Microclimate and transpiration of a greenhouse banana crop

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98 (2007) 66 – 78

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/issn/15375110

Research Paper: SE—Structures and Environment

Microclimate and transpiration of a greenhouse banana crop H. Demratia,, T. Boulardb, H. Fatnassib, A. Bekkaouic, H. Majdoubid, H. Elattirc, L. Bouirdend a

Ecole Polytechnique Prive´e d’Agadir, BP 805, Campus ISIAM, Quartier Dakhla, Agadir, Morocco INRA-URIH, 400, route des Chappes, BP 167, 06903 Sophia Antipolis, France c Institut Agronomique et ve´te´rinaire Hassan II, BP 6202 Rabat Instituts, Morocco d Laboratoire de Thermodynamique et Energe´tique, Universite´ Ibn Zohr, BP, 28/S Agadir, Morocco b

ar t ic l e i n f o

The interactions between a greenhouse banana crop and the surrounding environment were investigated within a large-scale type of greenhouse. This investigation took place

Article history:

through a whole development cycle of a mature banana crop in the region of Rabat,

Received 24 December 2005

Morocco. Two measurement series are presented: the first from 4 to 9 March 1998 is

Accepted 20 March 2007

representative of the Spring and Autumn conditions; the second which extends from 6 to

Available online 13 July 2007

26 July 1999 is representative of the Summer period. Space- and time-dependent air evolution (the evolution of air temperature in time (time course) and space (vertically at trees levels in the greenhouse)) leaf temperature and air humidity were analysed, and evidence was provided for the significant oasis effect generated by the crop in the greenhouse. In addition, leaf area and stomatal resistance measurements are analysed and discussed. With regard to the function of microclimate-dependent parameters, a model of leaf stomatal resistance was additionally defined and developed. The transpiration model of the banana crop under cover was validated with respect to the estimated rates of heat and mass transfer implied from indoor and outdoor climatic measurements. & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The import cost of tropical fruits into Morocco is high and is an important call on foreign currency. Greenhouse banana production can help meet market needs as it allows for an increase above the existing outdoor production together with an extension of the production period. However, it is essential to know and understand the interaction occurring between the crop and its environment in the Canaries’ shelters used in Morocco. At the outset of the 21st century, Mediterranean countries have about 174 000 ha of plastic greenhouses and around

28 000 ha of glasshouses (Jouet, 2003). In Morocco, the greenhouse surface area has increased from 2800 ha in 1988 up to 10 000 ha in 2000. Recently, large-scale Canaries’- type multispan plastic greenhouses rapidly expanded along the Atlantic shore line. It was thus necessary to characterise the production conditions, and mainly the induced microclimate, in order to improve their climatic control, management and profitability. The objective of this study was to develop and test models to enable banana crop transpiration flux to be determined, in real-scale greenhouse conditions. This is the most crucial parameter for both climate- and irrigationcontrol procedures for protected banana crops.

Corresponding author.

E-mail address: [email protected] (H. Demrati). 1537-5110/$ - see front matter & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2007.03.016

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Nomenclature

AS a, b b0 Cd Cp Cu Dsat Dsat,a

2

soil surface of the greenhouse, m constants

roof opening to side opening ratio discharge coefficient air specific heat at constant pressure, J Kg11C1 wind effect coefficient saturation deficit, mbar

saturation deficit relative to the air temperature, mbar Dsat,f saturation deficit relative to the leaf temperature, mbar Dsat,max maximum saturation deficit, mbar

df Fs Gv g Hs h ha ILA Lw Lf lf n RG RH Rnet r2 ra

leaf characteristic dimension, m thermal flux in the soil, W m2

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re ri ST Sh Sb T Ta Tr u v l g d y DT

67

equivalent average resistance, s m1 internal resistance of the leaf, s m1 total opening area of vents, m2 roof opening area, m2 lower opening area, m2 temperature, 1C air temperature, 1C transpiration flow, W m2 wind speed, m s1 air velocity in side the greenhouse, m s1 water latent heat of vapourisation, J kg1. psychometric constant, mbar1C1. slope of the saturation vapour pressure curve, mbar 1C1 wind direction, deg temperature difference, 1C

volume flow rate, m3s1 gravity constant, m s2 specific humidity of air, kg m3 height of the chimney, m heat transfer coefficient, W m2 leaf area index latent heat flux, W m2 greatest length of the leaf, m greatest width of the leaf, m exponent global radiation, W m2 relative air humidity, % net radiation, W m2 correlation coefficient aerodynamic resistance, s m1

