Experimental study on water transport in soils under temperature gradients

M. Mizoguchi, J. Masuda and Y. Hattori

INTRODUCTION 
Soil in its natural environment is continuously subject to changing 
temperature and is always under non-isothermal conditions. It is, 
however, surprising that although isothermal water transport in soil 
has been well investigated, a study of non-isothermal soil water 
transport has not been advanced due to the lack of experimental 
studies. The purpose of this study is to measure the soil water 
content profiles in horizontally closed unsaturated soil columns 
under linear temperature gradients in relation to the initial water 
content, testuration, and soil type.

EXPERIMENT
 A specially designed apparatus was built to measure the water 
content profiles under temperature gradients. It consists of two 
water reservoirs and eleven horizontal containers made by metal 
tubes 60 cm long and 2.8 cm inside diameter. Both end parts (20 
cm) of each tube are in a reservoir the right one heated and the left
 one cooled . Thus a forced linear temperature gradient is 
established in the middle part (20 cm) of container. The whole 
apparatus is surrounded by foam insulation. The materials used in 
this testing were sand and sandy loam. The soils,  previously 
conditioned to various water contents, were carefully packed into 
metal tubes 20 cm long and 2.2 cm inside diameter. The rubber 
stoppers were inserted in both ends of each tube to prevent 
moisture loss. They were then put aside for one or two days at 
approximately 15'C to allow moisture equilibration in the tubes. 
After that, ten compacted soil columns were placed horizontally in 
each container in which the expected linear temperature gradient 
had been established . After they were left under the temperature 
gradient for the desired hours, the columns were pulled out of the 
containers and were then promptly sliced 2.5 cm in thickness for 
the measurement of water content . The temperature in a soil 
column was measured continuously by thermocouples. 

RESULTS AND DISCUSSION 
(a)Water redistribution: 
Fig.1 shows typical water content profiles found at the end of the 
respective experiments under a temperature gradient. The water 
content in the cooler part of soil increased substantially with time, 
while that in the warmer one decreased . This observation indicates 
that thermal gradient causes soil water transport from warmer to 
cooler region. Graphical integration of the water redistribution gives
 the cumulative water flux as a function of time. (Fig.2) The 
cumulative water flux increased gradually with time. 
(b)Mobility of unsaturated soil water: 
Fig.3 shows water content profiles in sand at 140 hours for various 
initial water contents, and Fig.4 shows the relationship of 
cumulative water flux to initial water content. The cumulative mater
 flux varied with the initial water content, and was most at the 
medium water content. The water content for sandy loam was 
higher than that for sand. These results suggest that there is a 
water content susceptible to temperature gradient for the soil. 
(c)Mechanism cf transfer: 
The water content profile in the sandy loam column, which was 
sandwiched nets 1 cm thick at 5 cm intervals to prevent liquid 
transport, is shown in Fig.1. The water content was discontinuous 
at the part of each net. And as shown in Fig.2, the cumulative water
 flux was less than that without the net. These results indicate that 
thermal gradient causes liquid transport rather than vapor in the 
range of this water content. It is that soil attribute to conclude 
from the results of both (b) and (c) water movement under 
temperature gradient is the co-operated transport of liquid and 
vapor. 

CONCLUSION
 Measurement of soil water content profiles has been made in 
horizontally closed unsaturated soil columns under forced linear 
temperature gradients . Consequently, it was found that (a)Thermal 
gradients cause soil water transport from warmer to cooler region. 
(b)There is a water content susceptible to temperature gradient for
 the soil. ( c )The soil water movement under temperature gradients 
attribute to the co-operated transport of liquid and vapor.