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The radiation balance of a tropical pasture, I. The reflection of short-wave radiation
Agricultural Meteorology Elsevier Publishing Company, Amsterdam-Printed in The Netherlands
THE RADIATION BALANCE OF A TROPICAL PASTURE, I. THE REFLECTION OF SHORT-WAVE RADIATION
J. D. KALMA and R. BADHAM Division of Land Research, Commonwealth Scientific and Industrial Research Organization, Canberra A. C. T. (Australia) (Received Sept. 8, 1971)
ABSTRACT Kalma, J. D. and Badham, R., 1972. The radiation balance of a tropical pasture, I. The reflection of shortwave radiation. Agric. Meteorol., 10: 251-259. Short-wave reflection (albedo) was measured for three agricultural surfaces at Katherine, N. T., Australia, between December 18, 1969 and March 28, 1970. The seasonal albedo value for the pasture legume Townsville stylo was 0.19, for a mixed stand of two annual grass species 0.22, and for bare soil 0.21. Albedo of dry soil varied between 0.20 and 0.22. Wetting could lower albedo to 0.14. Maximum albedos for TownsviUestylo and grasses were 0.25 and 0.28. Minimum values (0.13 and 0.15, respectively) were found at plant establishment. Rapid increase in albedo was associated with periods of rapid g~owth (increasing crop cover and increasing leaf area index). Severe water stress occurred after February 20 and albedo for both vegetative surfaces consequently decreased through reduction in effective crop cover, maturing of inflorescences and gradual senescence. Diurnal variation in albedo for all surfaces can partly be attributed to the sensors used, but part of the increase is due to the nature of the surfaces themselves. The role of specular reflection and internal trapping is discussed.
INTRODUCTION The seasonal and diumal variation of radiative surface properties in a sown Townsville stylo pasture in tropical northern Australia has been studied as part of field studies of environmental aspects of this type of land use. The present paper deals with the reflection of short-wave radiation. In a second paper (Kalma, 1972) attention will be given to net all-wave and long.wave radiation. Net all-wave radiation (i.e., the radiation balance) controls the potential rate of water loss, whereas its photosynthetically active, visible component controls the potential rate of dry matter accumulation. Short-wave reflectivity or albedo is an important discriminant in the radiation balance of vegetative surfaces and a major objective of the present work is to determine to what extent albedo depends on the nature and status of the surface. In the last 40 years numerous workers have studied reflection by natural surfaces. But, as Oguntoyinbo (1970)p. oints out, relatively few measurements are available from tropical regions. No seasonal information is available on radiative properties of pasture surfaces in the savannah environment of northern Australia.
252
J.D. KALMAAND R. BADHAM
MATERIALS AND METHODS This work was carried out at Katherine Research Station, C.S.I.R.O., in the Northern Territory, Australia. Katherine (14°28'S 132°19'E, 108 m above M.S.L.)experiences a tropical savannah climate with 95% of the annual rainfall (90 cm) falling in the five summer months November to March. This distribution of the annual rainfall restricts most of the active growth in dry-land agriculture to the period November to April. Climatic characteristics of the region have been described by Slatyer (1960) and Fitzpatrick (1965). The soil at the Research Station is Tippera clay loam, a lateritic red earth overlying Cambrian limestone (Stewart, 1956). Measurements were made in a sown Townsville stylo pasture (Stylosanthes humilis H.B.K.) between December 18, 1969 and March 28, 1970. The experimental area, which was surrounded by similar pastures for at least 100 m in all directions, consisted of three plots. Two plots, both 800 m 2 in area, were cultivated in early December and one of them was subsequently sprayed with the herbicide dachthal. The herbicide killed the naturally occurring grass weeds Brachiaria ramosa and Digitaria ascendens. The unsprayed plot, which was a grassy area in the previous year, soon became completely dominated by these grasses. A third plot, 80 m 2 in area, was not cultivated but sprayed with herbicide and permanently kept bare by hand weeding. Ground slopes over the experimental area were uniformly 1 : 300, falling to the southeast. The nearest trees, 6 - 9 m high with open canopies, were 300 m to the west. Incident global radiation was measured with a Moll Gorczynski pyranometer manufactured by Kipp. Diffuse radiation was measured with a Kipp pyranometer, provided with a semi-circular shade ring as described by Drummond (1956) whose shade ring corrections were also used. Reflected short-wave radiation from all three surfaces was measured with inverted unshielded 180 ° Lintronic pyranometers (Monteith, 1959b), mounted at 2 m above the ground in the Townsville stylo area and in the grasses area and at 1 m in the centre of the bare soil plot. All instruments were inspected and maintained on a daily basis. It can be calculated from the above exposure heights (Reifsnyder, 1967) that 95% of the upward fluxes was received from an area of 254 m 2 for the vegetative surfaces and of 64 m 2 for the bare soil area. Measurements were made on about 100 days, but power failures, instrumental difficulties, maintenance, and special-purpose measurements reduced the number of complete days used for the present analysis to about 70, during which measurements were available made at 10-minute intervals between sunrise and sunset. The data logging system used in these measurements and the procedures followed in data storage and data conversion, have been described by Byrne et al. (1971). Calibrations and inter-comparisons were carried out in early December 1969 and early April 1970, using substandards which are regularly recalibrated at the Division of Meteoro. logical Physics, C.S.I.R.O., Aspendale, Australia.