Compared to previous works undertaken with regard to banana crop transpiration in open air (Brun, 1961; Hoffman, 1990; Kittas et al., 1998; Robinson & Bower, 1988), the current study focuses on the transpiration and microclimate of a mature banana crop in a large Canaries’ type greenhouse. The relevance of the present work was high because the greenhouse surface studied covered more than 1 ha. The challenge was to implement methods and tools that could both help estimate transpiration rates and be robust enough to be used in production conditions. Two main methods allowing transpiration estimation of a banana crop were investigated: (1) a classical method of transpiration estimation based on the Penman-Monteith approach and particularly on a sub-model of the climatic dependence of the stomatal resistance (Boulard et al., 1991); (2) a more recent method based on the balance of the greenhouse air water vapour content (Boulard & Baille 1995). As no direct measurements using a lysimetric device were available because of the banana trees great height (6 m), the Penman-Monteith model parameters were compared to those mentioned in the literature. To enable the interactions between cultivation and environment to be examined, the study was scheduled to match the main stages of the development of the banana crop. The focus of the study included the development of the major

Subscripts

a c e f i j l m max u sat S

air cover exterior leaf interior counter lower medium maximum upper saturation soil

physiological components which govern crop transpiration: the leaf area and the stomatal resistance.

2.

Theoretical considerations

2.1.

Greenhouse banana crop transpiration modelling

Most of the available studies concerning the banana crop focus on its development (Annadurai & Shanmugavelu, 1978; Hoffman, 1990) and they only rarely deal with the transpiration process (Brun, 1961). However, crop transpiration, mainly in water-stress conditions, was investigated by Robinson and Bower (1988), Hoffman (1990), and Kallarackal et al., (1990). Eckstein et al. (1995) measured crop transpiration and leaf stomatal resistance in laboratory conditions. Thomas and Turner (1998) determined leaf stomatal resistance in the field. In the present study, we first analyse the dependence of stomatal resistance on greenhouse climate and then we suggest a transpiration model considering indoor climate as boundary conditions.

2.1.1.

Stomatal resistance modeling vs. climatic factors

The stomatal resistance rs in s m1 mainly depends not only on radiation intensity at crop level RG in W m2 (Meidner &

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Mansfield, 1968), but also on leaf temperature Tf in 1C (Lange et al., 1971) and air saturation deficit Dsat in mbar (Monteith, 1963). A model combining these three climatic factors allows estimation of the stomatal resistance with regard to the climatic environment (Jarvis, 1976; Avissar et al., 1985; Stanghellini, 1987):     (1) rs Rg ; Tf ; Dsat ¼ rmin f 1 ðRG Þf 2 Tf f 3 ðDsat Þ,

Table 1 – Partial functions parameters aj and bj of the banana leaves stomatal resistance (lower face of the greenhouse banana leaves) versus the greenhouse climatic factors [Eqs. (4), (5a)–(5c)]

where rmin is the minimum internal resistance in s m1, f1, f2 and f3 are mathematical functions which represent the evolution of rs according to the considered climatic variable. For a greenhouse tomato cover, Boulard et al. (1991) argued that the stomatal resistance could be correctly modelled by means of only two climatic factors, one of which must be RG. Therefore, a two factor model was adopted where rs is calculated from either (RG, Tf) or (RG, Dsat): rs ¼ rmin f 1 ðx1 Þf 2 ðx2 Þ,

(2)

where      n f j xj ¼ 1 þ a0 exp a1 xj  a2

(3)

with n ¼ 1 for RG and n ¼ 1 for the two other climatic factors and j is a counter. This model was adopted to determine the stomatal resistance of the lower face of the leaves and then compared to the average resistance measured. The leaf temperature Tf, was considered as the average value for the three leaf levels. To sum up, then, we used the following: rs ¼ rmin f 1 ðRG Þf 2 ðx2 Þ,

(4)

where x2 is either the saturation deficit relative to the air temperature denoted Dsat,a or to the leaf temperature noted Dsat,f, or the leaf temperature Tf.  1 (5a) f 1 ðRG Þ ¼ 1 þ exp ða1 ðRG  b1 ÞÞ , f 2 ðDsat Þ ¼ 1 þ a2 exp ðb2 ðDsat  Dmax ÞÞ,

(5b)

     f 2 Tf ¼ 1 þ a2 exp b2 Tf  Tf ;max .

