A new device for continuous recording of the energy balance of natural surfaces

Agricultural Meteorology, 16 (1976) 71--84

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

A NEW DEVICE FOR CONTINUOUS RECORDING OF THE ENERGY BALANCE OF NATURAL SURFACES

A. PERRIER, B. ITIER, J. M. BERTOLINI and N. KATERJI* Station de Bioclimatologie de L 'I.N.R.A., C.N.R.A., Versailles (France) *Universit~ d 'Alep, Facult~ d 'Agronomie, Alep (Syria)

(Received July 25, 1975; accepted October 16, 1975)

ABSTRACT Perrier, A., Itier, B., Bertolini, J. M. and Katerji, N., 1976. A new device for continuous recording of the energy balance of natural surfaces. Agric. Meteorol., 16:71--84. A new device for continuous computing and recording of the energy balance terms is described with some of its principal advantages and characteristics. Hourly and daily printed mean results are compared with those of a sensible weighing lysimeter. Satisfactory performances were obtained during the two last past years, so it is actually used on any natural surfaces, instead of weighing lysimeter, with its simple package which only needs power. INTRODUCTION A m o n g the m e t h o d s w h i c h are c o m m o n l y used t h r o u g h o u t t h e w o r l d to d e t e r m i n e e v a p o r a t i o n u p till n o w , o n l y t w o s e e m t o give s a t i s f a c t o r y results. T h e s e are b a s e d o n e l a b o r a t e weighing l y s i m e t e r s ( P o p o v , 1 9 5 9 ; Van Bavel a n d Myers, 1 9 6 2 ; M c I l r o y and Angus, 1 9 6 4 ; G r e b e t , 1 9 6 5 ) and on a p p a r a t u s designed to m e a s u r e t h e c o m p o n e n t s o f t h e e n e r g y b a l a n c e ( L e m o n et al., 1 9 6 3 ; T a n n e r , 1960). O t h e r m e t h o d s b a s e d on a e r o d y n a m i c m e t h o d s or e d d y d i f f u s i v i t y d e t e r m i n a t i o n s (Webb, 1 9 6 5 ; D e n m e a d and M c I l r o y , 1 9 7 0 ; Perrier and Seguin, 1 9 7 0 ) h a v e also b e e n used b u t h a v e n o t given g o o d a g r e e m e n t w i t h t h e m o s t reliable technics, i.e. t h o s e using weighing l y s i m e t e r s . O f t e n , w i t h the a e r o d y n a m i c a n d e d d y d i f f u s i v i t y m e t h o d s , f l u c t u a t i o n s in t h e results a p p e a r larger t h a n t h o s e o b t a i n e d b y t h e e n e r g y b a l a n c e and l y s i m e t e r m e t h o d s (Perrier et al., 1 9 7 4 ) . A n o t h e r a p p r o a c h in the m e a s u r e m e n t o f e v a p o t r a n s p i r a t i o n is the e d d y c o r r e l a t i o n m e t h o d ( D y e r a n d Maher, 1965). This m e t h o d is c o m p l i c a t e d a n d needs v e r y small and fast r e s p o n d i n g sensors a n d o t h e r generally e x p e n s i v e e q u i p m e n t . Nevertheless, s o m e i n s t r u m e n t a t i o n has b e e n d e v e l o p e d to give c o n t i n u o u s r e c o r d i n g such as the f l u x a t r o n or e v a p o t r o n (Dyer, 1 9 6 6 ; D y e r et al., 1 9 6 7 ) a n d the a s s i m i t r o n ( I n o u e et al., 1969).

