The annual course of air temperature and near-surface soil temperature in a tropical Savannah environment

AgriculturalMeteorology- Elsevier PublishingCompany, Amsterdam- Printed in The Netherlands

THE ANNUAL COURSE OF AIR TEMPERATURE AND NEAR-SURFACE SOIL T E M P E R A T U R E

IN A TROPICAL SAVANNAH ENVIRONMENT

J. D. KALMA

Division of Land Research, Commonwealth Scientific and Industrial Research Organization, Canberra, A.C.T. (Australia) (Received June 5, 1970)

ABSTRACT

KALMA,J. D., 1971. The annual course of air temperature and near-surface soil temperature in a tropical savannah environment. Agr. Meteorol., 8: 293-303. By harmonic analysis periodic sine-functions were fitted to long-term five-day averages of maximum and minimum temperature of the air at screenheight and of the top 2 cm of bare soil for Katherine, N.T., Australia. A good simulation of the observed annual trend in all four elements was obtained by considering the first two harmonics only. The inclusion of higher harmonics did not lead to any significant improvement. The annual variation of extremes, mean values and ranges of air and soil temperatures and mean annual interrelationships have been studied and the interaction between soil-physical characteristics, rainfall pattern and other weather parameters discussed. A number of characteristic periods could be identified in the interrelationships between the soil and air temperature extremes, which are also climatologically well defined. It was concluded that wetting and drying of the soil are the major factors in determining the thermal regime of the topsoil and hence play an important role in the interrelationships between soil and air temperatures throughout the year. Some agronomic implications of the present climatological outline have been illustrated with reference to characteristic features in the life cycle of the pasture legume Stylosanthes humilis H.B.K., now well established in northern Australia.

INTRODUCTION I n n o r t h e r n Australia weather factors play a critical role during g e r m i n a t i o n a n d establishment of various crop a n d pasture species (NORMAN, 1960; WINKWORTH, 1969). Rainfall a n d e v a p o r a t i o n are traditionally considered the most i m p o r t a n t elements. SLATYER (1960) determined the primary characteristics of the growing season a n d the possibility of periods of water stress from rainfall, soil water characteristics a n d evapotranspiration. BYRNE a n d TOGNETTI (1969) considered the a m o u n t of rainfall to be the only factor determining the start of the growing season o f the pasture legume Townsville stylo. It is generally accepted that the thermal regime of seed bed a n d root env i r o n m e n t are of great i m p o r t a n c e in germination, seedling survival, crop es-

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J.D. KALMA

tablishment and subsequent growth (HARRINGTONand MINGES, 1954; BROUWER, 1962; VAN RIJN, 1968). In the present paper the annual variation in thermal conditions of the top 2 cm of soil, determined by the interaction between soil physical characteristics, rainfall pattern and other weather parameters, is studied for Katherine, N.T., Australia. SITE AND METHODS

Measurements were made within the meteorological enclosure of Katherine Research Station, C.S.I.R.O., (14.3°S, 132.2°E; 108 m + M.S.L.). Climatic characteristics of the Katherine region have been described in some detail by SLATYER(1960) and FITZPATRICK(1965).The environment is tropical savannah with a hot wet season from October to April and a warm dry winter from May to September. Annual rainfall is 90 cm, 76 cm falling in the four months December to March. The soil is Tippera clay loam, a lateritic red earth overlying Cambrian limestone (STEWART, 1956). From February 24, 1954 to November 26, 1967 a Negretti and Zambra thermograph was in operation at the site. The centre of its mercury-in-steel thermometer was 1.3 cm below the untilled bare soil surface. The diameter of the thermometer is 1.8 cm. Weekly charts were read and maximum and minimum temperatures for the 0.4-2.2 cm layer (subsequently referred to as soil temperatures) on each day listed. Comparison between this and other thermometers was carried out at yearly intervals. The overall accuracy of the system is estimated to be within + 0.4 °C. Mean values of maximum and minimum soil temperatures were computed for all 73 five-day intervals (pentads) of each year. Average values of these two extremes and their standard deviation for each pentad were calculated from all 14 years combined. By harmonic analysis, periodic sine-functions, consisting of two harmonics, were fitted to the 73 pentad-averages of maximum and minimum soil temperatures. Similarly daily values of maximum and minimum air temperatures for the same 14 years, read from two calibrated alcohol-in-glass thermometers at 2 m above the ground in the standard meteorological screen at 9 a.m. (hereafter: air temperatures), were averaged for each pentad of each year. These means were averaged over all years and by harmonic analysis periodic sine-functions with two harmonics were fitted to these data as well. Temperatures were recorded in °F and all calculations were done in the same units. The fitting by least squares of a series of sine-functions (Fourier-series) to an array of data arranged in their natural time-sequence has been described in Agr. MeteoroL, 8 (1971) 293-303

