Use of infrared thermometry for estimation of stomatal conductance as a possible aid to irrigation scheduling

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1 Agricultural and Forest Meteorology 95 (1999) 139±149 Use of infrared thermometry for estimation of stomatal conductance as a possible aid to irrigation scheduling Hamlyn G. Jones * Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland, UK Received 14 May 1998; received in revised form 10 March 1999; accepted 16 March 1999 Abstract This paper describes some new approaches for enhancing infrared thermometry (IRT) as a technique for detecting stomatal closure as a measure of plant water stress in humid environments. Although infrared thermometry has been widely used in arid climates for detecting plant stress (as indicated by stomatal closure) and for irrigation scheduling it has been found to be less reliable in more cooler humid climates. The use of wet and dry reference surfaces to reduce the method's sensitivity to environmental variation is described and indexes based on IRT measurements of the temperatures of individual leaves and of reference surfaces in the same environment are evaluated. Both an index that corresponds to Idso's original crop water stress index, but based on `wet' and `dry' reference leaves, and an index that is linearly related to leaf conductance were derived and shown to be closely related to measured leaf conductance in runner bean crops under a range of conditions. Various types of reference surface were evaluated and the use of non-transpiring and wet real leaves was found to be particularly convenient. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Crop water stress index; Infrared thermometry; Irrigation scheduling; Leaf temperature; Plant water stress; Stomatal conductance 1. Introduction Lack of water is frequently the major factor limiting crop production and there is an increasing premium on making the best use of available water for irrigation. Irrigation scheduling is most usually based on soil water balance approaches (see e.g. Jones, 1990), though there is increasing interest in the use of methods that depend on plant responses to water de cit. The most sensitive plant responses to soil water de cit, and hence those with the greatest potential value for irrigation scheduling, tend to be actual *Tel.: ; fax: ; h.g.jones@dundee.ac.uk growth rate and stomatal closure; these are generally much more sensitive to soil water status than is the leaf water potential (y l ) itself (e.g. Bates and Hall, 1981; Jones, 1990; Davies and Zhang, 1991). Although it is feasible to measure stomatal conductance directly by means of leaf porometers, the problems of calibration and of adequately sampling the population of leaves in a eld crop has restricted the use of porometers for practical irrigation scheduling. An important consequence of the stomatal closure that occurs when plants are subject to water stress is that energy dissipation is decreased so leaf temperature tends to rise; the idea of using leaf or canopy temperature as an indicator of plant water stress is not a new one (e.g. Tanner, 1963), but it was popularised /99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S (99)

2 140 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139±149 by Idso and colleagues (Idso et al., 1981; Jackson et al., 1981) in the early 1980s. On the basis of results obtained in the arid climate of Arizona they proposed a `crop water-stress index' (here given the symbol I CWSI ) based on the difference between canopy temperature, as measured using infrared thermometry (IRT), and that of a `non-water stressed baseline' representing the typical canopy temperature of a well watered crop. A range of empirical studies (e.g. Idso, 1982) have shown that there may be different non-water-stressed baselines for different crops and that ideally these need to be determined for each agroclimatic zone in which the crop is being grown. Although the theoretical basis of the approach is well established (see Jackson, 1982; Jones, 1992) and many studies have con rmed that in arid and semi-arid environments infra-red thermometry can provide a useful indicator of crop water stress and of yield loss, and even of leaf water potential (e.g. Hat eld, 1983; O'Toole et al., 1984), it does have some severe limitations in humid climates and in environments with signi cant climatic variability (Hipps et al., 1985). In particular, for any given stomatal conductance, the leaf-to-air temperature difference depends not only on the atmospheric water vapour pressure