72

However, in natural conditions, the extent of homogeneous area is generally limited and there is often a mosaic of natural obstacles and variations in the surface characteristics, so that advection is almost always present and the fluxes are non-conservative. Under these conditions, results obtained by methods based on gradients corresponding to conservative fluxes could be under- or overestimated. Generally, profile methods, such as the aerodynamic method, need several sensors in the height range 2 m above the canopy and eddy correlation determinations cannot be made at heights much below 1 m over any surface. Whatever the conditions, the weighing lysimeter gives a good local measurement and the energy balance method avoids some of the above difficulties by accepting gradient measurements in the height range 50 cm above a canopy and even down to 30 or 40 cm over soil or water. The first advantage of the energy balance method compared to that of weighing lysimeters is that the measure is more representative for surfaces of large areas in that it gives better spatial integration. Secondly, the perturbations due to instrumentation are considerably less than in the energy balance method. Although this method uses different kinds of sensors {net radiometer, fluxmeter, psychrometer), it can be moved and set up easily compared to stationary weighing lysimeters. Finally, by this method all the measures required to calculate the fluxes are obtained simultaneously.

NOTATION List o f s y m b o l s a

b C Cp

h K L M pr R Rn T

Ta Tw Z

A P 7 (Pc (Ps ¢PL

value stored in m e m o r y A value stored in m e m o r y B water vapour c o n c e n t r a t i o n specific heat o f air height o f the crop turbulent diffusivity c o e f f i c i e n t c o e f f i c i e n t o f latent heat o f evaporation molar mass o f air gradient o f saturated vapour pressure curve at T w universal gas c o n s t a n t net radiation air temperature d e w p o i n t temperature w e t bulb temperature height difference b e t w e e n t w o levels volumetric mass o f air pcpRT/LM, the psychrometric constant ground heat flux sensible heat flux latent heat flux

73 GENERAL PRINCIPLES The basic energy balance equations applicable for a layer 0--z (see the Notation), including vegetation and the air above it, can be expressed by" Rn + (Pc + (Ps(Z) + (PL(Z) = 0

(1)

Each term may be positive or negative; by convention, toward the surface is positive and away negative. Convective turbulent fluxes are expressed by the classical Boussinesq analogy with conduction, i.e.: (PL = L K ( z ) 5C 3z

(2)

~T (Ps = p c p K ( z ) ~ -

(3)

Eq. 1 assumes that: (1) Heat accumulation in the layer 0--z is negligible, even if this layer contains vegetation. Generally, neglecting this term gives a slight overestimation before 14h00 and an underestimation after this time. (2) The effects of photosynthesis and respiration on heat production and utilization are also negligible. (3) There is no advection, i.e.:

(P~(z) (P~(h) (PL(Z) = (PL(h) By following c o m m o n l y used procedure (Slatyer and McIlroy, 1961; Perrier and Seguin, 1970), the sensible and latent heat fluxes can be calculated from eqs. 1--3 assuming there is an equality between the turbulent diffusivities of sensible and latent heat. (ps = - ( R n +(Pc)

' ~" AT P (Tw) +~/ ATw

(PL = - ( R n + (pc + (ps) The relation of the coefficient ~,/[~/+ P' (Tw)] to T and Tw is given in Fig.1. This coefficient varies strongly with Tw in a non-linear manner. From eqs.1, 2, 3, 4, it can be seen that, knowing Rn, (Pc, AT, AT w and Tw, any of the fluxes can be calculated.

(4a) (4b)

74

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Fig.1. True values ( d o u b l e arrows) o f a = 7 / [ 7 + P'(Tw)] a c c o r d i n g to Tw for d i f f e r e n t c o m b i n a t i o n s o f T ( 0 ° - - 4 0 ° C ) and T d ( 0 ° - - 2 0 ° C ) . The curve gives t h e values i n t r o d u c e d into the B E A R N s y s t e m to c o m p u t e a as a f u n c t i o n o f T w : a = 0 . 0 0 0 1 4 Tw 2 - 0 . 0 1 0 7 4 Tw+ 0.60124

P R A C T I C A L USE O F THE E N E R G Y B A L A N C E R E L A T I O N S

A technique was developed by McIlroy (1971) to combine collection of the o u t p u t from sensors with a means of computing the fluxes in a relatively simple instrument. All computing functions are realized by an electric analogy using servo-balanced bridge elements with adjustable voltage supplies and bridge circuits. The m e t h o d presented here is numerical. It combines a conventional data collection system of 8 (with a possibility of 20) channels together with programmable memories functioning like a small c o m p u t e r which drives it. A flow chart (Fig.2) shows the principal operations together with their sequences (and some of main error diagnostics incorporated in the system).