THE RADIATIONBALANCEOF A TROPICAL PASTURE
253
RESULTS AND DISCUSSION Seasonal conditions Rainfall is the dominant factor in relation to dry-land plant growth at Katherine (Slatyer, 1960). Between cultivation (December 2)and the start of the experiment (December 18) 164 mm of rain was recorded. The major part of this fell after December 8 (155 mm). First germination was observed on December 11. From cultivation till the end of the experiment (March 28) 501 mm of rain fell, which may be compared with a long-term average of 760 mm for that period. Daily rainfall during the experimental period is shown in Fig. 1A. Substantial rainfall occurred only between December 18 and December 23 (109 mm) and between February 4 and February 10 (159 mm). The uneven distribution of less than normal rainfall caused total accumulated dry matter production at the end of the experimental period to be relatively low: 4,100 kg ha -1 in the grass-dominated plot and 2,170 kg ha -1 for the Townsville stylo plot or approximately 75% and 50%, respectively of "normal" yields. The seasonal development of leaf area index (LAI) and of accumulated dry matter production (DM) is given in Fig.lB. Fig.IC shows development of crop height and crop cover. It is evide-.t that most growth occurred in periods which followed heavy rain, viz. between December 18 and January 15 and between February 5 and March 1, approximately. Seasonal changes in albedo Daily values of albedo were calculated as ratios of daily totals of reflected and incoming solar radiation, effectively weighting measurements according to radiation intensity. By using only complete days of measurements, 69 daily values of albedo were obtained for Townsville stylo, 47 values for grass, and 67 for bare soil. Although measurements at solar elevations lower than 20 ° are known to be subject to considerable error, absolute amounts of radiation at those elevations are small. It is estimated on the basis of accuracies given by C.S.A.G.I. (Comit6 Special de l'Ann6e G6ophysique Internationale) (1957), that the mean error in daily albedo values is less than 5%. In Fig.lD all available albedo values have been plotted against date. The albedo of the bare soil, which was litter-free and uncultivated, was very much dependent on surface wetness of the soil. Albedo of dry soil varied between 20% and 23%. Thoroughly wetted soil (December 18-19; February 1 0 - 1 1 ) h a d albedo values of 14-16%. Moderate wetting caused albedo to be 2-4% lower than that of dry soil (January 12, 27, and 29; February 20). Generally, drying of the bare soil surface took 1 - 2 days. The present observations confirm findings by Monteith (1959a) and Haise et al. (1967).
254
J.D. KALMA AND R. BADHAM
The albedo of the Townsville stylo area showed a very similar trend to that of bare soil until February 5, except that the albedo is always 3 - 5 % lower. This suggests that till February 5, the albedo of Townsville stylo was strongly influenced by that of the underlying soil. The Townsville stylo was severely stressed throughout most of January. Under stress conditions its leaves show parahelionastism (J. E. Begg and B. W. R. Torssell, personal communication, 1970) and leaf distribution becomes non-random. The albedo of the cultivated underlying soil was lower than that of the smooth, cleared bare soil area. Rapid growth between February 5 and 15, marked by increases 60
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THE RADIATIONBALANCEOF A TROPICALPASTURE
255
in crop cover, in L.A.I. and in accumulated dry matter production, resulted in increased albedo. An increase in albedo with increasing L.A.I. has been observed for sugar beet (Monteith, 1959a)and cotton (Rijks, 1967). From February 22 onwards Towns~ille stylo is experiencing severe stress, slowing down growth, and resulting in a decrease in effective crop cover and hence a decreasing albedo. Flowering of TownsviUe stylo took place throughout March. It is unlikely to have had any effect on albedo, since its flowers are very small and only noticeable in the morning. After March 1 Townsville stylo albedo was lower than that of the bare soil surface. No grass albedo-values were available between January 11 and February 5. Till January 11 albedo of grass and bare soil were similar. The more complete cover in early January obviously compensated for the lower albedo of the cultivated underlying soil. Both grass and Townsville stylo covers had a non-random spatial leaf distribution. As is the case with Townsville stylo, rapid growth of grass between February 5 and 21 was associated with an increase in albedo to 27-28% while L.A.I. increased from 2.0 to 3.6. Flowering of grass took place between February 21 and 25, followed by some lodging of the stand and maturing of the inflorescences in the first two weeks of March, after which L.A.I. slowly declined with gradual senescence of the standing material. The surface characteristics of the bare soil did not change significantly during the experimental period, apart from some clearly defined short periods of wetting and drying. The near-constancy of the bare soil albedo (Fig.1 D)indicates that, on a seasonal basis, the effect on surface albedo of changes in solar declination and ratio of diffuse to total radiation is not very great. Seasonal changes in the albedos of grasses and Townsville stylo are therefore very likely to be due to changes in surface characteristics only, e.g. surface cover, leaf area index, spectral properties of leaves, etc. After interpolating for missing values throughout the whole experimental period, albedo values for all three surfaces have been applied to daily totals of incoming global radiation, as measured with a Rimco integrating pyranometer at the experimental site. In this way, albedo values could be weighted according to daily totals of global radiation. Weighted mean albedo for the experimental period was 0.209 for bare soil, 0.190 for Townsville stylo, and 0.223 for the grasses. No data are available on albedo of Townsville stylo pastures from other sources and seasonal studies have not been made in the same region previously. However, some comparable Australian work has been reported in the literature. Dyer (1967), in a combined water and energy balance study at Katherine, chooses values ranging from 0.18 in the middle of the wet season to 0.25 for the dry season. De Vries (1959) reports albedo values for both dry-land and irrigated pastures at Deniliquin, N.S.W. of 0.23. Fitzpatrick and Stern (1965)report a mean value of 0.18 for grass at Kimberley, W. A. The present data may be compared with results published for surfaces, which are in certain aspects similar to the Townsville stylo and grass surfaces at Katherine. Lin-sienchia (1967) reports albedo values of 0.19 fo¢ 30 em high Digitaria decumbens (Pangola grass) in Barbados. Budyko (1958) gives for meadows in the U.S.S.R. a range of 0.15-0.25. Stanhill (1970) reports for natural pastures in Israel a value of 0.25.