(5c)

The values chosen for the parameters was Tf,max of 33 1C, Dsat,maxof 15 mbar for Dsat,a and Dsat,max of 20 mbar for Dsat,f. For each function, the parameters aj and bj were statistically identified using multiple linear regression (Marquard, 1963). The values are given in Table 1.

2.1.2.

Greenhouse banana crop transpiration modelling

The modelling of greenhouse banana crop transpiration was realised using the method suggested by Penman (1948), then modified by Monteith (1963). It allows determination of the transpiration rate Tr , knowing the value of the easily measured climatic parameters, such as the net radiation at the top of crop, Rnet , together with the heat flow in soil, Fs , and the air saturation deficit, Dsat. For amphistomatic leaves Tr is given by: Tr ¼

rCp ILA =ra d ðRnet  Fs Þ þ Dsat d þ g d þ g

(6)

with   r gn ¼ g 1 þ i , ra

(7)

Coefficients

r2

a1 ¼ 0.0033 b1 ¼ 516.505

0.896

Global solar radiation RG and air saturation deficit Dsat,a

a2 ¼ 0.17 b2 ¼ 0.34

0.9

Global solar radiation RG and saturation deficit relating to Tf, Dsat,f

a2 ¼ 0.18 b2 ¼ 0.24

0.9

Global solar radiation RG and leaf temperature Tf

a2 ¼ 0.086 b2 ¼ 0.27

0.898

Variable Global solar radiation alone RG incident above the culture

where ri is the internal stomatal resistance of the leaves in s m1, ra the aerodynamic resistance in s m1, g is the psychrometric constant, equal to 0.66 mbar 1C1 and d is the slope of the saturation vapour pressure curve, mbar 1C1, ILA the crop leaf area index

2.1.3. Determination of the equivalent average resistance of the cover Banana leaf presents stomata only on its lower face (hypostomatic leaf) and the value of g* must therefore be modified:   r g ¼ 2g 1 þ i ra

(8)

The calculation of the aerodynamic resistance ra depends on air velocity (forced convection) and on the buoyancy effect due to the temperature difference between the leaves and air (Boulard et al., 1989):

ha ¼ 1:95

  v 0:5 C Tf  Ta 0:25 p þ 5:2 ¼ , d d ra

(9)

where Cp in J kg1 1C1 is the air specific heat at constant pressure; Ta is the air temperature in 1C; df in m is the leaf characteristic dimension considered equal to (Lf+lf)/2, Lf and lf in m are the leaf length and width, respectively, and v is the air velocity inside the greenhouse in m s1. The value of the characteristic air velocity is equal to the ratio of the ventilation flux Gv in m3 s1 divided by the surface of the opening section S in m2, S being the surface which stands perpendicular to the direction of the average air flow within the greenhouse (Wang et al. 1999). In the banana greenhouse equipped with fixed opening S is considered as the total opening surface ST (Demrati et al., 2001): v¼

Gv , ST

(10)

where Gv is calculated by the relation proposed by Kittas et al. (1998) (see appendix). The values of the parameters involved in this relation have already been statistically identified in the same greenhouse by Demrati et al. (2001).

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Thus, finally for canopy Tr ¼

dra ðRnet  Fs Þ þ rCp ILA Dsat , dra þ 2gðri þ ra Þ

2.1.4.

(11)

Validation of the transpiration model

The latent heat flux deduced from the previous transpiration model [Eq. (11)] was compared to the measurements of latent heat exchange between inside and outside the greenhouse.

