HydroUmcal Interactions Between Atmosphere, Soil and Vernation (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 204,1991.

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1 HydroUmcal Interactions Between Atmosphere, Soil and Vernation (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 204,1991. Theoretical and Experimental Analysis of the Relationship Between Crop Canopy Air Temperature and Vapour Pressure Deficit under Temperate Humid Conditions H.E. JENSEN, H. SVENDSEN, S.E. JENSEN & V.O. MOGENSEN The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Section of Soil and Water and Plant Nutrition, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark ABSTRACT Based on the steady state energy balance equation for a crop the theoretical relationship between crop canopy-air temperature and vapour pressure deficit is outlined. For a crop well supplied with water under actual climatical conditions this relationship constitutes a region which can be approximated by a straight line designated as the lower base line. From theoretical and experimental analyses it appears that determination of lower base line for crops grown under temperate humid conditions is made difficult due to the fact that relatively small temperature differences need to be measured within a narrow range of vapour pressure deficits and under fluctuating global radiation and wind speed. INTRODUCTION The use of crop canopy temperature to characterize crop water status and for control of crop water supply is based upon the assumption that transpiration cools the leaves. As water becomes limited transpiration is reduced and consequently the temperature of the leaves increases because of continued absorption of radiation. Ehrler (1973) suggested that leaf-air temperature differences could be used as a guide to irrigation scheduling. By using the steady state energy balance equation for a crop canopy Jackson et al. (1981) and Idso et al. (1981) developed a crop water stress index to be used for irrigation scheduling purposes based on relationships between leaf-air temperature differences and vapour pressure deficit. Originally this approach was developed and verified under warm and dry climatic conditions and only limited work (Keener & Kirchner, 1983; Pennington & Heatherly, 1989) has been conducted using the approach under humid conditions. The present study is concerned with canopy temperature of crops grown in a temperate humid climate. THEORETICAL CONSIDERATIONS Relationships between crop canopy surface temperature, crop water status and agrometeorological variables may be derived from the steady state energy balance equation for a crop, equation (1), in which R n is net radiation, G is soil heat flux, 137

2 H. E. Jensen et al. 138 H is sensible heat flux, and XE is latent heat flux as E is évapotranspiration and X is the heat of vaporization. R n = G + H + IE (1) The flux of sensible heat (H) from a crop can be written as equation (2) in which T c is crop canopy surface temperature, T a is air temperature, r a is aerodynamic resistance to vapour transport, p is air density and C is heat capacity of the air. In a similar way the flux of latent heat (XE) can be written as equation (3) in which e* is saturated vapour pressure at the temperature T c, e a is actual vapour pressure of the air, r c is the crop canopy resistance to vapour transport and y is the psychrometer constant. H = pc p (T c - T a )/r a (2) XE = pc p (e* - e a )/( Y (r a + r c )) (3) By assuming soil heat flux (G) to be negligible equation (1), (2) and (3) can be combined (Jackson et al. 1981) to give eqation (4) which relates the temperature difference (T c - T a ) between the crop canopy surface and the air to vapour pressure deficit (e* - e a ), net radiation (R n ), aerodynamic resistance (r a ) and crop canopy resistance (r c ). It is realized that aerodynamic resistance is closely related to wind speed and that the crop canopy resistance is closely related to the crop water status. In equation (4) A is the slope of the saturated vapour pressure temperature relation calculated as A = (e* - e*)/(t c - T a ), where e* is saturated vapour pressure at air temperature T a. T _ T = c a R n r a -K 1 + r c/ r a) < ~ *a p C p A + y(l + r c /r a ) A + Y(l + r c /r a ) For a crop subject to severe water stress the crop canopy resistance will assume a very high value in relation to the aerodynamic resistances. In this case equation (4) can be approximated by equation (5). In a diagram in which (T c - T ) is plotted versus (e* - e a ) the result is a horizontal line with intercept R n r a /(pc ). This upper limit has been designated as the upper base line (Hatfield, 1983). Va T c - T a = - ^ - (5) For a crop well supplied with water in the root zone the crop resistance is designated r. In this case equation (6) is obtained in which y* = y(l + r /r a ). This lower limit has been designated as lower base line (Hatfield, 1983). R r y* e* e T r-r\ n a ' a a /r\ - I - tfr) p C p A + y* A + y*

