Water transport in soil with a daily temperature wave. II. Analysis

Abstract

The results of a field experiment investigating water transport in soil with a strong daily temperature wave have been analysed according to theory, both theory and results having been given in Part I (C. W. Rose 1968a). Analysis yielded the history, for six days and nights following saturation, of separate net flux densities into two contiguous layers of the soil profile which were 1-3 cm and 3-12 cm below the soil surface. In this experiment, where maximum daytime temperature gradients in the soil approached 10 degCcm-1, net vapour flux densities into both soil layers were of comparable magnitude to transfer in the liquid phase, even at suctions as low as 200 cm of water. At suctions greater than about 5000 cm of water, changes in water content were achieved almost entirely by vapour transport. From the commencement of observations liquid phase water flux caused by gravity was small compared with that arising from gradients in soil water suction, and, as will commonly be the case, liquid flux arising directly from temperature gradients was also relatively unimportant. In contrast with the dominantly upward liquid phase movement through the profile, the direction of vapour flux oscillated in response to the diurnal temperature gradient, vapour flux therefore being downward during the day, except in a very shallow surface layer. It was upward during the night when the surface was cooler than the soil below, this being the main reason for the pronounced night-time increase in water content of the surface layers. Maximum errors arising from both experimental and analytical sources were analysed as uncertainties in a vapour flux enhancement factor defined in the theory. Despite these uncertainties there was some evidence of enhancement of the vapour flux, and analysis showed that the assistance to vapour flux that can be provided by a discontinuous liquid phase was the most likely cause of such enhancement in this experiment. Some implications of the work described are briefly reviewed.