Effect of yearly oscillating water table on soil moisture retention curves

Yearly oscillating water tables (WT) are not unusual in soils subjected to climates characterised by dry seasons alternating with wet seasons when the underground WT is not very deep (50–250 cm). This implies a time succession of moisture changes along the soil profile that can modify soil properties such as the fine pore porosity. The determination of the soil water retention curve (SWRC) in two fields over three seasons (1st winter, following summer, 2nd winter) highlighted significant shifts in the SWRC. These shifts could not be explained solely in terms of changes in physical parameters (mainly surface tension, entrapped air, water continuity, apparent contact angle). The discrepancy between theoretical and experimental results is usually greater in finer soils such as those under consideration. It is proposed that the observed changes are mainly due to reversible alteration of the chemical–physical and microbiological activity of the soil as supported by the high multiple regression coefficient found between the observed seasonal shifts of the SWRC as well as by the significance of the partial regression coefficients for clay–organic matter soil content, for depth of the WT from the sampling point and their interactions, independently of temperature effects.