3 139 Crop canopy air temperature and vapour pressure deficit Under actual temperate humid climatical conditions a certain range of R n, T, e a, r a and r may be encountered, which have to be taken into account when predicting the lower base line and the accuracy by which the lower base line may be estimated experimentally. Under such conditions equation (5) as well as equation (6) describe a region rather than a single line. In this case upper base line may be approximated as the mean value of the intercept of the upper region whereas the lower base line may be approximated as the best fit line through the lower region (O'Toole & Real, 1984). For a set of environmental conditions a theoretical lower region for a particular crop and the corresponding best fit line, equation (7), through that region can be established by a method which is explicitly described by Svendsen et al. (1990). (T c - T a ) = a + /?(e a - e a ) (7) The values of a and p are estimated to give the line through the center of the region that minimizes the sum of squares. An example of a theoretical lower base line is shown in Fig. 1. lb '. '. ' T a "C , is ,, ; Vapor pressure deficit kpa Vapor pressure deficit KPB FIG. 1 A. Relationship between crop canopy-air temperature difference (T c - T a ) and vapour pressure deficit (e* - e a ). Solid lines represent values of air temperature (T a ) varied at a given actual vapour pressure e a. The broken lines represent values of vapour pressure (e a ) varied at a given actual air temperature (T a ). R n = 600 W m~x r a = 15 s m -1, r cp = 20 s m -1. B. Region of predicted values of crop canopy-air temperature difference (T c - T a ) in relation to vapour pressure deficit (e* - e a ). R n = 600 W nt 2, r a = 15 s m -1, r cp = 20 s m -1, T a : C, e a : 0-3 kpa. Broken line is lower base line estimated as best fit line through the region.

4 H. E. Jensen et al, 140 For each of 27 combinations of R n (500, 600 and 700 W m" 1 ), r a (5, 15 and 25 s m -1 ) and r (20, 40 and 60 s m"" 1 ) the lower base line was calculated for a temperature range of C and a range of vapour pressure of 0-3 kpa. Within the ranges of parameters employed the analysis showed that net radiation has only limited influence on the slope whereas the intercept increases for increasing values of net radiation, that slope and intercept increase and decrease, respectively, for increasing values of the aerodynamic resistance whereas the slope as well as the intercept increase for increasing values of crop resistance. The ranges of intercept and slope were from 1.2 to 8.0 C and from -1.0 to -3.6 C kpa~, respectively. EXPERIMENTAL METHODS The experimental part of the present work was conducted during 1986 and 1987 under Danish climatic conditions using a field lysimeter with a sandy soil as well as with a sandy loam soil. Barley (Hordeum distichum L. cv. Lina) and rape (Brassica napus oleitera L. cv. Topas) grown in 1986, wheat (Triticum aestivum L. cv. Kraka) and perennial rye grass (Lolium perenne L. cv. Borvi) grown in 1987 were used as test crops. For each crop and soil type 4 plots were irrigated to keep soil water deficit less than approximately 20 mm, while in 4 other plots the crops were subjected to water stress at various periods by varying the amount of water applied which was made possible by protecting all plots from precipitation by an automatically moving glass roof. Measured values of air temperature, vapour pressure deficit, wind velocity and global radiation were obtained from the Departmental Climate and Water Balance Station at the immidiate vicinity of the field lysimeter. The crop canopy surface temperature was measured by determining the emission of radiation in the wavelength interval 8 < k < 14 jum using a handheld radiation thermometer (Everest Interscience Model 110) for all plots up to 4 times daily on measuring days during most of the particular growing season. In addition two radiation thermometers (Heiman Model KT 15) were placed over selected plots and the crop canopy surface temperature was measured every minute during measuring periods of various length on selected days. A view angle of 45 was used in all crop canopy surface temperature measurements. In 1986 net radiation was measured over the plots where the radiation thermometers were placed. The soil water content was determined two times weekly by using the neutron method. EXPERIMENTAL RESULTS Examples of instant values of crop canopy temperature for selected periods are shown in Fig. 2 for fully irrigated and stressed crops, respectively, grown in the sandy soil. The fully irrigated crops were assumed to have potential transpiration as the soil water deficits were less than 20 mm, whereas the stressed crop may be expected to have had a transpiration less than potential transpiration as the soil water deficit was in the range of mm.

