43 Agroforestry Systems 2:43-4 7. © 1984. Martinus Nifhoff/Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

T h e d e s i g n a n d t e s t i n g o f a s y s t e m f o r m o n i t o r i n g t h e a v a i l a b i l i t y

o f s o l a r r a d i a t i o n f o r i n t e r c u l t u r e

S.M. NEWMAN

Applied Biosystems Unit, Energy Research Group, Open University, Walton Hall, Milton Keynes, MK7 6AA, England

Key words: interculture, solar radiation, canopy, transmissivity, data logger, selenium cells

Abstract. Details of the design and testing, of a system, capable of monitoring the spatial and temporal variation in the availability of photosynthetically active radiation for interculture, are outlined. The system consisted of an array of filtered selenium photo- voltaic cells in specially built cosine corrected mounts. A data logger was used to record the output from these cells at preselected intervals in a form easily accessed by com- puter. The system was tested in a discontinuous canopy of pear (var: Conference) and found to be satisfactory.

Introduction

Interculture is a form of multi-layered, mixed cropping, where food or energy crops are grown within a stand of perennial, usually tree, crops. In some situations, there are economic and ecological advantages in using interculture rather than monoculture for the production of food, biomass or materials. Knowledge of the way in which the overstorey canopy reduces the amount of radiation available for interculture may be useful in the design of novel cropping systems or in the development of existing mono- cultures of perennial crops into interculture systems. Knowledge of both the temporal and spatial variation in the transmissivity of the canopy is necessary in order to make the best use of the available radiation. A system capable of supplying information on radiation availability within interculture systems was designed and tested in a pear orchard.

The design of the system

Specification for the sensors

Ideally, any sensor used to detect radiation available for interculture should have a spectral response similar to most crop species and respond only to wavelengths between 400 and 700nm. A sensor giving an output capable of being calibrated in units of energy would be most useful as this could be related to other energetic processes occurring within the agroecosystem

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and to standard radiation measurements. The sensors should be inexpensive, as many will be needed to cope with the complexity of the light climate found within discontinuous canopies. This complexity arises from the effects of the movement of leaves in windy conditions, the growth of the canopy, the movement of sun and clouds, and the variation of tree canopy density and size within the plantation. The high cost of many commercially available devices and the often inadequate information on their angular response necessitated the use of home built sensors. These were constructed from inexpensive, readily available components. The angular, spectral and current- illuminance responses of the sensor were all characterized.

Moving sensors were not employed due to problems associated with reducing the potential impact of understorey vegetation, wildlife, weather etc. on the sensor movement.

Construction of the sensor

Figure 1 shows the arrangement of the components in the sensor. The sensor consisted of a 25mm type B photovoltaic cell (Megatron

Limited, London), housed in a modified ~" P.V.C. straight coupling (Polyorc c/o Yorkshire Imperial Plastics Limited, Leeds). Two sensor hous- ings were made from each plastic fitting by parting the fitting on a centre lathe. An opal perspex diffusing disc was cemented into place on the flanged end of the housing. The container was then inverted and a volume of clear silicone elastomer (Sylgard 184) was poured into the housing, giving a layer 1 mm thick. Three cinemoid filters, steel blue, salmon pink and neutral density (Rank Strand Limited, London) were placed upon the hardened

4 ~ opal perspex diffuser

/~~'@filtersk-,~[_~

~ m ~ ~ - - ~ 2 5 = selenium cell

~ s i l i c o n e e las tomer p o t t i n g

compound

35 cm length of PVC pipe

modified PVC fitting

F~ure 1. Vertical section through sensor, showing arrangement of components,

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gel. The photo cell was then placed on top of the filters and soldered to a screened twin cable entering the housing through a hole drilled above the diffuser. The whole assembly was then waterproofed by topping up

3 tt with elastomer, leaving enough room for the insertion of a length of plastic pipe. This pipe could then be placed in the soil beneath the canopy to hold the sensor at a predetermined height. The screened twin cable ter- minated in a stereo jack plug. A ¼ Watt 200 Ohms load resistor was soldered across the terminals of the jack plug, in order to give a linear current illuminance relationship. 52 sensors were constructed in this way, in about 8 h, and each sensor cost about £4 for materials.

Performance of the sensor

The spectral response of the sensor was analysed and assessed in relation to the response for photosynthesis of most crop species [1]. The results o f this analysis are shown in Figure 2.

The relationship between illuminance short-wave irradiance, as measured by a Kipp and Zonen solarimeter, and the current output is shown in Figure 3.

The azimuthal response o f the sensor was about + 1% error over 360 ° with the incident light beam at an angle of 30°. The cosine response of the sensor is shown in Figure 4.

The monitoring system

Forty eight of the sensors were placed in various positions at a height of 30cm under a discontinuous canopy of Conference pear trees} Output

i00

5O

0 L L 1

300 400 500 600

Wavelength (nm)

I

7OO 8OO

Figure 2. Spectral response curve for photosynthesis ( . . . . ) (after McCxee, 1976) and the photosynthetically active radiation sensor ( ).

46

250

v

R

i

0 550

short wave irradiance (Wm -2)

Figure 3. The current output of the sensor in relation to irradiance (at 200 ~2 load).

o

o

14

0

1.2

1.0

0 . 8 i

O 0 4 5 o

angle of incidence 0

f 9 0 o

Figure 4. Angular response of sensor expressed as the ratio of output (unity for normal incidence) to the cosine of the angle of incidence.

from these sensors was recorded on a 60 channel Microdata 1600 data logger along with the output from 4 sensors and a Kipp solarimeter placed above the canopy. The output from the Kipp served as a calibration source. Output was recorded instantaneously from the 53 instruments at 5-min intervals, on a magnetic cartridge along with the time of day and position of each sensor. Data on the cartridge were replayed to a PDP.11 computer via a perex replay unit for further analysis.

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Performance of the system

No ageing of the sensors was detected over a two-month period. Water

did not enter any of the cells. The biggest problem of the system was that

that the cables became attractive to certain rodents which chewed completely

through the connections from time to time. Work is now being carried out

on designing inexpensive integrators that can be fitted within the cell housing.

Acknowledgements

The work was carried out under the support of an SRC studentship. I would

like to thank Dr. J.W. Palmer of the East MaUing Research Station, Mr. A. DeSilva of the Physics Department at the Open University, and Mr. G.E.

Bowman of the National Institute of Agricultural Engineering for their help.

Note

1. 'The number of sensors used will, generaUy, depend upon the nature of the canopy, the latitude of the experimental site, the aims of the investigator and the available funds/facilities. In the study of the pear orchard, it was found that there was little variation in transmissivity between different plots (rectangles with a tree at each corner) and little variation from day to day. This information is unlikely to be of use, however, for other systems. For correlations of overstorey canopy transmissivity with understorey crop growth daily total transmissivity values were found to be most useful. This is the reason for stating that the research priority is now to design inexpensive integrators for the sensors'.

Reference

1. McCree KJ (1976) Practical applications of action spectra. In: Smith H, ed, Light and Plant Development, pp 461-465, Butterworths, London