trunk sector heat balance methods, use the heat balance principle; the stem is heated electrically and the heat balance is solved for the amount of heat taken up by the moving sap stream, which is then used to calculate the mass flow of sap in the stem. In the heat-pulse method, rather than using continuous heating, short pulses of heat are applied and the mass flow of sap is determined from the velocity of the heat pulses moving along the stem. In addition, rates of sap flow can be determined empirically, using the thermal dissipation technique, from the temperature of sapwood near a continuously-powered heater implanted in the stem. Users must understand the theory underlying each of these methods, so that they can select the method most appropriate to their application and take precautions against potential sources of error. When attempting to estimate transpiration by stands of vegetation from measurements of sap flow in individual plants, users must also select an appropriate sampling strategy and scaling method.

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Steinberg, S., 1988, Dynamax Trunk-Flow Gauge Test - Technical Application Report 2. Insulation and Time of Attachment Test Houston, TexasDynamax Inc. pp

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Steinberg, S., van Bavel, C.H.M., et al. 1989, 'A Gauge to Measure Mass Flow Rate of Sap in Stems and Trunks of Woody Plants', Journal of the American Society for Horticultural Science, vol. 114, no. 3, pp. 466-472.

A stem flow gauge designed for herbaceous plants was adopted for measuring the absolute mass flow rate of sap in large stems and trunks of woody plants. The method uses a steady-state heat balance methoda in which a constant, known amount of heat is supplied to a stem segment. The axial and radial conductive hat luxes away from the heated segment are measured, as well as the rise in sap temperature. The device can be operated by commonly available dataloggers and does not require calibaration. In a greenhouse experiment with a small tree, the sap mass flow rate, as measured by the gauge, agreed with the measured transpiration rate within 4% when both were interegated over 24-hour periods or longer. Short term comparisons (≤4hr)were less accurate, dure to the changes in water content of the tree above the gauge, which cause a lag between transpiration rate and sap flow rate, The dynamic response of the tree and gauge system to sudden changes in sap flow was @20 min under midday conditions. Other than the insertion of temperature-sensing thermocouples 2mm into the trunk, the gauge components are noninvasive and do not disturb the tree physically or physiologically to a significant extent.

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Steinberg, S.L., van Bavel, C.H.M., et al. 1990, 'Improved Sap Flow Gauge for Woody and Herbaceous Plants', Agronomy Journal, vol. 82, pp. 851-854.

The internal wiring of an existing stem or trunk flow gauge was redesigned to obtain greater accuracy of the gauge itself, eliminate errors due to signal loss in connecting cables, and reduce the number of channels and of the computing required of the datalogger, Tests of the gauge conducted on bald cypress (Taxodium disticham) and Ficus retusa (L.) Nitida trees, in a greenhouse and in an urban backyard, and under well-watered and dry conditions gave daily sap mass flow rates that were within 5% of these obtained by direct weighing.

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Tan, C.S. and Buttery, B.R. 1995, 'Determination of the Water Use of two pairs of Soybean Isolines Differing in Stomatal Frequency using a Heat Balance Stem Flow Gauge', Canadian Journal of Plant Science, vol. 75, pp. 99-103.

Using heat-balance stem flow gauges, we were able to measure directly and continouously the sap flow rates in two pairs of soybean (Glycine max L.) Merr.] isolines differing in stomatal frequency. Plants with high stomatal frequency transpired significantly more water than the low stomatal frequency plants at high soil moisture levels. Under low soil moisture levels, the water use rate decreased greatly for the high stomatal frequency. Higher leaf temperatures associated with the low stomatal frequency plants were likely due to lower transpiration rates which reduced evaporative cooling especially under well-watered conditions.

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van Bavel, M.G. 1992, Stem-flow Gauges for Measurement of Crop Water Use, National Irrigation Convention Proceedings. pp. 59-72.

Over the last five years new advances in the techniques for sap-flow measurements have made the measurement of water use in crops, trees and vines a simple process to monitor. The method has been studied, and research has proven the utility of the constant heat stem-heat balance (SHB) method of detrmining transpiration (T) and measured in the field under realistic conditions, and studies are proving the utility of the method in a variety of applicatioins of Dynagages. Citrus and fruit growing application have recently been explored, and SHB is shown to be useful in determining water stress and in relating water consumed to the amount of hourly or daily evapotranspiration (ETP) demand. After several years of development, the need became apparent for evaluation for more numerous samples, and the author developed a system whereby up to 32 plants can be monitored simutaneously and real time results can be displayed. The Flow32^tm system makes crop and citrus applications for the farmer, horticulturist, irrigation specialist, and resarchers an indispensable addition to the tools needed to keep crop production at a peak, while minimizing water use, preventing drought stress, and monitoring growth patterns relative to environmental changes. Although many more species need to be tested using stem flow technology, many of the principal economic crops of the world have been shown to be effectively monitored using Dynagage. Many new applications are now enabled by having complete monitoring systems available to commercial as well as research stations.

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van Bavel, M.G. 1995, Advances in Microirrigation Control by Sap Flow Monitoring Systems., Proceedings of the Fifth International Microirrigation Congress, ASAE, Orlando, Florida. 2-6 April 1995 pp. 234-238.

Over the last seven years, new advances in the technique for sap-flow measurement have made the measurement of water in use in crops, trees and vines a simpler and more economical process. A new system designed is explained and a new microirrigation controller implementation is proposed in this paper. The method has been studied, and research has proven the utility of the heat balance sensor method of determining transpiration (T) and how it relates to sap flow (F). Many crops have been measured in the field under realistic conditions, and studies are proving the utility of the method in a variety of applications of stem gauge, a sensor and are shown to be useful in determining water stress and in relating water consumed to the amount of hourly or daily evapotranspiration (ET) demand. Two new sap flow monitoring systems have been recently produced which are being adapted for the closed loop, automatic control of water valve timing needed to keep crop production at a peak, while minimizing water use and preventing water stress. although many more species need to be tested using stem-flow technology, many ofther pricipal economic crops of the world have been shown to be effectively monitored using stem gauges.

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Wiebel, J. and Chacko, E.K. 1992, Sap Flow, Weight Loss and Transpiration in Mangosteen (Garcinia mangostana L.) Seedlings: a Preliminary Study, Sap Flow Workshop, Loxton Research Centre, Loxton S.A. 6 May 1992

Water loss from two year old mangosteen seedlings was estimated using Dynagage (sap flow), gravimetric method (weight loss) and LiCor 6200 (potential transpiration). The hourly and accumulated water loss measured during two days by the three systems were in close agreement. The results indicate that the rate of sap flow measured by Dynagage could be considered as a true indicator of canopy transpiration in mangosteen seedlings.

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