256
J.D. KALMA AND R. BADHAM
In a recent study (Oguntoyinbo, 1970) reflection coefficients ranging from 0.19 to 0.23 have been reported for a variety of Nigerian savannah grasses. As in the present study, a general increase in reflectivity is observed as cover changes from fresh green to dry. Lower reflectivity values for grasses in the tropics are partly ascribed to a smaller leaf area index and less ground cover and partly to greater solar elevations.
Diurnal changes in albedo In this study a general increase was observed in albedo with increasing zenith angle (Fig.2), as previously observed by various other workers (e.g. Rijks, 1967; Davies and
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Fig.2. Diurnal changes in albedo of Townsville stylo (solid circles), grasses (open circles), and bare soil (dotted line) on four days of comparable dally ratios of diffuse to total radiation (D/T). Hourly ratios of diffuse to total radiation have been indicated. Katherine, 1969-70. Buttimor, 1969). The diurnal variation in albedo can partly be attributed to the pyranometers used, but part of the increase with increasing zenith angle must be a real effect of the surfaces themselves. A number of errors, common to pyranometers, have recently been discussed by Flowers and Helfert (1966). The most important errors apparently are internal reflection, horizon light, and non-cosine response (Hart and Rosenberg, 1968). On the other hand, increased penetration of light into the standing crop at smaller zenith angles has often been considered as an important reason for the observed diurnal variation (Kalma and
257
THE RADIATION BALANCE OF A TROPICAL PASTURE
Stanhill, 1969). Lin-sien-chia (1967) suggests that specular reflection of direct radiation by leaves causes an increase of albedo at increasing zenith angle. He shows that for bare dry black soil, without specular reflection, albedo variation throughout the day is negligible, while under overcast conditions the albedo of vegetative surfaces is constant as well. For an ideal rough surface, the amount reflected is independent of the direction of the incoming radiation. Natural surfaces are not however perfect diffusers and specular reflection becomes more important at lower elevations. Not surprisingly, Kuhn and 0.5
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Fig.3. Diurnal change in albedo of Townsville stylo (solid line) and bare soil (dotted line) on a day with 20% diffuse light (A) and a day with 73% diffuse light (B). Hourly ratios of diffuse to total radiation have been indicated. Katherine, 1969-70.
Suomi (1958), using beam reflectors, and Hart and Rosenberg (1968), using inverted pyranometers with a cylindrical shield, report no diurnal variation in albedo of crop surfaces. Their instruments probably did not measure most speculady reflected light at low solar elevations. Fig. 2. shows the diurnal variation in albedo for the three surfaces on four days of similar cloud cover, as expressed by the ratio of dally diffuse radiation to daily global radiation (D/T). The ratios of hourly diffuse radiation to hourly global radiation have also been indicated in the figure. It appears that the dependence of albedo on zenith angle is smallest for bare soil, confirming similar observations by Lin-sien-chia (1967). The effect of changes in cloud cover (i.e. changes in the ratio diffuse to total radiation D/T) on albedo of Townsville style and bare soil has been illustrated in Fig.3. Incoming radiation on February 28 was 547 mWh cm -2, of which 27% was direct. Incoming radiation on March 2 was 748 mWh cm -2 with 80% direct radiation. Hourly ratios of diffuse to total radiation have been given in Fig.3. The diurnal variation in Townsville style albedo is on both days much greater than for bare soil. As shown in Fig.3A, a greater proportion of direct radiation causes the albedo of Townsville style to be relatively high in the early morning and late afternoon, as compared to the rest of the day.
