Groundwater - vegetation - atmosphere interactions in an intertidal salt marsh [electronic resource]
Keywords: Environmental Earth System Science

A large fraction of coastal wetlands worldwide have been severely impacted by development, resulting in among the highest losses of any wetland type. Necessary improvements in restoration and management of coastal wetlands require a better scientific understanding of the underlying plant-water interactions, or ecohydrology. This research developed a new conceptual model of intertidal salt marsh ecohydrology to define the relative roles of: tidal flooding, groundwater flow, vegetation zonation, and plant water uptake. Spatial and temporal variations in plant-water interactions were observed over three years at a field site in the Palo Alto Baylands, California. Three-dimensional numerical simulations of the coupled surface water and unsaturated groundwater flow and evapotranspiration (ET) at the site were used to explore the links between marsh vegetation and hydrology. /// Vegetation zonation is one of the most distinctive properties of salt marshes, yet had not been combined with physics-based hydrologic analysis prior to this research. Statistical analysis showed that vegetation zones at the field site were not correlated with traditional proxies for hydrologic influences such as elevation and distance-to-channel. Vegetation zonation was strongly correlated with a metric describing the spatial patterns of tidally-induced changes in salt marsh soil saturation and salinity. This metric was developed based on time-lapse imaging of bulk soil electrical conductivity and a new geophysical analysis method, Quantitative Differential Electromagnetic Induction imaging (Q-DEMI). /// Spatial variations in vegetation water use within and among vegetation zones were investigated in detail using centimeter-resolution thermal infrared (TIR) remote sensing. Well-established latent heat models were adapted to use spatially-variable canopy stomatal resistances. The detailed stomatal resistance maps were determined from the TIR data in a biophysically realistic manner by a new method. In principle, the stomatal resistance mapping method is applicable at scales from leaves (such as in this study) to landscapes. /// The dynamics of plant-water interactions originating at the leaf scale were also detectable in marsh-scale eddy covariance and meteorological field data. Alternating daytime tidal flooding and exposure shifted the marsh surface energy balance: from similar to a well-watered lawn during flooding, to similar to a sparse crop during exposure. The net ecosystem exchange of carbon dioxide was also temporarily suppressed in proportion to flood depth and duration, further indicating close plant-water coupling in the intertidal salt marsh environment. /// These spatial and temporal plant-water interactions occur within a larger context governed by the tidal regime and coastal groundwater flow. Continuous measurements of groundwater potential characterized marsh groundwater dynamics and provided evidence of sediment heterogeneity at the field site. In three dimensional, coupled groundwater-surface water simulations, the sediment heterogeneity affected both the balance between creek bank and interior marsh hydrologic processes and the spatial distribution of groundwater-surface water exchange. In the field, similar groundwater discharge zones were located in the tidal channels by fiber-optic distributed temperature sensing (DTS). The DTS data also provided the first description of the salt marsh benthic thermal regime, a system co-dominated by groundwater discharge and an ephemeral "tidal thermal blanket." /// Spatial variability in ET and rooting depth due to vegetation zonation were incorporated into a numerical model to represent the ecohydrologic system. The zonally-distributed ET and rooting depths caused notable spatial variations in hydrologic conditions in the marsh root zone, including significant variations in unsaturated pressure head and soil saturation. Modest control of salt marsh water table depth by vegetation following flooding tides was simulated throughout the field site, in accord with the prevailing conceptual model of salt marsh plant-water interactions. The simulations also suggested four additional classes of ecohydrologic dynamics apparent under conditions of prolonged marsh exposure. The four new classes of ecohydrological behavior were distinguished by combinations of relatively high or low soil permeability and high or low ET rate. Together, patterns in vegetation and soil permeability thus created distinctive "ecohydrological zones." In some cases, the contrast among such ecohydrological zones caused upward and downward groundwater flow regions to be spatially juxtaposed, suggesting future research into soil biogeochemistry at these sites may be interesting. /// In summary, a new conceptual model of salt marsh ecohydrology is based on a definition of "ecohydrological zones" as the relevant unit of structure and function within the salt marsh ecohydrological system. Distinctive ecohydrological zones are created by hydraulic interactions between groundwater, vegetation, and the atmosphere. The specific nature of each zone depends both on the soil hydraulic properties resulting from the local geomorphological history and on the plant water uptake and transpiration governed by each plant species' unique physiology. The set of ecohydrological zones within a salt marsh are nested, in turn, within a coarser hydrologic system structure imposed by the tidal regime and larger intertidal groundwater flow system.