A COMPONENT OF ATMOSPHERIC VAPOR IN THE WATER OF FLOODPLAIN VERTISOLS

Soil water is a vital ecosystem resource. The capacity of soils to retain water between rainfall events is critical for life on land. In addition, the availability of soil water is one of the most useful indicators of drought. Clay-rich soils are especially water-retentive and are thus important for productivity in dry climates. In this study, we evaluate and measure the absorption of atmospheric water vapor deep within Vertisols of the Trinity River floodplain in central Texas, USA. The high smectite clay content in these soils results in deep cracking during the summer and a large surface area making the soils highly hydrophilic. These cracks provide air-filled conduits through which atmospheric water vapor diffuses and condenses in the cool subsurface at depths of 0.8 to at least 1.4 meters below the ground surface. Oxygen and hydrogen isotope compositions of this deep soil water plot to the left of the local meteoric water line, confirming the presence of a diffused water vapor component. In addition, the deep soil is warmer than simulated with heat flow modeling during July and August, consistent with condensation and latent heating. Mass balance calculations suggest that during this time of year 3-4% of the soil water at 0.8 - 1 meters is condensed atmospheric water vapor, which may be able to enter dry ped interiors more easily than liquid water can. This downward flux of water vapor partially replaces water lost to transpiration and may therefore mitigate drought.