Estimation of the latent heat flow. The water vapour flux exchanged by ventilation between inside and outside of the greenhouse can be expressed by the following formula (Boulard, 1996):   Gv , Lw ¼ l Hs;i  Hs;e AS

(12)

where l is the water latent heat of vapourisation in J kg1, Hs,i and Hs,e are the inside and outside specific humidity, respectively, in kg water m3 and Gv is the greenhouse ventilation rate in m3 s1m2 (Demrati et al., 2001).

3.

Materials and methods

3.1.

Site and greenhouse description

Measurements were performed in a greenhouse located in a domain in the region of Rabat in Morocco (61570 W longitude, 331500 N latitude, 110 m altitude). The greenhouse was built and mounted by SODETRA Society over a surface of 1 ha (100 m width and length, 6–7 m high). It had 32 spans with metallic frames (Fig. 1) and was covered with a 200 mm polyethylene thermal film, with north–south oriented spans. The greenhouse was surrounded with lower shelters (3 m high) and protected from the dominant westerly wind (sea breeze) by a 7 m cypress hedge.

Greenhouse natural ventilation was performed by means of two opening types (Fig. 1): 608 m2 (6% of the greenhouse soil surface) of fixed openings consisted of 0.2 m width strips situated on the roof between the spans; and 266 m2 (2.7% of the greenhouse soil surface) of variable openings constituted by the greenhouse door (9 m2) and side openings obtained by rolling up the polyethylene film along the east and west sides.

3.2.

Crop

The greenhouse was occupied by a banana crop (cv. grande naine) planted on July, 1996 with a 0.2 plant m2 density. Crop cycle duration varied between 14 and 18 months, depending on the prevailing climatic conditions. In the studied greenhouse, two generations of fruits were spread over 6 months periods, from February to July 1998 and from March to August 1999. Irrigation was performed by means of circo-jets installed between the lines of plants.

3.3.

Climatic measurements

The crop and its climatic environment were studied throughout a development cycle; however, in this work, only two main measurement periods are presented; namely, from 4–9 March 1998, and from 6–15 July 1999. The main climatic variables such as indoor and outdoor temperature and humidity, outdoor global solar radiation, wind speed and direction were measured every 15 min. The following sensors were used: temperature and relative humidity sensors, HMP35D, Vaisala; pyranometer, SP1110, Campbell; anemometer, A100R, Vector; and wind vane, W200P, Vector). Outdoor air temperature and humidity, global radiation and wind speed and direction were measured on a mast situated 2 m above the greenhouse ridge. Indoor net radiation was monitored by means of a net radiometer (NR-LITE; Kipp & Zonen; Campbell Scientific Ltd, UK) situated between the top of the crop cover and the roof polyethylene film (0.5 m below

He

dg

32 spans

99 m

eo

fc

0.2m

Greenhouse tunnel

69

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yp

res s

3m banana crop 3m 6m

3m Fig. 1 – Scheme of the banana greenhouse and its immediate surrounding (the openings are in grey).

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(θ ,u)

R(G,e) Rner

(Te,R(H,e) ) Tc 5m T( f,u) T( f,m) 3m T( f,l)

Banana crop

1m Fs

Fs

(T5,R(H,5) )

(T3,R(H,3) )

(T1,R(H,1) )

Ts (−0.02m)

Fig. 2 – Schematic plan of the greenhouse with the measurement locations for climatic data: (Tj, HR,j): greenhouse air temperature and humidity at three levels 1, 3 and 5 m; RG outside global radiation; Te and HR,e outside air temperature and humidity; Tf,l Tf,m and Tf,u lower, medium and upper levels, respectively; Ts soil surface temperature; Tc roof cover temperature; Fs conductive heat flux at soil surface.

the roof), while the conductive flux at soil surface was measured by means of two flux meters (HFT3; Campbell Scientific Ltd., UK) situated 0.005 m below the soil surface. The soil temperature was measured by three platinum resistance thermistors (PT100) situated 0.2 m below soil surface. In addition to climatic variables, leaf temperature was monitored by means of copper-constantan (Cu–Cs) thermocouples stuck in the middle of the leaves at three levels, 3.4, 3.2, and 2.4 m from ground; namely, young, mature and old leaves, respectively (Fig. 2). Outdoor and indoor temperature and humidity were measured in passively ventilated shelters in order to promote air movement around the sensors and eventually prevent their heating due to solar radiation in daytime. It is of great importance to mention that air circulation in these shelters is induced by a ‘chimney’ effect, requiring a slight increase of the shelter air temperature with respect to outdoor air temperature. This is observed as a slight air temperature increase and eventually a weak reduction of relative humidity measured particularly at noon when solar radiation is at its highest. All these measurements were carried out every 15 min, then centralised on a data logger (CR23X, Campbell) before being processed. The schematic principle of measurements installed inside and outside the greenhouse is shown in (Fig. 2).