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AIR AND SOIL TEMPERATURE RELATIONS

various basic texts (e.g., BROOKS and CARRUTHERS, 1953). F o r the present study a digital c o m p u t e r p r o g r a m m e was used, r a t h e r similar to t h a t p r e p a r e d b y FITZPATRICK (1964). This p r o g r a m gives the m e a n o f the 73 values which m a k e u p the o b s e r v a t i o n a l a r r a y a n d calculates a m p l i t u d e a n d phase angle o f each h a r m o n i c . M o r e o v e r total variance, variance c o n t r i b u t e d by each h a r m o n i c , a n d c u m u l a t i v e variance c o n t r i b u t e d b y successive h a r m o n i c s are calculated. A d d i t i o n a l o u t p u t includes time(s) in r a d i a n s c o r r e s p o n d i n g to the first m a x i m u m a n d m i n i m u m value o f each h a r m o n i c . RESULTS A n n u a l a n d semi-annual trends are sufficiently accurately described b y the first a n d second harmonics. However, in the present study six h a r m o n i c s were calculated to enable the possible i m p r o v e m e n t f r o m inclusion o f some higher h a r m o n i c s to be determined. Table I presents the results o f F o u r i e r analysis o f p e n t a d - a v e r a g e s o f extreme soil a n d air t e m p e r a t u r e values. T a b l e I includes the n u m b e r o f those p e n t a d s in which the elements T a M a x ( m a x i m u m air t e m p e r a t u r e ) , TaMin ( m i n i m u m air t e m p e r a t u r e ) , T s M a x (maxim u m soil t e m p e r a t u r e ) a n d TsMin ( m i n i m u m soil t e m p e r a t u r e ) reach extreme TABLE I RESULTS OF CURVE FrIW1NG BY FOURIER ANALYSIS TO FOUR OBSERVATIONAL ARRAYS ( 1 ~ ) , CONSISTING OF

73

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(see text)

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Obs. variance Max. (1) Min. (1) Max. (2) Min. (2) Res. variance (1) Res. variance (2) Res. variance (3) Res. variance (4) Res. variance (5) Res. variance (6)

19.46 68 31 21, 58 39, 3 5.59 (28.7~) 0.35 (1.8~/o) 0.34 (1.8Yo) 0.32 (1.79/o) 0.31 ( 1 . 6 ~ ) 0.31 (1.69/o)

61.28 1 38 22, 58 40, 3 6.55 (10.7~) 1.46 ( 2 . 4 ~ ) 0.98 ( 1 . 6 ~ ) 0.84 ( 1 . 4 ~ ) 0.80 ( 1 . 3 ~ ) 0.79 ( 1 . 3 ~ )

72.93 60 23 22, 58 40, 4 24.40 (33.5~) 4.51 ( 6 . 2 ~ ) 4.31 ( 5 . 9 ~ ) 4.27 (5.9yo) 3.83 (5.3~o) 3.78 ( 5 . 2 ~ )

69.55 2 38 21, 58 40, 3 9.90 (14.2~) 1.74 (2.5%) 1.49 (2.2%) 1.39 ( 2 . 0 ~ ) 1.35 ( 1 . 9 ~ ) 1.28 ( 1 . 8 ~ )

1. Maximum air temperature (TaMax); TaMax = 93.22 + 5.27 sin (x + (2x + 4.381) °F. 2. Minimum air temperature (TaMin); TaMin = 67.62 + 10.46 sin (x + (2x + 4.295) °F. 3. Maximum soil temperature (TsMax); TsMax = 123.97 + 9.85 sin (x + (2x + 4.250) °F. 4. Minimum soil temperature (TsMin); TsMin = 67.85 + 10.92 sin (x + (2x + 4.348) °F.