75

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Fig.2. Flow chart describing the computing and control operations.

The classical calculations are based on eqs. 4a and 4b. However, we have i n t r o d u c e d t w o i m p o r t a n t n e w points: firstly, in the calculation, s e c o n d l y , in the m e t h o d o f d e p l o y m e n t of the psychrometers: {a) the c o e f f i c i e n t 9"/(P' + 9') is directly c o m p u t e d by the c o m p u t e r system at each m e a s u r e m e n t (see F i g . l ) ; (b) in order to increase the accuracy, the position of the p s y c h r o m e t e r s is inversed b e t w e e n each measurement. H o u r l y mean values of the four fluxes (W m 2) are printed as are the n u m b e r o f determinations included in the mean (30 if there were n o diagnostics). To give s o m e idea o f the ambient characteristics o f the air, the absolute dry and w e t bulb temperatures are also printed. This also gives a check o n the plausibility o f the calculations.

76 FIELD EQUIPMENT

The equipment is in two basic parts. (a) The BEARN system itself includes recording and computing elements together with control of the operations of the sensors and checks on their output. The BEARN system is placed in three racks, 67 cm high, 52 cm wide and 56 cm deep. The recording system consists of an analogue digital microvoltmeter (14,000 digits). Computing, checking and management functions are included in programmable memories. Presently, the equipment, which is usually situated in a shelter about 100 m from the sensors is operated by mains power or by car batteries (4 x 12V) which gives 10 to 20h of running time. (b) The calibration coefficients of each sensor are introduced into the calculator by manual counters. Net radiometer. The one used was manufactured by SWISSTECO, type $1, and was generally maintained I m above the top of the canopy or soil surface in order to keep conditions constant. Soil heat fluxmeter. With homogeneous soil surfaces such as bare soil or soil under grass, only one fluxmeter is needed, but beneath crop with shade and sunflakes, it is necessary to put several fluxmeters in series. Soil probe. The probe consists of a brass block which contains, together with a thermoresistance giving the reference temperature Ts, the reference junctions. Psychrometers. Dry and wet air temperature gradients are measured just above the top of the canopy (about 10 and 40 to 50 cm) with a double differential psychrometer system which uses thermocouple elements (Seck and Perrier, 1970) (Fig.3). Psychrometers are shielded and alimentated with water from special lateral water tanks by a constant head tank (Mariotte vase). Dry and wet air temperatures are obtained at the mean level by thermocouple junctions between one of the psychrometers and the brass block of the soil probe. At any time the program can be stopped by pressing a button "test c o m m a n d " . This results in a print out of the mean values of fluxes already computed. During this time, the voltage arriving from each of the sensors can be checked manually. The proper functioning of the psychrometers can be checked by letting them rest at the same level and observing if the differences between them are zero. It is advisable to make this check once every one or two weeks. The program can be resetted by pressing an "end of t e s t " button. The system can be left to work during 15 to 30 days without any other checks. The length o f this period depends on the rate at which water is evaporated and hence on the necessity to refill the supply tank, and eventually to change the wicks of wet bulb thermocouples.

77

Fig.3.A. Photograph of the recording and computing equipment. B, C. Photographs of field sensors.

78

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Fig.4. Daily e v o l u t i o n o f energy b a l a n c e t e r m s given by t h e B E A R N a n d b y t h e weighing l y s i m e t e r : A. y o u n g c r o p ; B. fully d e v e l o p e d c r o p ; C. y o u n g c r o p (see c o n d i t i o n s in t e x t ) .

