Dynamics of Very near-Surface Water Contents in An Arid Setting.

Very-near surface water content (0-3 cm depth) affects many environmental processes, from evapotranspiration and carbon flux rates to ecological vigor and global water circulation.  In this study, we report on a soil water content data set collected over a two-year period from 48 locations at a field site in the Mojave Desert, USA. Data were collected every four hours using dual-probe heat pulse (DPHP) sensors. Sensors were installed in three research plots (control, disturbed and undisturbed biological soil crusts, and irrigation versus non-irrigated conditions). Of the 48 sensors, 42 were installed at ground surface (24 beneath desert shrubs and 18 in bare soil interspaces) and six sensors were installed at 5-cm depth. Data were corrected for ambient temperature fluctuations. Spatio-temporal variation of water content was analyzed using the spatial mean and variance before and after precipitation events and during spring dry downs, the skewness of the probability density function, and the spatial and temporal stability of water content.  The results are presented as both histograms and plots of mean relative difference that show trends in specific, treated soil surfaces.  Water content trends at sensors installed at ground surface showed significant positive skewness with time, illustrating the short time periods when soil were above mean (and residual) water content levels.  Water contents at 5-cm depth show a tendency for temporal bi-modality, resulting from the slower drying rates than those found at ground surface. The results are important given the water dependency of different biological crusts present at the soil surface and the role that biological crusts may play in the carbon cycle of this desert region.