de cit, which is fully accounted for in the calculation of I CWSI, but it also depends on windspeed, on canopy surface roughness, and on net radiation. In humid climates these errors can lead to variation in the non-water stressed baseline becoming of the same order as the range of canopy temperatures over the full range of stomatal conductances. A further problem is that the cloudless conditions that are required for application of the original approach may not occur often enough in maritime climates such as in the United Kingdom to allow the regular measurements of I CWSI that are required for effective irrigation scheduling. Another dif culty that has commonly been found with the application of infrared thermometry to assess crop water stress has been the dif culty of separating measurements of leaf temperature from soil temperature, which is often many degrees higher than leaf temperature. One approach is to combine thermometry with the use of a spectral vegetation index to correct for the amount of soil in the eld of view (Moran et al., 1994), alternatively, as in the present study, one can avoid the problem by studying single leaves using a narrow acceptance angle IRT sensor. A number of approaches have been suggested for improving the sensitivity of infrared thermometry as a measure of crop water stress in humid regions. Fuchs and Tanner (1966) and Berliner et al. (1984) used well watered plots as a reference rather than the empirical non-water-stressed baseline, as a means of improving precision in humid environments. Unfortunately wellwatered plots are not usually available, so De Lorenzi et al. (1993), for example, have proposed modelling the behaviour of the well-watered crop by simulating the variation in canopy resistance as a function of weather conditions. Other approaches have involved extension of the I CWSI concept to include other environmental variables. For example, indexes that include net radiation as well as water vapour pressure de cit have been proposed (Jackson et al., 1981; Keener and Kirchner, 1983). Yet others have suggested a very different approach to detection of stress by infra-red thermometry based on variability in canopy temperature. For example, Aston and Van Bavel (1972) and Fuchs (1990) have pointed out that as a crop becomes water stressed canopy temperature becomes increasingly variable. The objective of the present study was to investigate the potential for improving the sensitivity of infrared thermometry for calculation of stress indexes, or even for the direct estimation of stomatal conductance, in temperate climates by using in-canopy measurements of similarly exposed wet and dry reference surfaces as a basis for comparison with measured leaf temperatures. 2. Materials and methods Theory The value of the I CWSI is de ned as (Idso et al., 1981; Jackson et al., 1981) I CWSI ˆ T s T base (1) T max T base where T s is the actual canopy surface temperature under given environmental conditions, T max is the upper bound for canopy temperature and equates to the temperature of a non-transpiring canopy such as

3 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139± would occur if the stomata were completely closed as a result of drought, while T base is the `non-waterstressed baseline' representing the `typical' canopy temperature when the stomata are fully open. If instead of referring temperatures to those of a well watered crop one uses a wet surface, it becomes straightforward to rearrange the basic energy balance equation to estimate the canopy resistance to water vapour transfer directly. The theoretical values for leaf or reference surface temperatures can be calculated using a standard rearrangement of the leaf energy balance (Jones, 1992; Eq. 9.6): r HR r aw r lw R ni T l T a ˆ c p r aw r lw sr HR Š r HR e (2) r aw r lw sr HR where T l T a is the leaf-to-air temperature difference, r lw is the leaf resistance to water vapour transfer (assumed to be largely determined by the stomatal resistance), r aw is the boundary layer resistance to water vapour, R ni is the net isothermal radiation (the net radiation that would be received by an equivalent surface at air temperature), e air water vapour pressure de cit, r HR is the parallel resistance to heat and radiative transfer, is the psychrometric constant, is the density of air, c p is the speci c heat capacity of air and s is the slope of the curve relating saturation vapour pressure to temperature. For the following analyses the leaf boundary layer resistance to heat transfer (r ah ) was estimated using the following