Water transport in soil with a daily temperature wave. I. Theory and experiment

The theory of water transport in soil is summarized for situations where flux can arise from gradients in temperature as well as in soil water suction and gravitational potential. The general differential equation describing such transport was doubly integrated to provide a form of equation suitable for the analysis of experimental data. An experiment is described in which profiles of water content and temperature were measured in the top 15 cm of bare soil for a period of 6 days and nights following saturation. High daytime radiation conditions resulted in maximum temperature gradients associated with the daily temperature wave in the soil approaching 10 degC cm-1, thus favouring vapour phase in relation to liquid phase water transport. The basic soil data necessary for the quantitative analysis of moisture fluxes into liquid and vapour components were determined. When water content at the eight sampling depths was plotted against time from saturation there was superimposed on a regular decline in water content an approximately sinusoidal fluctuation of daily period with a maximum around 0400-0600 hr local time and a minimum about 1600 hr. These results may be interpreted as indicating the existence of a vapour flux in the soil of magnitude comparable with the liquid flux. This hypothesis is examined quantitatively in Part II (C. W. Rose 1968).