258
1. D. KALMA AND R. BADHAM
A similar but less p r o n o u n c e d effect with lower m a x i m a and higher minima in albedo is clear f r o m Fig.3B. This p h e n o m e n o n has been observed t h r o u g h o u t the experimental period. It effectively substantiates the suggestion, referred to above, that specular reflection m a y be responsible for part o f the increase in albedo at low solar elevations. The increase of albedo with increasing cloud cover has been reported previously for various vegetative surfaces (Rijks, 1967; Kalma and Stanhill, 1969). It is, however, contrary to results reported by Munn (1966) and Lin-sien-chia (1967). ACKNOWLEDGEMENTS The authors wish to acknowledge the technical assistance o f Messrs. D. E. Watson and A. G. Swan, and the assistance o f Mrs K. Haszler in c o m p u t e r programming. REFERENCES Budyko, M. I., 1958. The Heat Balance o f the Earth's Surface. U.S. Dept. Commerce, Weather Bur., Washington D.C. (transl. from Russian), 259 pp. Byrne, G. F., Rose, C. W., Begg, J. E., Torssell, B. W. R. and McPherson, H. G., 1971 (in press). Instrumentation for crop-environment measurement in a tropical savannah climate. C.S./. R. O., Aust., Div. Land Res. Tech. Pap., 32:19 pp. C.S.A.G.I., 1957. Instruction Manual, Pt. VI. Radiation instruments and measurements. Ann. Intern. Geophys. Yr., pp. 367-466. Davies, J. A. and Buttimor, P. H., 1969. Reflection coefficients, heating coefficients and net radiation at Simcoe, southern Ontario. Agric. Meteorol., 6: 373-386. De Vries, D. A., 1959. The influence of irrigation on the energy balance and the climate near the ground. J. Meteorol., 16: 256-270. Drummond, A. J., 1956. On the measurement of sky radiation. Arch. MeteoroL Geophys. BioMirnatol., 7: 413-436. Dyer, A. J., 1967. A combined water and energy balance study at Katherine, Northern Territory. Aust. Meteorol. Mag., 15: 148-155. Fitzpatrick, E. A., 1965. Climate of the Tipperary area. In: General Report on Lands of Tipperary Area, Northern Territory, 1961. C.S.I.R.O., Aust., Land Res. Set., 13: 39-52. Fitzpatrick, E. A. and Stern, W. R., 1965. Components of the radiation balance of irrigated plots in a dry monsoonal environment, J. AppL Meteorol., 4: 449-460. Flowers, E. C. and Helfert, N. F., 1966. Laboratory and field investigations of Eppley radiation sensors. Monthly Weather Rev., 94: 259-264. Haise, H. R., Hanks, R. J. and Jensen, M. E., 1967. Solar reflectance for soil and crop surfaces. Unpublished report quoted in: Irrigation o f Agricultural Lands (Monograph 11, American Society of Agronomy). Madison, Wisc., p. 513. Hart, H. E. and Rosenberg, N. J., 1968. An extended cylindrical shield for use with an inverted Eppley pyranometer to measure albedo over small areas. In: Horticulture Progress Report 60. Agric. Expt. Sta., pp. 30-42. Univ. of Nebraska, Lincoln, Nebr. Kalma, J. D., 1972. The radiation balance of a tropical pasture, II. Net all-wave radiation. Agric. Meteorol., 10, in press. Kalma, J. D. and Stanhill, G., 1969. The radiation climate of an irrigated orange plantation. Solar Energy, 12: 491-508. Kuhn, P. M. and Suomi, V. E., 1958. Airborne observations of albedo with a beam reflector. J. MeteoroL, 15: 172-174. Lin-Sien-Chia, 1967. Albedos of natural surfaces in Barbados. Q. J. R. Meteorol. Soc., 93: 116-120. Monteith, J. L., 1959a. The reflection of short-wave radiation by vegetation. Q. J. R. Meteorol. Soc., 85: 386-392.
THE RADIATION BALANCE OF A TROPICAL PASTURE
259
Monteith, J. L., 1959b. Solarimeter for field use. J. Sc£ Instr., 36: 341-346. Munn, R. E., 1966. Descriptive Micrometeorology. Academic Press, New York, N.Y., 245 pp. Oguntoyinbo, J. S., 1970. Reflection coefficient of natural vegetation, crops and urban surfaces in Nigeria. Q. Z R. Meteorol. Soc., 96: 4 3 0 - 4 4 1 . Reifsnyder, W. E., 1967. Radiation geometry in the measurement and interpretation of radiation balance. Agric. Meteorol., 41: 255-265. Rijks, D. A., 1967. Water use by irrigated cotton in Sudan, I. Reflection of short-wave radiation. J. Appl. Ecol., 4: 561-568. Slatyer, R. O., 1960. Agricultural climatology of the Katherine area, N.T.C.S.I.R.O., Aust. Div. Land Res., Regional Surv., Tech. Pap., 1 3 : 3 9 pp. Stanhill, G., 1970. Some results of helicopter measurements of the albedo of different land surfaces. Solar Energy, 13: 5 9 - 6 6 . Stewart, G. A., 1956. Softs of the Katherine-Darwin region, Northern Territory. C.S.I.R.O., Aust. SoilPubl., 6: 68pp.