2.105) + 3.24 sin 1.545) + 3.19 sin 2.808) + 6.31 sin 1.508) + 4.04 sin

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values. Note that within the annual cycle the first harmonic has one maximum and one minimum (separated by 36-37 pentads) whereas the second harmonic has two maxima and two minima (successive extremes being separated by 18-19 pentads). The residual variance values given in Table I are total observational variances after removal of the variances accounted for by the various harmonics. The number in brackets indicates the number of harmonics being considered. ' Fig.1 presents the first and the second harmonic and the two harmonics combined, as calculated for the four elements. The calculated average values for all 73 pentads are also presented in this figure for TaMax, TaMin, TsMax, and TsMin and it may be noted that, when the first and second harmonics are combined, a good simulation of the observed annual trend in each of the four elements is obtained. It is believed that this method of data handling provides an objective way .

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Fig.2. Annual re]ationships between extremes of air temperature (A), extremes of soil temperature (B), minimum air temperature and minimum soil temperature (C) and maximum air temperature and maximum soil temperature (D) from long-term pentad-averages at Katherine, N.T. Roman numbers refer to characteristic periods. Some pentads are indicated by number.

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of describing and analysing the annual trend in TaMax, TaMin, TsMax and TsMin. In the subsequent discussion of interrelationships between the various elements, 73 pentad-values were calculated for each of the four elements, using the equations given in Table I. When pentad-values of maximum and minimum air temperature (TaMax and TaMin) are plotted against each other (Fig.2A), an annual cycle including all 73 points is obtained, very similar to those obtained for a number of other northern Australian localities. These cycles are especially pronounced for areas with welldefined dry and wet seasons. Closer inspection of Fig.2A reveals five characteristic periods. Period 1 (late June-late October) is marked by an increase in TaMax and in TaMin; during period H (late October-late November) TaMin still increases while TaMax drops; in period III (late November-middle of February) TaMax drops more rapidly than TaMin; period I V (middle of February-late March) is characterized by a slight increase in TaMax and a rapid drop in TaMin; and finally period V (late March-late June) is marked by decreasing TaMax and TaMin. The same, though more pronounced, annual cycle is evident in Fig.2B where TsMax is plotted against TsMin. The secondary minimum TsMax in the middle of February is more pronounced than that in TaMa.¥ at the same time of the year. Period IV lasts a fortnight longer in Fig.2B. However, some exaggeration of the trends of the relationships of Fig.2A and Fig.2B around pentad 9 may occur because of the limited number of harmonics (2) used in this analysis. Fig.2C shows that TaMin and TsMin approach a 1 : 1 relationship. Throughout the year absolute differences between the five-day averages of minimum air and soil temperatures are never more than 0.8°C. Recent experimental results obtained at Katherine suggest that surface temperature minima are very unlikely to be more than 0.6°C lower than those at 13 mm below the surface (TsMin). It therefore follows that a close relationship exists throughout the year between TaMin, TsMin and the minimum surface temperature. Fig.2D shows more clearly that the differences between the annual cycles of Fig.2A and 2B are almost completely due to a distinct annual cycle in the relationship between TsMax and TaMax. DISCUSSION The important contributions of the first and second harmonics in simulating the annual variation of the four temperature extremes indicate the importance of distinct annual and semi-annual tendencies. The residual variances presented in Table I indicate that the inclusion of higher harmonics does not lead to significant improvement in the simulation. In contrast to the very low variability of maximum and minimum air temperature throughout the year observed by SLATYER (1960) and reconfirmed Agr. MeteoroL, 8 (1971) 293-303

