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A note on temparature variation in the muguga lysimeter system
Agricultural Meteorology - Elsevier Publishing C o m p a n y , A m s t e r d a m - Printed in The Netherlands
A NOTE ON T E M P A R A T U R E VARIATION IN THE M U G U G A L Y S I M E T E R SYSTEM 1:. J. WANGATI
East African Agriculture and Forestry Research Organization, Kikuyu (Kenya) (Received March 3, 1964)
The height of the water column balancing the pressure exerted by the lysimeter on the bolsters can only be used as a measure of change in the weight of the lysimeter if there are no significant errors due to thermal expansion of the water in the column. In the Muguga installation every possible care has been taken to avoid temperature fluctuations; the measuring column is water jacketed, enclosed in a vermiculite-filled tube inside an insulated stone outhouse. Temperatures were measured to 0.1°C by copper-constantan thermocouples recording continuously on a multichannel recorder. One thermocouple was near the meniscus, a second was placed 18 inches below the meniscus, a third on the buried connecting pipe and a fourth affixed to the surface of one of the bolsters with a piece of flat plastic and araldite. This last thermocouple was heavily insulated with pads of wool to reduce the effect of changes of ambient temperature in the pit. During the period of observation no rain fell, radiation values up to 650 cal./cm z day were recorded while mean air temperature was 16°C with a diurnal range at screen height of 12°C.
RESULTS
Maximum temperature variations amounted to 4.5cC in the top 6 inches of the measuring column and 3.3°C some 18 inches down, lagging about 6 h behind maximum air temperature; the temperature of the connecting pipe and the surface of the bolsters varied respectively by 0.7°C and 0.5°C with an 8 h lag. These results may be used to estimate fluctuations in height of the balancing column due to temperature. Neglecting the consequence of temperature change at the bolsters, and assuming that column height hi, at temperature T1, becomes h e at temperature Te:
gp2h2 =
gpihl
where p, the density of water, changes by 0.00015/~C, and g is the acceleration due to gravity. Agr. Meteorol., 2 (1965) 53-54
54
F. J. WANGA~J
Substituting P2 = Pl (1 - - 0.00015 A T) and h 2 A h 0.00015h 1 .............. A T 1 --0.00015 A T
=:
h 1 ~' A h :
0.00015h 1
F o r the underground connecting pipe A T
0.7;C and h, ~:- 2 m:
A h .... 0.21 m m F o r the measuring column, assuming mean temperature variation o f 3°C and h ...... 1 m: A h -- 0.45 mm Hence total fluctuation: Ah :
0.66 m m
CONCLUSION
Despite the care taken in lagging the measuring column, the diurnal variation in c o l u m n height due to thermal expansion is measurable, and the lysimeter system cannot therefore be used for hourly observations. However, in field applications the consequences o f temperature change can be minimised by observation o f the column-height at a suitable standard time each day; for more detailed measurements throughout the course o f a day, compensation for fluctuations in column height due to thermal expansion must be effected,
Agr. Meteorol,, 2 (1965)
53-54
A NOTE ON T E M P A R A T U R E VARIATION IN THE M U G U G A L Y S I M E T E R SYSTEM 1:. J. WANGATI
East African Agriculture and Forestry Research Organization, Kikuyu (Kenya) (Received March 3, 1964)
The height of the water column balancing the pressure exerted by the lysimeter on the bolsters can only be used as a measure of change in the weight of the lysimeter if there are no significant errors due to thermal expansion of the water in the column. In the Muguga installation every possible care has been taken to avoid temperature fluctuations; the measuring column is water jacketed, enclosed in a vermiculite-filled tube inside an insulated stone outhouse. Temperatures were measured to 0.1°C by copper-constantan thermocouples recording continuously on a multichannel recorder. One thermocouple was near the meniscus, a second was placed 18 inches below the meniscus, a third on the buried connecting pipe and a fourth affixed to the surface of one of the bolsters with a piece of flat plastic and araldite. This last thermocouple was heavily insulated with pads of wool to reduce the effect of changes of ambient temperature in the pit. During the period of observation no rain fell, radiation values up to 650 cal./cm z day were recorded while mean air temperature was 16°C with a diurnal range at screen height of 12°C.
RESULTS
Maximum temperature variations amounted to 4.5cC in the top 6 inches of the measuring column and 3.3°C some 18 inches down, lagging about 6 h behind maximum air temperature; the temperature of the connecting pipe and the surface of the bolsters varied respectively by 0.7°C and 0.5°C with an 8 h lag. These results may be used to estimate fluctuations in height of the balancing column due to temperature. Neglecting the consequence of temperature change at the bolsters, and assuming that column height hi, at temperature T1, becomes h e at temperature Te:
gp2h2 =
gpihl
where p, the density of water, changes by 0.00015/~C, and g is the acceleration due to gravity. Agr. Meteorol., 2 (1965) 53-54
54
F. J. WANGA~J
Substituting P2 = Pl (1 - - 0.00015 A T) and h 2 A h 0.00015h 1 .............. A T 1 --0.00015 A T
=:
h 1 ~' A h :
0.00015h 1
F o r the underground connecting pipe A T
0.7;C and h, ~:- 2 m:
A h .... 0.21 m m F o r the measuring column, assuming mean temperature variation o f 3°C and h ...... 1 m: A h -- 0.45 mm Hence total fluctuation: Ah :
0.66 m m
CONCLUSION
Despite the care taken in lagging the measuring column, the diurnal variation in c o l u m n height due to thermal expansion is measurable, and the lysimeter system cannot therefore be used for hourly observations. However, in field applications the consequences o f temperature change can be minimised by observation o f the column-height at a suitable standard time each day; for more detailed measurements throughout the course o f a day, compensation for fluctuations in column height due to thermal expansion must be effected,
Agr. Meteorol,, 2 (1965)
53-54