THE RADIATION BALANCE OF A TROPICAL PASTURE, I. THE REFLECTION OF SHORT-WAVE RADIATION
J. D. KALMA and R. BADHAM Division of Land Research, Commonwealth Scientific and Industrial Research Organization, Canberra A. C. T. (Australia) (Received Sept. 8, 1971)
ABSTRACT Kalma, J. D. and Badham, R., 1972. The radiation balance of a tropical pasture, I. The reflection of shortwave radiation. Agric. Meteorol., 10: 251-259. Short-wave reflection (albedo) was measured for three agricultural surfaces at Katherine, N. T., Australia, between December 18, 1969 and March 28, 1970. The seasonal albedo value for the pasture legume Townsville stylo was 0.19, for a mixed stand of two annual grass species 0.22, and for bare soil 0.21. Albedo of dry soil varied between 0.20 and 0.22. Wetting could lower albedo to 0.14. Maximum albedos for TownsviUestylo and grasses were 0.25 and 0.28. Minimum values (0.13 and 0.15, respectively) were found at plant establishment. Rapid increase in albedo was associated with periods of rapid g~owth (increasing crop cover and increasing leaf area index). Severe water stress occurred after February 20 and albedo for both vegetative surfaces consequently decreased through reduction in effective crop cover, maturing of inflorescences and gradual senescence. Diurnal variation in albedo for all surfaces can partly be attributed to the sensors used, but part of the increase is due to the nature of the surfaces themselves. The role of specular reflection and internal trapping is discussed.
INTRODUCTION The seasonal and diumal variation of radiative surface properties in a sown Townsville stylo pasture in tropical northern Australia has been studied as part of field studies of environmental aspects of this type of land use. The present paper deals with the reflection of short-wave radiation. In a second paper (Kalma, 1972) attention will be given to net all-wave and long.wave radiation. Net all-wave radiation (i.e., the radiation balance) controls the potential rate of water loss, whereas its photosynthetically active, visible component controls the potential rate of dry matter accumulation. Short-wave reflectivity or albedo is an important discriminant in the radiation balance of vegetative surfaces and a major objective of the present work is to determine to what extent albedo depends on the nature and status of the surface. In the last 40 years numerous workers have studied reflection by natural surfaces. But, as Oguntoyinbo (1970)p. oints out, relatively few measurements are available from tropical regions. No seasonal information is available on radiative properties of pasture surfaces in the savannah environment of northern Australia.
252
J.D. KALMAAND R. BADHAM
MATERIALS AND METHODS This work was carried out at Katherine Research Station, C.S.I.R.O., in the Northern Territory, Australia. Katherine (14°28'S 132°19'E, 108 m above M.S.L.)experiences a tropical savannah climate with 95% of the annual rainfall (90 cm) falling in the five summer months November to March. This distribution of the annual rainfall restricts most of the active growth in dry-land agriculture to the period November to April. Climatic characteristics of the region have been described by Slatyer (1960) and Fitzpatrick (1965). The soil at the Research Station is Tippera clay loam, a lateritic red earth overlying Cambrian limestone (Stewart, 1956). Measurements were made in a sown Townsville stylo pasture (Stylosanthes humilis H.B.K.) between December 18, 1969 and March 28, 1970. The experimental area, which was surrounded by similar pastures for at least 100 m in all directions, consisted of three plots. Two plots, both 800 m 2 in area, were cultivated in early December and one of them was subsequently sprayed with the herbicide dachthal. The herbicide killed the naturally occurring grass weeds Brachiaria ramosa and Digitaria ascendens. The unsprayed plot, which was a grassy area in the previous year, soon became completely dominated by these grasses. A third plot, 80 m 2 in area, was not cultivated but sprayed with herbicide and permanently kept bare by hand weeding. Ground slopes over the experimental area were uniformly 1 : 300, falling to the southeast. The nearest trees, 6 - 9 m high with open canopies, were 300 m to the west. Incident global radiation was measured with a Moll Gorczynski pyranometer manufactured by Kipp. Diffuse radiation was measured with a Kipp pyranometer, provided with a semi-circular shade ring as described by Drummond (1956) whose shade ring corrections were also used. Reflected short-wave radiation from all three surfaces was measured with inverted unshielded 180 ° Lintronic pyranometers (Monteith, 1959b), mounted at 2 m above the ground in the Townsville stylo area and in the grasses area and at 1 m in the centre of the bare soil plot. All instruments were inspected and maintained on a daily basis. It can be calculated from the above exposure heights (Reifsnyder, 1967) that 95% of the upward fluxes was received from an area of 254 m 2 for the vegetative surfaces and of 64 m 2 for the bare soil area. Measurements were made on about 100 days, but power failures, instrumental difficulties, maintenance, and special-purpose measurements reduced the number of complete days used for the present analysis to about 70, during which measurements were available made at 10-minute intervals between sunrise and sunset. The data logging system used in these measurements and the procedures followed in data storage and data conversion, have been described by Byrne et al. (1971). Calibrations and inter-comparisons were carried out in early December 1969 and early April 1970, using substandards which are regularly recalibrated at the Division of Meteoro. logical Physics, C.S.I.R.O., Aspendale, Australia.