equation (see Fig. 3.6 in Jones, 1992) s d r ah ˆ 100 (3) u where resistances are expressed in s m 1, d is the leaf characteristic dimension (m) and u is windspeed (m s 1 ). For a dry surface having the same radiative and aerodynamic properties the sensible heat loss will equal the net radiation absorbed, so that using the concept of net isothermal radiation (see Jones, 1992) one can estimate T dry ( ˆ T max in Idso's formulation) from T dry T a ˆ rhrr ni c p (4) The temperature of the corresponding wet surface (T wet ) can be calculated from Eq. (2) by setting r lw equal to zero which gives r HR r aw R ni T wet T a ˆ c p r aw sr HR Š r HR e r aw sr HR (5) Subtracting Eq. (5) from Eq. (4) and dividing by the difference between Eqs. (2) and (4), reduces to T dry T wet T dry T l ˆ rlw r aw s= r HR (6) r aw s= r HR which can be rearranged to give a simple expression for the leaf resistance r lw ˆ r aw s= r HR T l T wet (7) T dry T l An important feature of Eq. (7) is that r lw can be determined solely from a combination of measurements of the temperatures of the leaf and of equivalent wet and dry surfaces and a term that depends only on the resistance to heat and water loss through the leaf boundary layer. It is notable that the multiplier in this equation is independent of net radiation absorbed or of air vapour pressure de cit and only weakly dependent on temperature. Because the term (T dry T l ) in this equation tends to be quite small and is the difference between two quantities that are each quite variable in the eld, the behaviour of this equation can be rather unstable, so it is generally preferable to use its reciprocal, the leaf conductance (g lw ) T dry T l g lw ˆ (8) f T l T wet r aw s= r HR g In much of what follows we will use conductances rather than resistances because transpiration rates are more closely related to conductances. For compatibility with much of the micrometeorological literature, results are presented using mass units for stomatal conductances (mm s 1 ), though approximate conversion to the molar units (mol m 2 s 1 ) used in most physiological literature can readily be achieved by multiplication by 0.04 (noting that this factor is slightly temperature sensitive: Jones, 1992). In addition to the basic crop water stress index (I CWSI ) de ned in Eq. (1), it follows that one can de ne further indexes using the above equations.

4 142 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139±149 One possibility is to use an index of analogous form to I CWSI but substituting T dry for T max and T wet for T base, giving I 2 ˆ T l T wet (9) T dry T wet An alternative is to de ne an index (I 3 ) which is proportional to stomatal resistance. From Eq. (7), therefore, one can write I 3 ˆ T l T wet (10) T dry T l Finally, from Eq. (8) one can de ne an index (I 4 ) which is proportional to stomatal conductance (and hence decreases as stomata close) as I 4 ˆ T dry T l (11) T l T wet 2.2. Field measurements In order to test the principles underlying the use of infrared thermometry for estimation of stomatal conductances, a number of studies were conducted in the eld. The studies reported here were conducted on crops of runner beans grown on a slightly sandy loam overlying calcareous gravel at Horticulture Research International, Wellesbourne, Warwick UK ( W, N, 45 m a.m.s.l.) during the summers of 1995 and In each year there were three replicates of runner bean with three levels of irrigation (A, B and C) in a fully guarded randomised block arrangement (40 experimental plants per plot). Soil moisture was monitored using one neutron probe per plot in 1995 and by theta-probes (Delta-T Devices, Burwell, Cambs, UK) in Plants were sown in groups of four and grown up 2.5 m canes arranged in wigwams with four canes per wigwam. The wigwams were arranged in double rows 1.2 m apart and 2.6 m between wigwams in each row, an arrangement which approximates normal commercial practice in the region. The ground was slightly sculpted into broad ridges before planting to ensure effective runoff of rainfall from experimental plots using clear polyethylene sheeting laid on the soil; this largely excluded rainfall. All plots were provided with seep hose along the rows for irrigation. Well irrigated treatments were irrigated two to three times per week (aiming to maintain soil moisture close to eld capacity), mild stress treatments were irrigated at half the rate of the well watered plots and no water was applied to the drought plots. Treatments were switched between plots once or twice each season to ensure that a range of