3.4. crop

Physiological parameter measurements of the banana

3.4.1.

Leaf area measurement

The banana crop leaf area index (ILA , ratio of total leaf surface to greenhouse soil surface) was estimated by means of a twosteps procedure: (1) in the first step, the correlation between the greatest width lf and greatest length Lf of the leaves and their surface area by means of destructive measurements, was established

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(2) in a second step (non-destructive), the average leaf surface per plant (mother-plant and rejects (young plants)) has been estimated by summing the elementary leaf surfaces of the cover deduced from the previous linear relationship and using their corresponding dimensions Lf and lf .

3.4.2.

Stomatal resistance measurement

A porometer (type MK3, Delta T Devices) was likewise used to measure the stomatal resistance of the lower face of the leaves. These measurements were carried out on leaves located at the same levels as the thermocouples (3.4 m high for young leaves, 3.2 m high for average-age leaves and 2.40 m high for the old leaves, respectively). These measurements were repeated several times for each position until a mean constant value was obtained. The measurements were performed continuously, day and night, during a 40 h period every 30 min for each position. Parallel to stomatal resistance measurements, air temperature, humidity and radiation at the same levels were also measured. These measurements were later used to derive a climate dependent stomatal resistance model.

4.

Results and discussion

4.1.

Microclimatic characterisation

4.1.1.

External conditions

The time course of external temperature, relative humidity, external radiation and wind speed and direction are presented in Figs. 3(a and b) and Fig. 4. In summer time [Fig. 3(a) and (b)], the daily course of outdoor global radiation, air temperature and humidity and even wind speed and direction (Fig. 4) varied periodically in a very similar way from one day to another. Only the night outdoor temperature showed noticeable changes. The wind direction was also strongly related to the time of the day, as shown in Fig. 4.

4.1.2.

Development of indoor climate parameters

(a) Vertical profile of temperature and relative humidity Fig. 5 shows the greenhouse air temperature at three levels (1, 3 and 5 m above ground) together with the outdoor air temperature for the period 4 to 9 March. In daytime it is evident that air temperature increases very significantly with height in the greenhouse particularly a temperature increase of 4 1C, between 1 m above the ground (under the crop) and 3 m, and of 9 1C between 3 m above ground (in the canopy) and 5 m (above the crop). Fig. 6 shows the time course of indoor air relative humidity evolution at three levels (1, 3 and 5 m above ground), together with outdoor air relative humidity for the same period. In daytime, the lowest value of air relative humidity was observed at the highest level, where the air temperature, was highest too. Conversely, during nighttime, the highest value of air relative humidity was observed near the soil surface, where the air temperature was low. In summary, these temperature and humidity profiles are characteristic of the greenhouse banana crop climate. For the diurnal period, the air temperature gradient increases with the height of the greenhouse. It is due to the progressive

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700

100

600 80

500

60

400 300

40

200 20

100

0

0 0

20

40

60

80 Time, h

100

120

140 120

800

100

700

80

600 500

60

400

40

300 200

20

100

0

0 10

60

110 Time, h

160

210

Fig. 3 – Variation of outside climatic conditions with time: relative air humidity, HR,e; global radiation RG;. (a) from March 4 to 9, 1998 and (b) from July 6 to 15, 1999.

air temperature, Te; and

6

900 800

5

700

4

600 500

3

400

2

300 200

Wint speed u, m s−1

Global solar radiation RG,e, W m−2

900

Global radiation R G,e, W m−2 Wind direction θ , deg

Relative humidity HR,e, % Temperature Te, °C

120

Relative humidity HR,e, % Temperature Te, °C

Global solar radiation RG,e, W m−2

800

1

100

0

0 0

10

20

Fig. 4 – Time course of: wind speed, u; direction at 9 m high; from July 6 to 9, 1999.