AIR AND SOIL TEMPERATURERELATIONS

299

in the present study, a much larger coefficient of: variation of TsMax was observed over the 14 years of the present study, which shows a marked seasonal trend with maxima of about 12 ~o in the rainy summer season and minima of about 4 ~o in the dry winter. The coefficient of variation of TsMin was about 3 ~o in summer and 11 ~ in winter. Presence of rainfall apparently leads to increased variability in TsMax and decreased variability in TsMin. Mean daily air temperature (Ta) is normally estimated from (maximum air temperature -t- minimum air temperature)/2. This method gives also an acceptable, though less accurate estimate of mean daily soil temperature (Ts). F r o m the equations of Table I, Ta and Ts were thus calculated for all pentads, and the daily amplitudes of air and soil temperatures were also obtained (ATa and ATs, respectively). Fig.3 shows the annual course of Ta, Ts, ATa, and ATs, and also the mean total rainfall for the same 14 years for each pentad. Note that the minima in Ta and Ts coincide (first week of July) but that their maxima are separated by about a fortnight (around the first week of November). As shown earlier (Fig.2C)

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J.D. KALMA

TaMin and TsMin are very similar. Any seasonal trend in (Ts-Ta) is therefore primarily due to differences between TsMax and TaMax. Fig.3 shows that a maximum in (Ts-Ta) of about 11 °C is reached just before the onset of the rainy season, and a minimum of 7°C ,when the rainfall reaches its culminating point. The annual trend in (Ts-Ta) is very likely to be caused by the annual variation of soil moisture content and hence, especially during daytime, by variation in the size of the temperature gradients in the top-soil and of the diurnal temperature fluctuation at the soil surface. The heat content of air and soil columns extending a certain distance above and below the soil surface changes at a rate equal to the sum of the net rates at which radiative energy is absorbed, sensible heat is exchanged with higher and lower layers and surroundings, and latent heat is released by condensation or used by evaporation. Net radiative energy at the surface is used during daytime for warming air, evaporating water and warming soil, whereas at night the loss of radiative energy is compensated by sensible heat transfer to the surface mainly from deeper soil layers and latent heat release by condensation. Both thermal conductivity (2) and heat capacity (C) increase with increasing soil moisture content. Thus, as the soil becomes wetter more heat is required for evaporation, while in addition to heat conduction, heat may also be transferred by mass movement of water as liquid and vapour transport in the soil (DE VRIES, 1963). As shown in Fig.3, the above is confirmed by the annual variation in ATs. VAN WIJK and DE VRIES (1963) clearly show that in the absence of free convection, the ratio of partitioning of net radiative energy between soil and air increases as the soil becomes wetter. Under those circumstances the soil "consumes" more energy (greater storage and transmission properties for heat) which will lead, together with an increase in latent heat release by evaporation, to less heating of the soil surface and hence smaller (Ts-Ta) values. The fact that ATa starts decreasing about 40 days earlier than ATs is difficult to explain. The overall annual trends in Ts and Ta, presented in Fig.3, are very similar (except for the secondary minimum and maximum in Ts in February). They show that temperatures generally increase rapidly between early July and early November, are rather constant from late January to late March, and decrease during the rest of the year. The climatic descriptions of the Katherine area given by SLATVER (1960) and FITZPATRICK (1965) indicate that the five characteristic periods of Fig.2 are also climatologically clearly defined. The rainless period I is marked by a gradual trend to warmer conditions until the onset of the summer period characterized by the wane of the S.E. trade winds. During this period total increase in daily global radiation is about 100 ly and average daytime cloudiness increases rapidly (BUREAU OF METEOROLOGY,1961). The soil is very dry. The occasional afternoon thunderstorms of the transitional period H charAgr. Meteorol., 8 (1971) 293-303