5 141 Crop canopy air temperature and vapour pressure deficit VAPOR PRESSURE DEFICIT WIND SPEED 1.4- VAPOR PRESSURE DEFICIT 1.2- ^ E I k aoo 1 goo i TEMPERATURE MINUTES IRRIGATED/* 1B- AIR le TEMPERATURE STRESSED MINUTES FIG. 2 Canopy temperature of stressed and fully irrigated rape crop (left) and barley crop (right) and several agrometeorological variables during an hour of measurement. Surface temperatures of both the fully irrigated and the stressed rape crop measured before flowering on 24 June at h are shown in Fig. 2, together with corresponding measured values of global radiation, air temperature, wind speed and vapour pressure deficit. During the hour considered global radiation, air temperature and vapour pressure deficit were relatively constant, the latter at a relatively high level, while wind speed fluctuated as is common under Danish climatic conditions. The surface temperatures of the stressed and fully irrigated crop were 23.5 C and 20.0 C, respectively, both fluctuating with an amplitude of up to 2 C. The difference between crop surface temperature and air temperature (T c - T a ) was approximately 4.5 C and 1.0 C for the stressed and fully irrigated crop, respectively. The fluctuations of the crop surface temperature were inversely related to fluctuations in wind speed in such a way that an increase in wind speed caused a decrease in crop surface temperature. Rapid changes in wind speed may also influence surface temperature as a result of canopy movements. In such cases the measured temperature is underestimated as shaded parts of the canopy are exposed to the instrument. This may be considered as an inevitable error in such surface temperature measurements. Surface temperatures of both the fully irrigated and the stressed barley crop measured before heading on 23 June at h are shown in Fig. 2 together with corresponding measured values of global radiation, air temperature, wind speed and vapour pressure deficit. During most of the hour considered global radiation, air temperature and vapour pressure deficit were constant and the short time variations in surface temperature related to the short time variations in wind speed resulting in a similar variation pattern as found for the rape crop. During a period of a few minutes a passing cloud caused a considerable decrease in global radiation which did not effect air temperature. However, the decrease in global radiation caused a considerable decrease in crop surface temperature within a period of few minutes. During short periods with considerable fluctuations in

6 H. E. Jensen et al. 142 global radiation the system did not conform to steady state conditions and the condition for equation (1) was not fulfilled. Consequently surface temperatures measured during such periods are not used in subsequent analyses. However, it is assumed that a few minutes after the cloud passage the system again conformed to a steady state condition (Wiegand & Swanson, 1973; Pennington & Heatherly, 1989) after which variations in surface temperature mostly were related to variations in wind speed. Surface temperatures of both the fully irrigated and the stressed wheat crop measured in the preheading period on 22 June at h are shown in Fig. 3 together with corresponding measured values of global radiation, air temperature, wind speed and vapour pressure deficit. During the hour considered only small changes occurred in air temperature and vapour pressure deficit while wind speed fluctuated. The preheading period was characterized by relatively cold and wet conditions which applies also to 22 June the day of the present measurements. During the hour of measurements global radiation varied from approximately 300 W m during periods with total cloud cover to approximately 1000 W m -2 during periods with partial cloud cover. During the hour considered surface temperatures of the fully irrigated and stressed wheat crop were almost identical and apparently no significant effects of wind speed or crop movements on surface temperature were observed. The surface temperatures of the fully irrigated and stressed wheat crop were identical during periods with total cloud cover as well as during periods with partial cloud cover; this is considered to be the result of the very small vapour pressure deficit. The difference between crop surface temperature and air temperature (T c - T a ) varied between close to 0 C during periods with total cloud cover to approximately 6 C during periods with partial cloud cover. As the difference between crop surface temperature and air temperature (T c - T a ) was different at various levels of global radiation the values of (T c - T a ) conform to different lower base lines each characterized by the level of global radiation VAPOH PRESSURE DEFICIT WIND SPEED VAPOR PRESSURE DEFICIT 0- t» B00 ' E I TEMPERATURE 0 28 TEMPERATURE MINUTES FIG. 3 Canopy temperature of stressed and fully irrigated wheat crop (left) and rye grass crop (right) and several agrometeorological variables during an hour of measurement.

