Modeling spatial and temporal variations of surface moisture content and groundwater table fluctuations on a fine-grained beach, Padre Island, Texas

Abstract

The basic goals of this study are to document, represent and model beach surface moisture dynamics. Achieving these goals requires that the dynamics be understood within the context of the key associated processes including evaporation and groundwater table fluctuations. Atmospheric parameters including wind speed, air temperature and relative humidity, evaporation, beach surface moisture content, groundwater table fluctuations and tidal oscillations were directly monitored in an eight-day field experiment. Field measurements demonstrated that beach surface moisture content has a relatively high degree of variability in the cross-shore direction and a relatively low variability in the alongshore direction. The highest levels of variability were found in the middle beach, where daily fluctuations of up to 30% (volume) were common. Long-term variations in surface moisture content are controlled by water table fluctuations, while short-term variations are dominated by either evaporation or groundwater table fluctuations depending on the local water table depth. Two traditional methods to estimate potential evaporation rates were tested, the mass-transfer method and the combined (energy-budget and mass-transfer together) approach. Results showed that the mass-transfer method produces consistent large errors in simulations, even with recalibration of the constants. Simulations utilizing the Penman equation provide much better agreement with field data. It was found that groundwater table fluctuations at the studied beach are mainly forced by tidal oscillations. The numerical solution of the linealized Boussinesq equation provides an accurate approach to simulate tide-forced beach groundwater table fluctuations. These simulations were found to be significantly improved by modeling the system as a sloping beach rather than using the traditional vertical beach approach. Spatial and temporal variations in beach surface moisture were modeled using the numerically solved Richard’s equation and the Force-Restore method. The Force-Restore method underestimates surface moisture contents when the water table is relatively shallow owing to an inherent defect in the model itself. The simulations employing the numerically solved Richard’s equation agree closely surface moisture content from the field. This study represent perhaps the first, certainly the most comprehensive, attempt that has been made to date, to explain intermediate-scale variability in beach surface moisture content in light of the microscale process that drive these dynamics. The findings of this study should be applicable to longer time periods, and larger spatial areas with similar environmental settings. However, more investigations regarding hydraulic properties of local sediment are needed to enhance the model applicability.