AIR AND SOIL TEMPERATURE RELATIONS

301

acterized by variable winds, start wetting the soil and cause a decrease in diurnal amplitudes (higher maximum and lower minimum temperatures) while the soil temperature maxima drop more rapidly than maximum air temperature. Total daily solar radiation in this period increases with another 30 ly while daytime humidity (SLAXVER, 1960) and cloudiness both increase gradually. The rapid increase of rainfall, reaching its maximum in late December/early January, is covered by period IlL In early January the Inter-tropical Convergence Zone (I.T.C.Z.) reaches the Katherine area with, irregularly, very moist maritime air causing rain and overcast skies. During this period atmospheric moisture content and cloudiness increase rapidly and incoming solar radiation, after a slight drop over December, stays constant at about 510 ly/day. Total soil moisture content increases rapidly throughout this period. Period I V is a second transitional period during which the I.T.C.Z. returns northward, rain becomes less frequent, and cloudiness decreases. Occasional thunderstorms occur. Incoming solar radiation is still at 510 ly/day but the decreasing cloudiness results in greater radiative cooling of the surface at night. This period is marked by successive periods of wetting and drying of the top soil. Period V is characterized by a drop of about 70 ly/day in incoming solar radiation. Cloudiness and atmospheric moisture content decrease. Rain is usually negligible after the end of April and the return of the south-easterlies in late April/early May brings settled, dry and sunny periods. During this period the soil dries continuously. The periodicity of air and soil temperatures in the Katherine region is especially complicated through annual and diurnal variation in the properties of advected air, transported from other regions and flowing over a surface to whose thermal and moisture conditions it has not yet become adjusted (VAN WIJK and BORGHORST, 1963). This advection of un-adjusted air may be from distant sources (FITZPATRICK, 1968) or local s o u r c e s (SELLERS, 1965). The southward progression of the Inter-tropical Convergence Zone is an example of distinct change in macroadvectional conditions. Microadvection occurs throughout most of the year because the average surface temperature of the bare soil within the meteorological enclosure is very likely to be higher than that of the surrounding vegetative surface, so that the meteorological enclosure is being supplied with colder air from its surroundings. A quantitative assessment of the role advection plays in the interrelationships between soil and air temperature is hardly possible. As pointed out earlier, the present study is primarily concerned with interactions between the physical environment of the top 2 cm of the soil and the prevailing weather. Bare undisturbed soil is for a greater part of the year rather unnatural but in contrast to tilled soil or soil under a vegetative cover, it may be readily described and reproduced. Attention is directed mainly to annual temperature interrelationships. The long series of measurements, the five-day periods used and the method of data Agr. Meteorol., 8 (1971) 293-303

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J.D. KALMA

handling, inevitably restrict the present study to a climatological outline of the mechanisms involved. Some of the agronomic implications of the present results may be illustrated for the pasture legume Townsville stylo (Stylosanthes humilis H.B.K.). This selfseeding annual is now well established in northern Australia. The characteristic features of the life cycle of Townsville stylo have been given by TORSSELLet al. (1968). The bare soil conditions maintained in the present study throughout the year are, to a fair approximation, equivalent to Townsville stylo pastures during the late dry season especially after heavy grazing (end of period I), during the transition period between the dry and the wet season (period H) and the early wet season (first half of period III). Germination and seedling establishment of this legume take place in p e r i o d / / a n d the first half of period IlL Fig.2 and 3 demonstrate clearly that germination and pasture establishment take place during a period of rapid, but often erratic change in environmental conditions. At the beginning of period H maximum air and soil temperatures are at their highest (about 38 °C and 60 °C, respectively) and the soil is still extremely dry. Thereafter, all temperatures decrease rapidly whereas rainfall increases and optimum conditions for germination and establishment are obtained. The variability of maximum soil temperatures is highest at this time of the year and is mainly caused by the very pronounced variation of rainfall within any one season. Germination and successful seedling establishment indicate the start of the growing season. SLATYER (1960) estimated that the growing season normally starts on a mean date of December 17 (i.e., in pentad 71) with the standard deviation from the mean being + 2.3 weeks. BYRN~and TOGNETT1(1969) presented simulated yield curves for eight seasons showing the start of the growing season in different years as far apart as 40 days. Against the background of the present climatological outline, the interactions between environmental factors such as rainfall and radiation and the thermal root-environment at Katherine has been studied in great detail on the basis of a recent series of short-term temperature profile measurements. Results of this analysis will be reported in a future paper. ACKNOWLEDGEMENTS

For their continued interest and criticisms of a draft of this paper, I am indebted to my colleagues Dr. J. E. Begg, Dr. D. A. Rose and Dr. B. W. R. Torssell. The assistance of Mrs. A. Komarowski in data extraction and of Mrs. K. Haszler in data processing is gratefully acknowledged.