stomatal conductances were available for testing the sensitivity of infrared thermometry. Initial trials with a number of various sized rectangular `model' leaves showed that they responded much more slowly than real leaves to environmental changes. The most satisfactory reference surfaces were found to be real attached leaves. `Wet' and `dry' reference surfaces, respectively, were obtained by either wetting both sides thoroughly using a handheld sprayer containing water plus a small amount of detergent to act as wetter, or by covering both surfaces with a thin coating of petroleum jelly (Vaseline) to prevent transpiration. Leaf temperature measurements were made using a `Scheduler 1 Crop Stress Monitor', kindly provided by Agrichandlers TM (Emma Nash), Hartley Witney, Basingstoke, UK. The quoted angle of view was 2.58 and the spectral range was 8±14 mm nominal. Estimates of Idso's crop water stress index (I CWSI ) were obtained using necessary variables measured by the Scheduler and the inbuilt software (using the calculations for grape as there was no calculation available for runner bean). Reference leaves were also measured with the Scheduler, with a wet and a dry leaf reading being taken before and after each pair of readings on experimental leaves, and the appropriate reference values averaged for calculation of stress indexes. Direct measurements of the temperature of some leaves were also made using 40 gauge copper-constantan thermocouples inserted into minor veins on the underside of leaves and xed using glue from medical sticking plasters supplemented by 3 mm squares of plaster at least 5 mm from the thermocouple junction. Outputs were recorded on a DL2 data logger (Delta-T Devices, Burwell, Cambs, UK) along with meteorological data (air temperature, humidity, solar radiation (Kipp solarimeter) and windspeed as recorded by a Delta-T Devices Meteorological station). The meteorological station was situated adjacent to the trial, no more than 20 m from any individual plot. Stomatal conductances were measured using a steady-state porometer (EGM-1, PP-Systems, Stotfold, Beds,

5 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139± UK) on the same leaves as used for infrared measurement within 2 to 4 min following the infrared measurements (during which time stomatal conductance is unlikely to have changed signi cantly, Barradas and Jones, 1996). 3. Results 3.1. Theoretical sensitivity and effect of comparison with different reference surfaces The sensitivity of leaf temperature to a given change in stomatal (or `surface') conductance sets the limit to the resolution of infrared thermometry. This sensitivity is strongly dependent, not only on the value of the leaf conductance, but also on environmental conditions. The magnitude of the change in leaf temperature for a 20% reduction in leaf conductance from a `typical' value of 5 mm s 1 (200 mmol m 2 s 1 )is shown for a range of values of net radiation, windspeed (and hence boundary layer resistance) and humidity in Fig. 1. These values were calculated using Eq. (2) and assuming T air ˆ 293 K, a windspeed of 0.5 m s 1, a characteristic leaf dimension of 0.05 m, and a relative humidity of 66%, unless otherwise stated. It is clear from this gure that the sensitivity of leaf temperature to changes in conductance increases with net radiation absorbed, with decreasing windspeed (equivalent to increasing leaf size and increasing boundary layer resistance) and with increasing vapour pressure de cit. Although the leaf temperature change for a given change in stomatal conductance is, of course, independent of whether it is measured as (T l T a ) or (T l T dry )or(t l T wet ), it is worth noting that the sensitivity of each of these measures to changes in environment can be very different depending on stomatal conductance and environmental conditions (Jones, 1994; Jones et al., 1997) Use of reference leaves A key feature of natural environments is that leaf temperatures uctuate rapidly as radiation, windspeed and air temperatures vary. The example in Fig. 2 shows typical natural variation in the eld of bean leaf temperature from a moderately stressed plot, with changes of 58 or more occurring within minutes. Also shown in this gure are the corresponding dynamics of similarly exposed wet and dry reference leaves showing that the temperature dynamics of these were similar to those of the experimental leaf with all leaves having time constants of the order of 10 s at the typical windspeeds involved. Although the time constant would be expected to depend slightly on the