THE RADIATIONBALANCEOF A TROPICAL PASTURE
253
RESULTS AND DISCUSSION Seasonal conditions Rainfall is the dominant factor in relation to dry-land plant growth at Katherine (Slatyer, 1960). Between cultivation (December 2)and the start of the experiment (December 18) 164 mm of rain was recorded. The major part of this fell after December 8 (155 mm). First germination was observed on December 11. From cultivation till the end of the experiment (March 28) 501 mm of rain fell, which may be compared with a long-term average of 760 mm for that period. Daily rainfall during the experimental period is shown in Fig. 1A. Substantial rainfall occurred only between December 18 and December 23 (109 mm) and between February 4 and February 10 (159 mm). The uneven distribution of less than normal rainfall caused total accumulated dry matter production at the end of the experimental period to be relatively low: 4,100 kg ha -1 in the grass-dominated plot and 2,170 kg ha -1 for the Townsville stylo plot or approximately 75% and 50%, respectively of "normal" yields. The seasonal development of leaf area index (LAI) and of accumulated dry matter production (DM) is given in Fig.lB. Fig.IC shows development of crop height and crop cover. It is evide-.t that most growth occurred in periods which followed heavy rain, viz. between December 18 and January 15 and between February 5 and March 1, approximately. Seasonal changes in albedo Daily values of albedo were calculated as ratios of daily totals of reflected and incoming solar radiation, effectively weighting measurements according to radiation intensity. By using only complete days of measurements, 69 daily values of albedo were obtained for Townsville stylo, 47 values for grass, and 67 for bare soil. Although measurements at solar elevations lower than 20 ° are known to be subject to considerable error, absolute amounts of radiation at those elevations are small. It is estimated on the basis of accuracies given by C.S.A.G.I. (Comit6 Special de l'Ann6e G6ophysique Internationale) (1957), that the mean error in daily albedo values is less than 5%. In Fig.lD all available albedo values have been plotted against date. The albedo of the bare soil, which was litter-free and uncultivated, was very much dependent on surface wetness of the soil. Albedo of dry soil varied between 20% and 23%. Thoroughly wetted soil (December 18-19; February 1 0 - 1 1 ) h a d albedo values of 14-16%. Moderate wetting caused albedo to be 2-4% lower than that of dry soil (January 12, 27, and 29; February 20). Generally, drying of the bare soil surface took 1 - 2 days. The present observations confirm findings by Monteith (1959a) and Haise et al. (1967).
254
J.D. KALMA AND R. BADHAM
The albedo of the Townsville stylo area showed a very similar trend to that of bare soil until February 5, except that the albedo is always 3 - 5 % lower. This suggests that till February 5, the albedo of Townsville stylo was strongly influenced by that of the underlying soil. The Townsville stylo was severely stressed throughout most of January. Under stress conditions its leaves show parahelionastism (J. E. Begg and B. W. R. Torssell, personal communication, 1970) and leaf distribution becomes non-random. The albedo of the cultivated underlying soil was lower than that of the smooth, cleared bare soil area. Rapid growth between February 5 and 15, marked by increases 60
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THE RADIATIONBALANCEOF A TROPICALPASTURE
255
in crop cover, in L.A.I. and in accumulated dry matter production, resulted in increased albedo. An increase in albedo with increasing L.A.I. has been observed for sugar beet (Monteith, 1959a)and cotton (Rijks, 1967). From February 22 onwards Towns~ille stylo is experiencing severe stress, slowing down growth, and resulting in a decrease in effective crop cover and hence a decreasing albedo. Flowering of TownsviUe stylo took place throughout March. It is unlikely to have had any effect on albedo, since its flowers are very small and only noticeable in the morning. After March 1 Townsville stylo albedo was lower than that of the bare soil surface. No grass albedo-values were available between January 11 and February 5. Till January 11 albedo of grass and bare soil were similar. The more complete cover in early January obviously compensated for the lower albedo of the cultivated underlying soil. Both grass and Townsville stylo covers had a non-random spatial leaf distribution. As is the case with Townsville stylo, rapid growth of grass between February 5 and 21 was associated with an increase in albedo to 27-28% while L.A.I. increased from 2.0 to 3.6. Flowering of grass took place between February 21 and 25, followed by some lodging of the stand and maturing of the inflorescences in the first two weeks of March, after which L.A.I. slowly declined with gradual senescence of the standing material. The surface characteristics of the bare soil did not change significantly during the experimental period, apart from some clearly defined short periods of wetting and drying. The near-constancy of the bare soil albedo (Fig.1 D)indicates that, on a seasonal basis, the effect on surface albedo of changes in solar declination and ratio of diffuse to total radiation is not very great. Seasonal changes in the albedos of grasses and Townsville stylo are therefore very likely to be due to changes in surface characteristics only, e.g. surface cover, leaf area index, spectral properties of leaves, etc. After interpolating for missing values throughout the whole experimental period, albedo values for all three surfaces have been applied to daily totals of incoming global radiation, as measured with a Rimco integrating pyranometer at the experimental site. In this way, albedo values could be weighted according to daily totals of global radiation. Weighted mean albedo for the experimental period was 0.209 for bare soil, 0.190 for Townsville stylo, and 0.223 for the grasses. No data are available on albedo of Townsville stylo pastures from other sources and seasonal studies have not been made in the same region previously. However, some comparable Australian work has been reported in the literature. Dyer (1967), in a combined water and energy balance study at Katherine, chooses values ranging from 0.18 in the middle of the wet season to 0.25 for the dry season. De Vries (1959) reports albedo values for both dry-land and irrigated pastures at Deniliquin, N.S.W. of 0.23. Fitzpatrick and Stern (1965)report a mean value of 0.18 for grass at Kimberley, W. A. The present data may be compared with results published for surfaces, which are in certain aspects similar to the Townsville stylo and grass surfaces at Katherine. Lin-sienchia (1967) reports albedo values of 0.19 fo¢ 30 em high Digitaria decumbens (Pangola grass) in Barbados. Budyko (1958) gives for meadows in the U.S.S.R. a range of 0.15-0.25. Stanhill (1970) reports for natural pastures in Israel a value of 0.25.