30

40

50 Time, h

direction, h; and

absorption of solar radiation by the crop canopy, together with a limitation to the vertical air exchanges between the regions above and below the crop canopy. As a consequence, air relative humidity is higher below the crop cover than above.

60

70

80

90

global radiation, RG; h, u are the wind speed and

The greenhouse banana crop is thus characterised by a strong vertical microclimatic gradient, with high temperature and low humidity at the top of the crop cover and much more moderate climatic conditions at its base.

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Temperature, °C

40 35 30 25 20 15 10 0

10

20

30

40

50 Time, h

60

70

80

Fig. 5 – Time course of air temperature at three levels (1, 3 and 5 m), T1 ; T3; and the exterior air temperature Te. For clarity only 4 days, from March 4 to 7, 1998 are presented.

(b) Leaf temperature Fig. 7 shows the diurnal changes in the vertical profile of leaf temperature. During daytime, as solar radiation is mainly intercepted by the highest leaves, this leaf temperature is warmer than the greenhouse air (typically 6 1C more), particularly at noon. At night, air and leaf temperatures are very similar at the top and in the middle of the crop (young and average age leaves) whereas, as already stated by Papadakis et al. (1994) for greenhouse tomato crop, air temperature is higher than that of leaves (old ones) at the base of the canopy. A similar gradient can be observed between the interior air temperature Ti and the roof cover temperature Tc , and the ground surface temperature Ts (Fig. 8). At daytime, Tc is higher than air temperature because of the high absorption of solar radiation by the plastic cover. On the contrary, at night Tc is significantly lower than the outdoor air temperature because the cover radiates to the open sky. (c) Soil heat flux and net radiation Fig. 9 attests to the weakness of energy exchanges between the soil and the air (40 W m2 maximum compared with a maximum incident solar flux of 500 W m2) and stresses that energy exchanges were performed by ventilation through the openings and by conduction through the plastic cover.

4.2.

Study of the transpiration parameters

4.2.1.

Leaf area index estimate

Surface S in m2 and characteristic dimensions Lf and lf in m of the average banana leaf are linked by a linear relationship having following form: S ¼ 0:64ðLf lf Þ þ 1416:82

(13)

with r2 ¼ 0:94 This relationship was used to determine the evolution of the leaf area index of the banana crop throughout the 8 month study period (March–October 1997), in a non-destructive way . Fig. 10 displays the evolution of ILA for the period (March–June), which follows the banana harvest (started in

90

T5, respectively; and

December) and the mother plant picking. It corresponds to low ILA values of the new plants, followed by rather fast growth of the index.

4.2.2.

Stomatal resistance

Figs. 11 and 12 show both the daily variations of the leaf stomatal resistance measured by the porometer, together with the evolutions of indoor air temperature and humidity and outdoor radiation along the same period. These stomatal resistance values are averages of the measurements performed at the three different leaf levels (higher, average and lower leaves). During the night the stomatal resistance is roughly constant and reaches a maximum value of approximately 15 s cm1, corresponding thus to a situation where the stomata are closed. In the day the resistance decreases as the light level increases and then oscillates between 2 and 5 s cm1 during the whole diurnal period. The minimum values of the stomatal resistance were measured during the day as 2 s cm1 which can be compared with values of 1.12 s cm1 without water stress (Turner, 1994) and between 0.7 s cm1 during the summer and 2.8 during winter in open air under subtropical conditions (Robinson & Bower, 1988). More recently, Thomas and Turner (1998) measured stomatal resistance of the banana leaf (cv Williams) with respect to the leaf age. They found out rs values equal to 1.74, 1.52 and 1.71 s cm1 for the first, the fifth and the ninth leaves, respectively.

4.2.3.

Stomatal resistance modelling

Table 1 summarises the statistically identified parameter values for each function. Two climatic factors are sufficient to describe variations of leaf stomatal resistance. As it explains 89% of the variance (Fig. 13), the stomatal resistance was expressed as a unique function of the global radiation according to the relation: rs ¼ rmin ½1 þ exp 0:0033ðRG  516:505Þ1 .