RESULTS AND DISCUSSION During t h e last t w o years, data given by the B E A R N s y s t e m at La Mini~re w e r e c o m p a r e d with t h o s e o b t a i n e d by a sensible w e i g h i n g l y s i m e t e r w h i c h had proved a reliable p e r f o r m a n c e for 9 years (Perrier et al., 1 9 7 4 ) w i t h an a c c u r a c y o f + 0 . 0 3 m m w h i c h c o r r e s p o n d s t o a + 20 W m -2 a c c u r a c y in the m e a s u r e o f h o u r l y latent heat flux. T h e s e m e a s u r e m e n t s were m a d e t h r o u g h o u t t h e g r o w i n g period o f an irrigated corn crop b e t w e e n the beginning o f J u n e t o t h e e n d o f S e p t e m b e r u n d e r all w e a t h e r c o n d i t i o n s .

Hourly comparisons Fig.4 shows the energy balance for a very y o u n g corn canopy (Zea mays L.) and Fig.4B for a fully developed one. There is a good agreement in the results obtained by t h e t w o m e t h o d s and the daily evolution of the energy balance. The good agreement unde r variable conditions allows a study of the diurnal variation o f evapotranspiration and hence, from the work of Perrier et al.

8O (1975), estimates of the mean stomatal resistances can be calculated. The comparison between different crop conditions can also be undertaken as shown in Fig.4C where the curve given by the BEARN was obtained five days after the last rain while that of the lysimeter was obtained at the same time but under irrigated conditions. Fig.5 shows a comparison of the results obtained by the BEARN system with those of the lysimeter. This ~)lot contains two hundred mean hourly values taken at random. The correlation coefficient is 0.88 and the regression slope 0.92. The correlation might be better if the sensitivity (_+ 20 W m 2) of the lysimeter was greater. This is seen by the vertical alinement of the points.

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Fig.5. Comparison between hourly values given by the BEARN and the weighing lysimeter (200 random values among those obtained during 1973 and 1974).

81

Daily comparison

During the year 1973 the system was placed over a fully developed canopy for a period of 36 days. During the year 1974, measures were made over a full growing season between the end of May till the beginning of September. All the daily values obtained by the BEARN for this period were compared to those of the lysimeter (Fig.6).

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The global results are quite satisfactory, the m a x i m u m discrepancy being 1.5 mm. As the slopes of the daily and hourly regression are the same, the sample of 200 points taken for hourly comparisons was sufficient to cover the variance of the data. The reason why the slope is less than 1 is probably due to the fact that maize was surrounded by dry bare soil and hence advection was possible• Table I shows the percentage of values falling in the + 10, 20, 30 and 40% range of the lysimeter for hourly and dally estimates. It shows that even if the correlation coefficients are the same, the daily estimates are more accurate. However, 40% of the daily values lie outside the _+20% accuracy range. This is not only due to inaccuracy of the BEARN system, but also

82 TABLE I Percentage o f values included b e t w e e n the accuracy range o f _+ x % _+10%

-+20%

_+30%

_+40%

22 43

37 61

50 78

60 90

Hourly Daily

to the inaccuracy of the lysimeter and the fact that the representativity of a lysimeter to that of the total crop is generally greater than 10% (Perrier et al., 1974).

Total cumulative comparisons If we compare separately the accumulated data for each year (36 days in 1973 and the whole growing season in 1974, i.e. 99 days) (Fig.7), the 6E

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Fig.7. Comparison between cumulative evapotranspiration given by the BEARN and the weighing lysimeter for a corn crop (Zea reals) during July and August 1973 (A), and May till August 1974 (B).

errors decrease and become less than 10%. The random daily variation seems to give a very good cumulative evapotranspiration and allows to follow the water consumption of a crop during its growing period.

CONCLUSION With this portable equipment satisfactory estimates of all energy balance terms are continuously made. Results are as good as those given by sensible weighing lysimeters and provide an easy way to evaluate the energy balance of an actively growing crop under natural conditions with only one to four checks on performance per month. This instrument can be used to compare two different plots, for example, one set of sensors being placed on the first (irrigated) and the other on the second (non irrigated), with alternative periods of recording at half hourly intervals. Presently, by using only one more measurement (CO 2 gradient at same levels), we can calculate also the CO2 flux above the canopy using the calculated values of (Ps and AT.