7 143 Crop canopy air temperature and vapour pressure deficit Surface temperatures of the fully irrigated and stressed rye grass crop, measured on 20 August at h are shown in Fig. 3 together with corresponding measured values of global radiation, air temperature, wind speed and vapour pressure deficit. During the hour air temperature and vapour pressure deficit were almost constant while global radiation and wind speed fluctuated considerably. Under partial cloud cover the global radiation was approximately 750 W m~ 2 whereas during passing cloud cover global radiation decreased to approximately 200 W m~ 2. Variations in the crop surface temperature were closely related to those in global radiation, in contrast the effect of wind speed was insignificant. Between passing clouds the surface temperature of the fully irrigated as well as of the stressed crop was significantly greater than the air temperature while during cloud cover the crop surface temperature decreased and, for the fully irrigated crop, was close to the air temperature. Furthermore between passing clouds the surface temperature difference between the stressed and irrigated crop increased to approximately 5 C whereas during passing cloud cover the difference decreased to approximately 2 C. A diurnal change in crop surface temperature has been demonstrated by Hatfield (1983). In the present study it has been demonstrated, Fig. 2 and Fig. 3, that the crop surface temperature shows substantial short time changes which are closely related to short time changes in global radiation and wind speed and to the level of vapour pressure deficit. Data for the fully irrigated crops have been used in an attempt to estimate lower base lines by applying a linear model, equation (9), in which a and b have been estimated by linear regression. The data used for this purpose were 5 -minute average values of crop canopy temperature, air temperature and vapour pressure, respectively, selected from periods with limited temporal changes in global radiation and wind speed to ensure approximate steady state conditions. (T c - T a ) = a + b(e* - e a ) (8) Regression analysis has been carried out on data for the total growing season as well as on data for the preheading and postheading period, respectively. Furthermore regression analysis has been carried out on data split according to levels of global radiation and wind speed, respectively. As examples the canopy-air temperature differences (T c - T a ) for the fully irrigated rape and barley crop are plotted against vapour pressure deficit (e* - e a ) in Fig. 4 using data split according to level of global radiation and wind speed, respectively. The corresponding linear regression parameters are shown in Table 1. Under more arid climatic conditions Idso etal.(1981) found a unique lower base line for alfalfa grown at various locations in the United States whereas Kirkham et al (1983) found different base lines for alfalfa grown at the same location in two different years described as wet and dry, respectively. Dividing our data according to crop development stage or irradiance or wind speed did not improve the regression coefficient consistently. In a few cases a reasonable value of the regression coefficient was obtained, e.g. barley at the highest level of global radiation. However, it is realized that by splitting the data the number of observations may become too small to justify a regression analysis. At high level of global radiation the effects of fluctuations in other agro-