256
J.D. KALMA AND R. BADHAM
In a recent study (Oguntoyinbo, 1970) reflection coefficients ranging from 0.19 to 0.23 have been reported for a variety of Nigerian savannah grasses. As in the present study, a general increase in reflectivity is observed as cover changes from fresh green to dry. Lower reflectivity values for grasses in the tropics are partly ascribed to a smaller leaf area index and less ground cover and partly to greater solar elevations.
Diurnal changes in albedo In this study a general increase was observed in albedo with increasing zenith angle (Fig.2), as previously observed by various other workers (e.g. Rijks, 1967; Davies and
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Fig.2. Diurnal changes in albedo of Townsville stylo (solid circles), grasses (open circles), and bare soil (dotted line) on four days of comparable dally ratios of diffuse to total radiation (D/T). Hourly ratios of diffuse to total radiation have been indicated. Katherine, 1969-70. Buttimor, 1969). The diurnal variation in albedo can partly be attributed to the pyranometers used, but part of the increase with increasing zenith angle must be a real effect of the surfaces themselves. A number of errors, common to pyranometers, have recently been discussed by Flowers and Helfert (1966). The most important errors apparently are internal reflection, horizon light, and non-cosine response (Hart and Rosenberg, 1968). On the other hand, increased penetration of light into the standing crop at smaller zenith angles has often been considered as an important reason for the observed diurnal variation (Kalma and
257
THE RADIATION BALANCE OF A TROPICAL PASTURE
Stanhill, 1969). Lin-sien-chia (1967) suggests that specular reflection of direct radiation by leaves causes an increase of albedo at increasing zenith angle. He shows that for bare dry black soil, without specular reflection, albedo variation throughout the day is negligible, while under overcast conditions the albedo of vegetative surfaces is constant as well. For an ideal rough surface, the amount reflected is independent of the direction of the incoming radiation. Natural surfaces are not however perfect diffusers and specular reflection becomes more important at lower elevations. Not surprisingly, Kuhn and 0.5
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Fig.3. Diurnal change in albedo of Townsville stylo (solid line) and bare soil (dotted line) on a day with 20% diffuse light (A) and a day with 73% diffuse light (B). Hourly ratios of diffuse to total radiation have been indicated. Katherine, 1969-70.
Suomi (1958), using beam reflectors, and Hart and Rosenberg (1968), using inverted pyranometers with a cylindrical shield, report no diurnal variation in albedo of crop surfaces. Their instruments probably did not measure most speculady reflected light at low solar elevations. Fig. 2. shows the diurnal variation in albedo for the three surfaces on four days of similar cloud cover, as expressed by the ratio of dally diffuse radiation to daily global radiation (D/T). The ratios of hourly diffuse radiation to hourly global radiation have also been indicated in the figure. It appears that the dependence of albedo on zenith angle is smallest for bare soil, confirming similar observations by Lin-sien-chia (1967). The effect of changes in cloud cover (i.e. changes in the ratio diffuse to total radiation D/T) on albedo of Townsville style and bare soil has been illustrated in Fig.3. Incoming radiation on February 28 was 547 mWh cm -2, of which 27% was direct. Incoming radiation on March 2 was 748 mWh cm -2 with 80% direct radiation. Hourly ratios of diffuse to total radiation have been given in Fig.3. The diurnal variation in Townsville style albedo is on both days much greater than for bare soil. As shown in Fig.3A, a greater proportion of direct radiation causes the albedo of Townsville style to be relatively high in the early morning and late afternoon, as compared to the rest of the day.