84 REFERENCES Denmead, O. T. and McIlroy, I. C., 1970. Measurement of carbon dioxide exchange in the field. In: Z. Sestak, J. Catsky and P. G. Jarvis (Editors), Plant photosynthetic Production -- Manual of methods. Junk, The Hague, pp. 467--516. Dyer, A. J., 1966. The evapotron. An eddy flux instrument for measuring natural evaporation. In: Conference on Instrumentation for Plant Environment Measurements, Session IV. Oxygen, CO2 and Water Vapour. C.S.I.R.O., Div. Meteorol. Phys., Aspendale, p. 39. Dyer, A. J. and Maher, F. J., 1965. Automatic eddy flux measurement with the evapotron. J. Appl. Meteorol., 4(5): 622--625. Dyer, A. J., Hicks, B. B. and King, K. M., 1967. The fluxatron. A revised approach to the measurement of eddy fluxes in the lower atmosphere. J. Appl. Meteorol., 6(2): 408--413. Grebet, Ph., 1965. Evapotranspirom~tre pesable par dynamom~tres ~lectroniques. C. R. Acad. Agric., Ft., 51(15): 1026--1032. Inoue, E., Uchijima, Z., Saito, T., Isobe, S. and Vemura, K., 1969. The "assimitron". A newly devised instrument for measuring CO2 flux in the surface air layer. J. Agric. Meteorol., 25(3): 165--172. Lemon, E. R., Shinn, J. M. and Stoller, J. H., 1963. Experimental determination of the energy balance. In: The Energy Budget at the Earth's Surface, Part I. Prod. Res. Rep., 71, USDA, Washington, D. C., pp. 7--27. McIlroy, I. C., 1971. An instrument for continuous recording of natural evaporation. Agric. Meteorol., 9(1/2): 93--100. McIlroy, I. C. and Angus, D. E., 1964. Grass, water and soil evaporation at Aspendale. Agric. Meteorol., 1(3): 201--224. Perrier, A. and Seguin, B., 1970. M~thodes et techniques de d~termination des coefficients de transfert et des flux dans l'air. In: Techniques d'~tude des Facteurs Physiques de la Biosphere. Inst. Nat. Rech. Agron., Paris, pp. 425--445. Perrier, A., Archer, P. and Blanco de Pablos, A., 1974. Etude de l'~vapotranspiration r~elle et maximale de diverses cultures: dispositif et mesure. Ann. Agron., 25(5): 697--731. Perrier, A., Itier, B., Bertolini, J. M. and Blanco de Pablos, A., 1975. Mesure automatique du bilan d%nergie d'une culture: exemple d'application. Ann. Agron., 26(1): 19--40. Popov, O. V., 1959. Lysimeter and hydraulic soil evaporimeters. In: Colloque de Hannoversch-Mfinden, t.2: Lysim~tres. Sept. 1959. Assoc. Int. Hydrol. Sci., Publ. 49, (Gentbrugge), pp. 26--37. Seck, M. and Perrier, A., 1970. Description d'un psychrom~tre a thermocouples. Son application £ la mesure des gradients d'humidit~. In: Techniques d%tude des Facteurs Physiques de la Biosphere. Inst. Nat. Rech. Agron., Paris, pp. 223--234. Slatyer, R. O. and McIlroy, I. C., 1961. Practical Microclimatology. C.S.I.R.O., (Australia), UNESCO. Tanner, C. B., 1960. Energy balance approach to evapotranspiration from crops. Soil Sci. Soc. Am. Proc., 24: 1--9. Van Bavel, C. H. M. and Myers, L. E., 1962. An automatic weighing lysimeter. Agric. Eng., 13(10): 580--583. Webb, E. K., 1965. In: Agricultural Meteorology, Meteorol. Monogr., (28): 27--58.