surface conductance (see Eq. 9.11; Jones, 1992), it appears that for practical purposes the dynamics of these different leaves are similar. Temperature measurements with the Scheduler 1 of bean leaves and of the `wet' and `dry' reference bean leaves were consistently within 0.58C of the temperatures measured using thermocouples with no evidence for any consistent differences between the three types of leaf. The various arti cial models tested all had signi cantly longer time constants (data not shown) than the `wet' or `dry' bean leaves so were abandoned for subsequent studies Performance of stress indexes Assuming for illustrative purposes that the conductance of fully open stomata is 16 mm s 1 (KoÈrner et al., 1979), one can use Eq. (2) to calculate the appropriate value of T base for calculation of the I CWSI and hence, by substituting into Eq. (1), get the dependence of I CWSI on leaf conductance and windspeed. Typical results are shown in Fig. 3(a) which was calculated assuming a net radiation of 200 W m 2, a temperature of 293 K, a leaf characteristic dimension of 0.1 m, a maximum conductance (g lw ) for fully open stomata of 16 mm s 1, and a relative humidity of 66%. The dependence on leaf conductance is clearly non-linear with the degree of curvature increasing with decreasing windspeed. If instead of referring temperatures to that of a surface with open stomata, one refers them to that of a wet surface (g lw ˆ1; i.e. I 2 ), the lines diverge further (Fig. 3(b)), with the degree of curvature again being strongly dependent on windspeed. As pointed out above, I 3 is somewhat unstable so a more convenient index is provided by its reciprocal (Eq. (11)). Fig. 4(a) shows that this index (I 4 ) is linearly related to the leaf conductance (g lw ), while Fig. 4(b) shows that the slope is a function only of temperature and the boundary layer conductance (g ah, which is de ned as the reciprocal of r ah ).

6 144 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139±149 Fig. 1. The calculated dependence on environmental conditions of the sensitivity of the change in leaf temperature as stomatal conductance decreases by 20% from 5 mm s 1. (a) dependence on net radiation absorbed and windspeed (ÐÐÐÐÐÐ ˆ 0.1 m s 1 ; ÐÐÐ ÐÐÐ ˆ 0.3 m s 1 ; ÐРЈ 0.9 m s 1 ; ±±±ˆ 2.7 m s 1 ); (b) dependence on air water vapour pressure deficit (e) and net radiation (ÐÐÐÐÐÐ ˆ 0Wm 2 ; ÐÐÐ ÐÐÐ125 W m 2 ;ÐÐЈ 300 W m 2 ; ±±±ˆ 450 W m 2 ), and (c) dependence on windspeed and net radiation (symbols as for b). Scheduler and porometer data were obtained on the runner bean crops on 16 dates in 1995 and 13 in 1996 to allow testing of the various stress indexes. Some contrasting examples taken from days when a good range of stomatal conductances were available are shown in Fig. 5 to illustrate how the different indexes perform under typical UK conditions. Generally both I 2 and I 4 showed substantially better relationships to measured stomatal conductance than did the I CWSI calculated by the Scheduler. There was greatest disparity between methods on cooler days with less sunshine, with the Scheduler I CWSI being particularly variable on 4 September 1995 when air temperature was between 19 and 218C and solar radiation between 150 and 400 W m 2. Similarly 14 August 1996 was much cooler and cloudier (ca C and 200± 300 W m 2 ) than 19 August (ca. 308C and 400± 800 W m 2 ). On consistently hot and sunny days all methods gave reasonable relationships with measured stomatal conductance (data not shown). On each occasion the best discrimination of stomatal conductance was achieved with I 4, which was linearly related to measured conductance. The discrimination was especially good at the higher conductances (compare Fig. 5(i) and (f)). Note that the indexes often show values in ranges not expected

7 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139± Fig. 2. Typical dynamics over a period of about 40 min of (a) the temperatures of a runner bean leaf (), a `wet' reference bean leaf (Ð Ð Ð), a `dry' reference bean leaf (ÐÐÐÐÐÐ) and of the air (), together with (b) variation in incident short-wave irradiance (W m 2 ). Fig. 3. (a) Calculated dependence of Idso's crop water stress index (I CWSI ) on leaf conductance (g lw ) and wind speed (ÐÐÐÐÐÐ ˆ 0.1 m s 1 ;ÐÐЈ 0.3 m s 1 ;±±±ˆ 0.9 m s 1 ; ˆ2.7 m s 1 ) for a net radiation of 200 W m 2, a temperature of 293 K, a leaf characteristic dimension of 0.1 m, a maximum conductance (g lw ) for fully open stomata of 16 mm s 1, and a relative humidity of 66%, (b) Corresponding variation in I 2 calculated from Eq. (9).