(14)

The addition of a second climate parameter such as air temperature or humidity marginally improves the fit (r2 ¼ 0. 9, Table 1).

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100 Relative humidity HR, %

90 80 70 60 50 40 30 20 10 0

5

10

15

20

39

44

63

68

Time, h 100 Relative humidity HR, %

90 80 70 60 50 40 30 20 10 24

29

34 Time, h

100 Relative humidity HR, %

90 80 70 60 50 40 30 20 10 48

53

58 Time, h

Fig. 6 – Time course of air relative humidity at three levels (1, 3 and 5 m) HR,1; RH,e; (a) 4 March; (b) 5 March; and (c) 6 March, 1999. and the exterior air humidity

4.3.

Transpiration model and validation

Fig. 14 presents the variation of the greenhouse banana crop transpiration [Eq. (11)]. It shows that transpiration varies: (1) during the day transpiration rate is high and reaches values between 200 and 270 W m2 corresponding to periods when the incident solar radiation is high and the canopy stomatal resistance is low; and (2) during the night the stomatal

HR,3; and

HR,5 respectively;

resistance is much higher and the radiative component almost zero; consequently, transpiration is very small. Fig. 14 also highlights the similarity of the variation of the banana crop transpiration flux Tr calculated using the model, and of the latent heat flow Lw exchanged between indoor and outdoor greenhouse. The statistical analysis of this correspondence (Fig. 15) shows that the slope of the linear regression between Lw and Tr is very close to unity and that

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Temperature, °C

40 35 30 25 20 15 0

10

20

30

40

50

60

70

Time, h Fig. 7 – Variation of leaf temperature in 1C with time at three levels: at lower level Tf,l; at higher level Tf,h; and the interior air temperature Ti; from July 6 to 8, 1999.

at medium level Tf,m;

40

Temperature, °C

35 30 25 20 15 10 0

10

20

30

40

Fig. 8 – Time course of roof cover temperature Tc; Ti in 1C; from July 6 to 9, 1999.

50 Time, h

60

70

80

soil surface temperature Ts; and

600

90

interior air temperature

150

500 400 50

300 200

0

100

Soilheat flux, Wm–2

Net radiation Rnet, Wm–2

100

–50 0 –100

0

50

100

150

200

–100

Time, h Fig. 9 – Time course of net radiation , Rnet over the banana cover in the greenhouse and of the soil heat flux at soil , in W m2; from July 6 to 15, 1999. surface,

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4.5 4 Leaf area of crop ILA

3.5 3 2.5 2 1.5 1 0.5 0 March

April

May

June

July

August

September October

Fig. 10 – Seasonal variation in 1999 of the leaf area index ILA of the greenhouse banana crop.

Stomatal resistance rs , s cm−1

18 16 14 12 10 8 6 4 2 0 5

10

15

20

25 Time, h

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35

40

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40

300

30

200

20

100

10

Temperature Te , °C Relative humidity HR,e , %

Global solar radiation RG,e , W m−2

Fig. 11 – Time course of the foliar stomatal resistance (from 00.00 hours on July 21 1999).

0

0 5

10

15

20

25 Time, h

30

35

40

Fig. 12 – Time course of temperature and relative humidity of the greenhouse air and outside global solar radiation during the period of measurement of foliar stomatal resistance (from 00.00 hours on July 21 1999).

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Stomatal resistance rs , s cm−1

18 16 14 12 10 8 6 4 2 0 0

100

200 300 400 Global solar radiation RG , W m−2

500

600

Fig. 13 – Stomatal resistance variation with radiation RG: experimental (dots) and regression (line) Equation 16.

Transpiration Tr , W m−2 Latent heat f low Lw, W m−2

350 300 250 200 150 100 50 0 10

30

50 Time, h

70

90

Fig. 14 – Comparison between the values of transpiration Tr ( ) calculated by Eqn (11) and those of latent heat flux ( measured between the inside and outside of the greenhouse; from July 6 to 9, 1999.

