Annals of Arid Zone-16(2), 195-2, 1977 Soil temperature variations in the red soil at Bangalore B.Y. RAMANA RAO AND R.Y. RAMAMOHAN Department of Physics University of Agricultural Sciences, Bangalore-5624. (Received on 12.5.75) ABSTRACT The air temperature and the soil temperatures at Scm, lscm, and 3cm depths observed at 72 hrs and 142 hrs 1ST in the red soil at Bangalore for the years 1971 to 1974 were analysed. The soil temperature gradients between Scm and IScm and IScm and 3cm depths were calculated. It is observed that the peaks in the soil temperature at different depths are similar to the peaks in the air temperature. However, the minimum values in the soil temperature do not occur in the same months as the air temperature. The soil temperature gradients are negative in the morning and positive in the afternoons. The temperature gradients are found to be uniform during the southwest monsoon. The importance of the soil temperature gradients in understanding the heat movement through the soil is discussed. INTRODUCTION Soil temperature plays a significant role in controlling seed germination, soil moisture availability and microbiological activities of the soil. The study of soil temperature with depth and its fluctuations throughout the year is a vital study in agriculture. Smith (1932) and Padmanabhamurty and Subrahmanyam (1963 ) analysed the soil temperature and its seasonal fluctuations. In the present. study, the air temperatures and the soil temperatures in the bare red soil of the Agricultural College farm, Hebbal, Bangalore '(Latitude 12 58'N, longitude 77 35'E and altitude 99 metres above M.S.L.) were analysed for different months. The heat movement through the soil depends upon the temperature gradients which will be ultimately controlled by the moisture content and thermal conductivity of the soil. Hence the mean temperature gradients for different months at different depths were also analysed. MATERIAL AND METHODS The daily soil temperature measurements were made at depths 5cm, 15 cm and 3 cm using bent bulb soil thermometers for the years 1971 to 1974. The air temperatures were also observed using a dry bulb thermometer exposed in standard
196 : B.V. RAMANA RAO AND R.V. RAMAMOHAN Stevenson's screen. The observations were taken twice a day at 72 hrs I.S. T. and 142 hrs I.S.T. regularly. The average daily soil temperatures for different depths and different months for the years 1971 to 1974 were calculated The mean daily air temperatures for different months were plotted in figure 1 The mean daily soil temperatures for different depths for different months were plotted in figure II. The mean temperature gradients for different months at different depths at 72 hrs I.S.T. and 142 hrs I.S.T. are given in Table 1. 72 Hrs I S T 142 Hrs I S T 34 3 IaI : 26 ::I t- «:. III A. 22 IaI t- o: <{ 18 14 J F MAMJ J A SON D MONTH FIG I. THE MEAN DAILY AIR TEMPERATURES DURING DIFFERENT MONTHS AT BANGAlORE
SOIL TEMPERATURE VARIATIONS IN THE RED SOIL AT BANGALORE : 197 45 INDEX 72Hrs Scm 1Scm -.-.- 3cm - - '42 Hrs -..---- -.----- 4 u 35 UJ : l- e{ : 3 UJ Q. UJ I- VI 25 2 15 J F M A M J J MONTH FIG 11. MEAN DAILY SOIL TEMPERATURES AT 5em,15em AND 3 em DURIN G DIF FERENT MONTHS A:r BAN GALORE A SON RESULTS AND DISCUSSION From figure I, it caijbe seen that there are two peaks during April and September months in the air temperature distribution both in the morning and afternoon values, the prominant peak being in the month of April. The minimum value in the morning temperature occurs during the month of January whereas the minimum value in the afternoon temperature occurs during the month of December. The variations in the morning temperature is particularly very less during the months July to October compared to the variations in the afternoon temperatures during the same months.
198: B. V. RAMANA RAO AND R. v. RAMAMOHAN From figure II, it can be seen that there are two peaks in the soil temperature at different depths during the months of April and September, the prominant peak being in the month of April due to excessive heating of the soil in summer. These two peaks can be seen for both morning and afternoon observations. The minimum value of the morning soil temperature at 5 cm depth occurs in the month of January just as the air temperature. However, the minimum values of the morning soil temperature at 15 cm and 3 em depths occur during the month of December. The sub-surface soil temperature at 5 cm depth is minimum during the month of August at 142 Hrs. I.S.T. but not in the winter months. Similar kinks can be seen in the air temperature and the soil temperature distribution curves for all the other depths during the month of August, though it does not coincide with the minimum value. The minimum values in the afternoon soil temperatures at 15 cm and 3 cm depths occur during the month of December only as in the case of air temperature. It is also noticed that the soil temperature fluctuations are less at 15 cm and 3 cm depths during the rainy season (June to October). In general, the variation in the soil temperature from morning to the afternoon is quite uniform at 3 cm depth which may be due to little variation in the moisture content at that depth during different months. It can be seen from Table 1, that the temperature gradients are negative in the morning and positive in the afternoon. The temperature gradients in the morning between 5 cm and 15 Cm depth are less and uniform for the months June to December varying from -O.3 Cj cm to -O.7 Cj cm. This period corresponds the south-west monsoon and the post-monsoon seasons and there will be moisture present in the soil, which increases the thermal conductivity (Baver, 1972) and reduces the temperature gradient. The temperature gradients in the morning between 15 cm and 3 cm depths are higher varying from -O.14 Cj cm to -O.24 Cj cm throughout the year except during the month of May and are almost uniform. However, there is slight decrease in the temperature gr adients in the afternoons at this depth during June to October. The soil temperature gradients in the afternoons between 5 cm and 15 cm depths are high in summer and low and uniform during the southwest monsoon season (June to September) and slightly more during the winter months. During the southwest monsoon season, the moisture content in the soil will be high which decreases the temperature gradients and hence the heat flow. The fluctuations in the temperature gradients are uniform varying between +.31 o Cj cm to + O.43 Cj cm during
SOIL TEMPERATURE VARIATIONS IN THE RED SOIL AT BANGALORE : 199 the months July to October instead of June to September as observed in the. shallow depths. In the absence of the thermal properties of wet soil, the soil temperature gradients will throw light on the heat movement through the soil as described in this paper. ACKNOWLEDGEMENTS The authors are thankful to Dr. B.S. Mann, Director, Central Arid Zone Research Instiute, Jodhpur. for valuable suggestions and encouragement given by him in preparing this article. The authors are also thankful to the Assistant Meteorologist, Main Research Station, Hebbal,.Bangalore for providing the necessary information used in the present study. REFERENCES Baver, L.D. 1972. Soil Physics, 5th Edn. John Wiley & Sons, New York pp. 273. Padmanabhamurty, B. and Subrahmanyam, V.P. 1963. Analysis of soil temperatures at Waltair. Indian. J. Met. Geophys. 14: 339-342. Smith, A. 1932. Seasonal sub-soil temperature variations. J. agric. Res. 44: 421.
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