8 146 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139±149 Fig. 4. (a) Dependence of I 4 ( ˆ (T dry T l )/(T l T wet )) on leaf conductance (g lw,mms 1 ) and windspeed (ÐÐÐÐÐÐ ˆ 0.1 m s 1 ; ÐÐЈ 0.3 m s 1 ;±±±ˆ 0.9 m s 1 ; ˆ2.7 m s 1 ), and (b) the dependence of the multiplier in Eq. (8) ( ˆ 1/((r aw (s/ g)r HR )) on the boundary layer conductance to heat transfer (g ah ) and temperature (T; ˆ158C;±±±ˆ 208C; ˆÐÐÐ258C; ÐÐÐ ÐÐÐ ˆ 308C; ÐÐÐ ˆ 358C). Fig. 5. Comparisons of I CWSI (as calculated by the Scheduler using the built in calibration for grape) (a, b, c), I 2 (calculated as (T l T wet )/ (T dry T wet )) (d, e, f), and I 4 (calculated as (T dry T l )/(T l T wet )) (g, h, i) using data for runner beans. Different symbols indicate different irrigation treatments.

9 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139± Fig. 6. (a) Calculated dependence on stomatal conductance of I 3 (Eq. (10)) for the data for 4 September 1995, and (b) Conductances estimated from Eq. (8) for the data of 4 September 1995; the solid line shows the 1 : 1 relationship. The different symbols indicate different irrigation treatments. from the theory. I CWSI and I 2 should be in the range 0± 1, while I 3 and I 4 should not fall below zero. Values outside these ranges indicate that the measured leaf temperature is out of the range de ned by the wet and dry references used. Such variation is only to be expected as stomata close and T l approaches T dry. The extreme sensitivity of I 3 to observations where leaf temperature exceeds the temperature of the dry reference surface are illustrated for the 4 September 1995 data in Fig. 6(a) (compare with other indexes calculated for the same data and shown in Fig. 5(a),(d) and (g), of which all except I CWSI appear more reliable). Although accurate estimates of the boundary layer resistance corresponding to individual infrared readings were not available for the present study, an approximation was obtained by assuming that the wind speed estimates at the meteorological station approximated the values within the plot and that r ah could be calculated from Eq. (3) assuming a leaf breadth of 15 cm. As can be seen from Fig. 6(b), these estimates of stomatal conductance were of the same order as the values measured with the porometer. 4. Discussion Major limitations to the use of infrared thermometry as an indicator of stomatal closure and hence as a basis for irrigation scheduling have included the sensitivity of leaf temperature to environmental conditions (e.g. Walker and Hat eld, 1983) and the small absolute temperature changes that occur as stomata close in the more humid environments and low levels of incident radiation that are common in more temperate or maritime climates. The purpose of the work described here was to provide a method to improve the reliability of infrared-based stress indexes in such conditions by reducing the error associated with estimation of the necessary values of T max and T min. The approach was based on replacing theoretical estimates of T max and T base by measured temperatures of appropriate reference surfaces. Across a wide range of weather conditions at a site in the UK the new indexes were clearly better, or at least equivalent to, the original Crop Water Stress Index (Idso et al., 1981; Jackson et al., 1981; Idso, 1982). The results also show that Eq. (11) can even be used to estimate g lw from IRT measurements, but it requires reliable estimates of the g ah. A number of questions arise, however, when one attempts to optimise such an approach. Perhaps the most important is the choice of reference surface. Preliminary trials established that the typically rapid leaf temperature uctuations in the runner bean canopy at Wellesbourne, ruled out the use of slowly responding physical model reference surfaces, leading to the use of real leaves as wet and dry references. Because of the localised turbulence structure within crop canopies any reference surfaces need to be physically close to the sampled leaves and with similar orientation and exposure. Rather than using automatic

