ABSTRACT
II. Water Vapor Flow Under a Temperature Gradient...................................................................... 225 A. Simple Model....................................................... 225
1. Isothermal Diffusion of Water Vapor in Soils ................................................ 225
2. Enhancement Factor by One-Way Diffusion in Soils .......................................... 228
3. Nonisothermal Diffusion of Water Vapor in Soils ................................................ 230
4. Limitation of the Simple Model ................... 232 B. Modified Model.................................................... 233
1. Mechanistic Enhancement Factors of Water Vapor Diffusion.............................. 233
2. Condensation and Evaporation in Water Island Model ...................................... 235
III. Liquid Water Flow Under a Temperature Gradient...................................................................... 241 A. Driving Forces Due to a Temperature
Gradient............................................................... 241 1. Gradient of Total Head rct.......................... 241 2. Gradient of Matric Head rcm...................... 242 3. Gradient of Osmotic Head rco .................... 244
B. Coefficients of Flow Under a Temperature Gradient............................................................... 245
C. Liquid Water Flow Due to Water Content Gradient, Solute Concentration Gradient, and Temperature Gradient................................. 246
IV. Observations of Water Flow in Soils Under Temprature Gradients ............................................... 246 A. Observations in Closed Soil Columns ................ 246 B. Observations in Open Soil Columns .................. 248 C. Observations in a Field....................................... 250 D. An Observation with Salt Effects....................... 253
Appendix: Practical Estimation of Vapor Flux .................. 255 References............................................................................ 257
I. TEMPERATURE PROFILE AND SOIL WATER FLOW IN FIELDS
The temperature gradient of soil is a driving force for the movement of both liquid water and water vapor. Therefore, a daily change in the temperature profile may influence water movement in the soil. Figure 6.1 shows a typical daily change of wind velocity, net radiation from the atmosphere, relative humidity, and air temperature near the ground, measured simultaneously in a sand dune field in summer (1). The heights of each measurement are indicated in the figure. Figure 6.2 is the corresponding daily change of soil temperature and volumetric soil water content at each depth. Both the highest temperature and the lowest temperature appear at the land surface, being 488C at 1:00 p.m. and 238C at 5:00 a.m., respectively. Although the total amount of water in this soil is decreasing during 24 h of measurement, it should be noted in Figure 6.2 that there is an evident temporal increase in volumetric water content in the surface
soil at midnight, although it disappears with sunrise. This type of temporal moisture increase in the surface zone may be preferable for plant growth in such fields under arid and semiarid weather conditions. Both liquid water and water vapor move in such a sand dune field by the driving forces of gravity, matric potential gradient, temperature gradient, and, presumably, solute concentration gradient.