8 H. E. Jensen et al. 144 WIND SPEED 2-4 m s-1 WIND SPEED 4-6 m s W m-s W m VAPOR PRESSURE DEFICIT. kps VAPOR PRESSURE DEFICIT, kpa FIG. 4 Canopy-air temperature plotted against vapour pressure deficit for a fully irrigated rape crop (left) and barley crop (right) measured at various levels of global radiation and wind speed. meteorological variables such as air temperature, wind speed and vapour pressure, on the crop surface temperature are damped resulting in the smaller variation about the regression line. Nevertheless, in general values of the regression coefficient obtained in this study were much less than typical values obtained under arid climatic conditions (Idso, 1982). Under Danish climatic conditions the maximum vapour pressure deficit measured in the period was approximately 3 kpa (Hansen et al ). In the present study the maximum values of vapour pressure deficit were about 1 TABLE 1 Linear regression parameters calculated for (T c - T a ) versus (e* - e a ) for rape and barley at various levels of global radiation (Sj) and wind speed (/x). N is number of observations, a and b is intercept and slope, respectively, s a and s b is standard deviation of intercept and slope, respectively, while R is regression coefficient. S;(W m -2 ) H(m s *) a s a b s b R 2 N Rape Barley

9 145 Crop canopy air temperature and vapour pressure deficit and 2 kpa in 1987 and 1986, respectively, which is very small compared with vapour pressure deficits up to 7 kpa which were commonly found when determining lower base lines for crops grown under arid conditions (Idso, 1982). The narrow range of vapour pressure deficits makes it difficult to determine lower base lines with a reasonable accuracy. Furthermore global radiation fluctuates considerably during the growing season and likewise wind speed the latter of which makes it difficult to adjust canopy air temperature measured over a range of global radiation to a value for a single reference level of global radiation (Pennington & Heatherly, 1989). Thus under the present humid climatic conditions application of canopy-air temperature to characterize crop water status is complicated by the fact that relatively small temperature differences need to be measured within a narrow range of vapour pressure deficits and under fluctuating global radiation and wind speed. REFERENCES Ehrler, W.L.(1973) Cotton leaf temperature as related to soil water depletion and meteorological factors. Agron. J. 65, Hansen, S., Jensen, S.E. & Aslyng, H.C. (1981) Jordbrugsmeteorologiske observationer. Statistisk analyse og vurdering Hydroteknisk Laboratorium, Den Kgl. Veterinasr- og Landbohojskole, K0benhavn Hatfield, J.L. (1983) The utilization of thermal infrared radiation measurements from grain sorghum crops as a method of assessing their irrigation requirements. Irrig. Sci. 3, Idso, S.B. (1982) Non-water-stressed baselines. A key to measuring and interpreting plant water stress. Agric. Meteorol. 27, Idso, S.B., Reginato, R.J., Reicosky, D.C. & Hatfield, J.L. (1981) Determining soil induced plant water potential depressions in alfalfa by means of infrared thermometry. Agron. J. 73, Jackson, R.D., Idso, S.B., Reginato, R.J. & Pinter, P.J. Jr. (1981) Canopy temperature as a crop water stress indicator. Water Resour. Res. 17, Keener, M.E. & Kirchner, P.L. (1983) The use of canopy temperature as an indicator of drought stress in humid regions. Agric. Meteorol. 28, Kirkham, M.B., Johnson, D.E. Jr., Kanemasu, E.T. & Stone, L.R. (1983) Canopy temperature and growth of differentially irrigated alfalfa. Agric. Meteorol. 29, O'Toole, J.C. & Real, J.C. (1986) Estimation of aerodynamic and crop resistances from canopy temperature. Agron. J. 78, Pennington, D.A. & Heatherly, L. (1989) Effects of changing solar radiation on canopy-air temperature of cotton and soybean. Agric. Forest Meteorol. 46, Svendsen, H., Jensen, H.E., Jensen, S.E. & Mogensen, V.O. (1991) Theoretical analysis of the relationship between crop canopy-air temperature and vapour pressure deficit under temperate humid conditions. Acta Agric. Scand. 41.

10 H. E. Jensen et al. 146 Wiegand, CL. & Swanson, W.A. (1973) Time constants for thermal equilibration of leaf, canopy, and soil surfaces with change in insolation. Agron. J. 65,