Latent heat flow of water Lw, Wm–2

350

)

First besecting y = 1.0298x + 4.2509 r2 = 0 .9082

300 250 200 150 100 50 0

0

50

100

150 Tr model,

200

250

300

Wm–2

Fig. 15 – Regression lines between the values of transpiration Tr calculated by Eq. (11) and those of latent heat flux measured between the inside and outside of the greenhouse.

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their correlation coefficient is high (r2 ¼ 0.91). Irrigation through the circojets system contributes to the increase in the wet ground surface and the evapotranspiration flux, which could explain why during the day (Fig. 14), the water vapour flux exchanged between inside and outside the greenhouse is slightly larger than the transpiration flux calculated from the model. These results show that at least two methods can be used to estimate the transpiration flux of the banana crop under full-scale greenhouse conditions. From the point of view of the experimental devices required, the direct estimation of the transpiration flux is the simplest method because it only needs the measurements of air temperature and humidity in the greenhouse, together with the determination of ILA and rs. However, as mentioned above, given the important range of variation of rs found in the literature, important variations can ensue from an inappropriate estimation of this parameter. The water vapour balance requires measurements (with better accuracy) of more numerous climate parameters (indoor and outdoor air temperature and humidity, wind speed and direction) and knowledge and awareness of the state of greenhouse (window opening surface), but there is less uncertainty as to the value of the parameters involved (Bailey, 2000).

5.

Conclusions

This study emerged from the need to develop reliable methods to estimate transpiration flux of a banana crop in greenhouse conditions and, more generally, analyse the influence of latent heat flux on the indoor climate. The microclimate study and the transpiration flow were performed in a 1 ha commercial greenhouse banana crop. The micro-climatic study of a greenhouse banana crop has helped show the following: (1) existence of a strong and stable (low temperature below) vertical thermal stratification during the day, (2) air humidity is also stratified (high air water vapour content below), (3) these two phenomena give rise to a strong oasis effect at the level of the plant cover. In parallel, a climate-dependent model of stomatal resistance of banana leaves was developed. It shows that solar intensity RG is the most important climatic factor, which, by itself, explains the essential aspect of stomata regulation through the day. This stomatal resistance model, combined with a model of leaf aerodynamic resistance, allowed computation of an equivalent resistance of the banana crop cover to water vapour transfer and derivation of a model of banana crop transpiration depending on greenhouse climate and crop characteristics (ILA). There is good agreement between the calculated transpiration flux, and the latent heat flow exchange between the inside and the outside of the greenhouse. This demonstrates that, given the size of banana trees, both approaches are better adapted to the estimation of transpiration flux than the direct lysimetric method.

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98 (2 007 ) 6 6 – 78

Finally, these models should allow estimation, with better accuracy, of the water requirements of the banana crop under cover and improve water saving in regions where water is the main limiting factor for agriculture.

Acknowledgements We deeply and warmly thank the joint Franco-Moroccan Inter-Academic Committee that contributed to finance the exchange and the work that resulted in this article. We also thank the National Center of Coordination and Planning of Science and Technical Research of Morocco that, too, partly contributed to the financing of this work through the PARS Program. Our thanks also go particularly to the Manager and Staff of the S.O.D.E.A. Domain (Company of Agricultural Development) in Rabat who gracefully allowed us to use their greenhouse and helped us throughout the realisation of the measurements involved in this study.

Appendix The ventilation rate was calculated by two different formulas depending on wind speed u (m s1): for uo2 m s1  "   0:5 # ST 2 DT h 2 þ Cu u 2g Gv ¼ Cd (A.1) Ti 2 2 for uX2 m s1 Gv ¼

ST pffiffiffiffiffiffi C Cu u, 2 d

(A.2)

where ST ¼ Sh þ Sb , ¼

b¼

pffiffiffiffiffiffi 2 2b ð1 þ bÞð1 þ b2 Þ0:5 Sh , Sb

(A.3) ,

(A.4)

(A.5)

h in m being the vertical height between the high and low openings (height of the chimney), Sh and Sb are the high and low openings surfaces, respectively, in m2, DT is the temperature difference between inside and outside air and Ti in K is the air greenhouse temperature, b is the roof opening to side opening ration. R E F E R E N C E S

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