10 148 H.G. Jones / Agricultural and Forest Meteorology 95 (1999) 139±149 recording of reference surface temperatures, it was found convenient to use the IRT to measure temperatures of `wet' and `dry' reference leaves established within the canopy. Most effective minimisation of errors could be achieved by averaging reference temperatures measured immediately before and after the sample. Although the I CWSI or any of the other leaf temperature-based measures described in this paper can provide a useful indication of the need for irrigation, or its timing, they are less well adapted to estimation of the amount of irrigation water that is needed. Estimation of amount of irrigation requires empirical calibration for different systems. The use of infrared thermometry, and the modi cations described in the present paper, are therefore most suited as a readily portable system for spot measurements in crops as a check for monitoring the success of routine soil moisture-based irrigation scheduling schemes. Establishing the representative wet and dry reference leaves in the crop that are needed for the present approach only takes a few minutes at any site. The availability of such a spot check is particularly valuable because calculations of soil water balance can be subject to signi cant cumulative errors (Doorenbos, 1984). It is also possible for infrared thermometry-based approaches to be applied to the automatic control of trickle irrigation or other irrigation systems where small amounts of water are applied frequently. Indeed Wanjara et al. (1992) have described an automated trickle irrigation system based on continuous infrared monitoring of canopy temperature. The present approach of using reference surfaces could potentially improve the reliability of such automated control systems for humid environments, though it would probably be necessary to use physical models as reference surfaces which would need to be monitored using thermocouples. Our preliminary studies indicated that it would be necessary for these reference surfaces to have similar time constants to those of real leaves. 5. Concluding remarks The results presented above support the concept that the sensitivity of infrared thermometry as an indicator of plant stress (acting through changes in stomatal aperture) can be improved for humid environments by the use of arti cial wet and dry reference surfaces and careful choice of the stress index used. The improvement in sensitivity arises from a reduction in errors associated with short-term variability of the values of T base and T max which is not accounted for in the usual formulation of Idso's crop water stress index. Reformulation of the energy balance equation to derive an index proportional to stomatal conductance was found to be particularly stable, and with some further assumptions could be used to estimate stomatal conductance directly from infrared thermometry. Acknowledgements I am grateful to the Ministry of Agriculture, Fisheries and Food, the Horticulture Development Council and Agrichandlers Ltd., for funding some of this work through the Sustainable Agriculture LINK scheme, to Roy Drew and Nick Parsons for help with the experimental work, and to David Aikman and Terry McBurney for useful discussions. References Aston, A.R., Van Bavel, C.H.M., Soil surface water depletion and leaf temperature. Agron. J. 64, 368±373. Bates, L.M., Hall, A.E., Stomatal closure with soil moisture depletion not associated with changes in bulk water status. Oecologia 50, 62±65. Barradas, V.L., Jones, H.G., Responses of CO 2 assimilation to changes in irradiance: laboratory and field data and a model for beans (Phaseolus vulgaris L.). J. Exp. Botany 47, 639±645. Berliner, P., Oosterhuis, D.M., Green, G.C., Evaluation of the infrared thermometer as a crop stress detector. Agric. Forest Meteorol. 31, 219±230. Davies, W.J., Zhang, J., Root signals and the regulation of growth and development of plants in drying soil. Ann. Rev. Plant Physiol. Plant Mol. Biol. 42, 55±76. Doorenbos, J., Guidelines for predicting crop water requirements. FAO Irrigation and Drainage paper 24, FAO, Rome. De Lorenzi, F., Stanghellini, C., Pitacco, A., Water shortage sensing through infrared canopy temperature: timely detection is imperative. Acta Horticulturae 335, 373±380. Fuchs, M., Infrared measurement of canopy temperature and detection of plant water stress. Theor. App. Climatol. 42, 253± 261. Fuchs, M., Tanner, C.B., Infrared thermometry of vegetation. Agron. J. 58, 597±601.

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