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REFERENCES BROOKS, C. E. P. and CARRUTHERS, N., 1953. Handbook of Statistical Methods in Meteorology. Her Majesty's Stationary Office, London, 412 pp. BROUWER, R., 1962. Influence of temperature of the root medium on the growth of seedlings of various crop plants. In: Jaarboek Inst. Bodemk. Studies, Wageningen, pp.11-18. BUREAU OF METEOROLOGY, 1961. Climatological Survey - Region 1, Darwin - Katherine, Northern Territory. Dept. of Supply, Melbourne, 50 pp. BYRNE, G. F. and TOGNETTI, K., 1969. Simulation of a pasture-environment interaction. Agr. Meteorol., 6: 151-163. DE VRIES, D. A., 1963. Thermal properties of soils. In: W. R. VAN WUK, (Editor), Physics of Plant Environment. North Holland, Amsterdam, pp.210-235. FITZPATRICK, E. A., 1964. Seasonal distribution of rainfall in Australia analysed by Fourier methods. Arch. Meteorol., Geophys. Bioklimatol., Ser. B, 13(2): 270-286. 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., Land Res. Ser., 13: 39-52. FITZPATRICK, E. A., 1968. An appraisal of advectional contribution to observed evaporation in Australia using an empirical approximation of Penman's potential evaporation. J. Hydrol., 6(1): 69-94. HARRINGTON, J. F. and MINGES, P. A., 1954. Vegetable Seed Germination. Univ. Calif. Ext. Serv., 11 pp. (mimeo). NORMAN, M. J. T., 1960. The establishment of pasture species on arable land at Katherine, N.T. C.S.I.R.O., Div. Land Res., Regional Surv., Tech. Papers, 8:18 pp. SELLERS, W. D., 1965. Physical Climatology. Univ. Chicago Press, Chicago, London, 272 pp. SLATYER,R. O., 1960. Agricultural climatology of the Katherine area. C.S.I.R.O., Div. Land Res., Regional Surv., Tech. Papers, 13:39 pp. STEWART, G. A., 1956. Soils in the Katherine-Darwin region, Northern Territory. C.S.I.R.O., Div. Land Res., Regional Surv., Soil Publ., 6 : 6 8 pp. TORSSELL, B. W. R., BEGG, J. E., ROSE, C. W. and BYRNE, G. F., 1968. Stand morphology of TownsviUe lucerne. Australian J. Exptl. Agr. Animal Husbandry, 8: 533-543. VAN RUN, P. J., 1968. Ecological aspects of weed control in cotton in the Ord River valley, W.A., 1. Conditions affecting germination of weeds. Australian J. Exptl. Agr. Animal Husbandry, 8: 620-624. VAN WIJK, W. R. and BORGHORST, A. J. W., 1963. Turbulent transfer in air. In: W. R. VAN WUK, (Editor), Physics of Plant Environment. North Holland, Amsterdam, pp.236-276. VAN WIJK, W. R. and DE VRIES, D. A., 1963. Periodic temperature variations in a homogeneous soil. In: W. R. VAN WIJK, (Editor), Physics of Plant Environment. North Holland, Amsterdam, pp.102-143. WINKWORTH, R. E., 1969. Germination of Townsville lucerne (Stylosanthes humilis H.B.K.) in relation to weather at Katherine, N . T . J . Australian Inst. Agr. Sci., 35: 201-204.

Agr. Meteorol., 8 (1971) 293-303