258
1. D. KALMA AND R. BADHAM
A similar but less p r o n o u n c e d effect with lower m a x i m a and higher minima in albedo is clear f r o m Fig.3B. This p h e n o m e n o n has been observed t h r o u g h o u t the experimental period. It effectively substantiates the suggestion, referred to above, that specular reflection m a y be responsible for part o f the increase in albedo at low solar elevations. The increase of albedo with increasing cloud cover has been reported previously for various vegetative surfaces (Rijks, 1967; Kalma and Stanhill, 1969). It is, however, contrary to results reported by Munn (1966) and Lin-sien-chia (1967). ACKNOWLEDGEMENTS The authors wish to acknowledge the technical assistance o f Messrs. D. E. Watson and A. G. Swan, and the assistance o f Mrs K. Haszler in c o m p u t e r programming. REFERENCES Budyko, M. I., 1958. The Heat Balance o f the Earth's Surface. U.S. Dept. Commerce, Weather Bur., Washington D.C. (transl. from Russian), 259 pp. Byrne, G. F., Rose, C. W., Begg, J. E., Torssell, B. W. R. and McPherson, H. G., 1971 (in press). Instrumentation for crop-environment measurement in a tropical savannah climate. C.S./. R. O., Aust., Div. Land Res. Tech. Pap., 32:19 pp. C.S.A.G.I., 1957. Instruction Manual, Pt. VI. Radiation instruments and measurements. Ann. Intern. Geophys. Yr., pp. 367-466. Davies, J. A. and Buttimor, P. H., 1969. Reflection coefficients, heating coefficients and net radiation at Simcoe, southern Ontario. Agric. Meteorol., 6: 373-386. De Vries, D. A., 1959. The influence of irrigation on the energy balance and the climate near the ground. J. Meteorol., 16: 256-270. Drummond, A. J., 1956. On the measurement of sky radiation. Arch. MeteoroL Geophys. BioMirnatol., 7: 413-436. Dyer, A. J., 1967. A combined water and energy balance study at Katherine, Northern Territory. Aust. Meteorol. Mag., 15: 148-155. Fitzpatrick, E. A., 1965. Climate of the Tipperary area. In: General Report on Lands of Tipperary Area, Northern Territory, 1961. C.S.I.R.O., Aust., Land Res. Set., 13: 39-52. Fitzpatrick, E. A. and Stern, W. R., 1965. Components of the radiation balance of irrigated plots in a dry monsoonal environment, J. AppL Meteorol., 4: 449-460. Flowers, E. C. and Helfert, N. F., 1966. Laboratory and field investigations of Eppley radiation sensors. Monthly Weather Rev., 94: 259-264. Haise, H. R., Hanks, R. J. and Jensen, M. E., 1967. Solar reflectance for soil and crop surfaces. Unpublished report quoted in: Irrigation o f Agricultural Lands (Monograph 11, American Society of Agronomy). Madison, Wisc., p. 513. Hart, H. E. and Rosenberg, N. J., 1968. An extended cylindrical shield for use with an inverted Eppley pyranometer to measure albedo over small areas. In: Horticulture Progress Report 60. Agric. Expt. Sta., pp. 30-42. Univ. of Nebraska, Lincoln, Nebr. Kalma, J. D., 1972. The radiation balance of a tropical pasture, II. Net all-wave radiation. Agric. Meteorol., 10, in press. Kalma, J. D. and Stanhill, G., 1969. The radiation climate of an irrigated orange plantation. Solar Energy, 12: 491-508. Kuhn, P. M. and Suomi, V. E., 1958. Airborne observations of albedo with a beam reflector. J. MeteoroL, 15: 172-174. Lin-Sien-Chia, 1967. Albedos of natural surfaces in Barbados. Q. J. R. Meteorol. Soc., 93: 116-120. Monteith, J. L., 1959a. The reflection of short-wave radiation by vegetation. Q. J. R. Meteorol. Soc., 85: 386-392.
THE RADIATION BALANCE OF A TROPICAL PASTURE
259
Monteith, J. L., 1959b. Solarimeter for field use. J. Sc£ Instr., 36: 341-346. Munn, R. E., 1966. Descriptive Micrometeorology. Academic Press, New York, N.Y., 245 pp. Oguntoyinbo, J. S., 1970. Reflection coefficient of natural vegetation, crops and urban surfaces in Nigeria. Q. Z R. Meteorol. Soc., 96: 4 3 0 - 4 4 1 . Reifsnyder, W. E., 1967. Radiation geometry in the measurement and interpretation of radiation balance. Agric. Meteorol., 41: 255-265. Rijks, D. A., 1967. Water use by irrigated cotton in Sudan, I. Reflection of short-wave radiation. J. Appl. Ecol., 4: 561-568. Slatyer, R. O., 1960. Agricultural climatology of the Katherine area, N.T.C.S.I.R.O., Aust. Div. Land Res., Regional Surv., Tech. Pap., 1 3 : 3 9 pp. Stanhill, G., 1970. Some results of helicopter measurements of the albedo of different land surfaces. Solar Energy, 13: 5 9 - 6 6 . Stewart, G. A., 1956. Softs of the Katherine-Darwin region, Northern Territory. C.S.I.R.O., Aust. SoilPubl., 6: 68pp.