Measurement and modeling of the spatiotemporal dynamics of beach surface moisture content

The spatiotemporal dynamics of surface moisture exert a significant influence on the operation of aeolian transport systems at many beaches. However, we currently lack the detailed understanding of variability in surface moisture required to incorporate it into aeolian models. This problem is addressed here through direct measurements and simulation modeling of beach surface moisture over a twelve-day period, and through quantification of the relative contributions of evaporation, condensation and groundwater inputs to surface moisture. It was found that the beach surface can be characterized spatially in terms of three moisture zones: a consistently dry zone (<3%); a variable zone (3% to saturation); and a consistently wet zone (>40%). The relative influence of groundwater inputs was found to decrease moving landward, as the depth of the water table increased and the amplitude of tidally-induced water table fluctuations decreased. The critical pressure head (groundwater depth) at which evaporation begins to impose a demonstrable influence on surface moisture variability was found to be 90-100 cm. Temporally, beach surface moisture is a function of the lunar tidal cycle at longer-term time scales (weekly), and diurnal tidal and evaporation cycles. A numerical model was used to simulate capillary transfers of moisture from the water table to the surface, and moisture losses due to evaporation. The model was found to reliably replicate the measured spatiotemporal variability in surface moisture. In the dry zone, where most aeolian transport would be expected to occur, simulated moisture contents were typically within ±0.2% of measured volumetric contents.