Groundwater – Vegetation – Atmosphere Interactions in an Intertidal Salt Marsh

Project Leads:

Kevan Moffett
Steven Gorelick

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 five 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 at the site were used to explore the links between marsh vegetation and hydrology.

1. Salt Marsh Vegetation Zonation and Soil Conditions

Vegetation zonation is one of the most distinctive properties of salt marshes, yet is seldom examined together with physics-based analysis of salt marsh hydrologic dynamics. 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 (ECa) and a new geophysical analysis method, Quantitative Differential Electromagnetic Induction (Q-DEMI) mapping.

Moffett, KB, DA Robinson, and Gorelick SM. (2010) Relationship of salt marsh vegetation zonation to spatial patterns in soil moisture, salinity and topography. Ecosystems, 13: 1287-1302. doi:10.1007/s10021-010-9385-7.

2. Biophysical Stomatal Control of Salt Marsh Vegetation Water Use

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, one-dimensional 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.

Moffett, KB, and SM Gorelick. (2012) A method to calculate heterogeneous evapotranspiration using sub-meter thermal infrared imagery coupled to a stomatal resistance submodel. Water Resources Research, 48, W01545, doi:10.1029/2011WR010407.

3. Effects of Tidal Flooding on Salt Marsh Evapotranspiration and Net Ecosystem Exchange

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.

Moffett, KB, A Wolf, JA Berry, and Gorelick SM. (2010) Salt marsh - atmosphere exchange of energy, water vapor, and carbon dioxide: effects of tidal flooding and biophysical controls. Water Resources Research, 46, W10525. doi:10.1029/2009WR009041.

4. Locations and Timing of Groundwater - Surface Water Interactions in Salt Marsh Tidal Channels

In the field, 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.”

Moffett KB, Tyler SW, Torgersen T, Menon M, and Gorelick SM. (2008) Processes controlling the thermal regime of saltmarsh channel beds. Environmental Science & Technology, 42(3): 671-676. doi:10.1020/es07130

5. Effects of Vegetation Zonation and Soil Heterogeneity on Salt Marsh Ecohydrological Function: Numerical Simulations

Spatial and temporal interactions between plants and water and between surface water and groundwater 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. Simulations used the modeling software HydroGeoSphere (U of Waterloo).

Spatial variability in evapotranspiration and rooting depth due to vegetation zonation were incorporated into the numerical model to better represent the ecohydrologic system. The zonally-distributed evapotranspiration 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 surprising new classes of ecohydrological 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 evapotranspiration rate. Together, patterns in vegetation and soil permeability thus created distinctive “ecohydrological zones.” In some cases, the contrast among such ecohydrological zones created potential for plant-induced lateral groundwater flow and caused upward and downward groundwater flow regions to be spatially juxtaposed, suggesting exciting future research into soil conditions at these locations.

Moffett, KB, SM Gorelick, RG McLaren, and EA Sudicky. (2012) Salt marsh ecohydrological zonation due to heterogeneous vegetation – groundwater – surface water interactions. Water Resources Research, 48, W02516, doi:10.1029/2011WR010874

6. Alternative Stable States

"Intertidal marshes develop between uplands and mudflats, and develop vegetation zonation, via biogeomorphic feedbacks. Is the spatial configuration of vegetation and channels also biogeomorphically organized at the intermediate, marsh-scale? We used high-resolution aerial photographs and a decision-tree procedure to categorize marsh vegetation patterns and channel geometries for 113 tidal marshes in San Francisco Bay estuary and assessed these patterns’ relations to site characteristics. Interpretation was further informed by generalized linear mixed models using pattern-quantifying metrics from object-based image analysis to
predict vegetation and channel pattern complexity. Vegetation pattern complexity was significantly related to marsh salinity but independent of marsh age and elevation. Channel complexity was significantly related to marsh age but independent of salinity and elevation. Vegetation pattern complexity and channel complexity were significantly related, forming two prevalent biogeomorphic states: complex versus simple vegetation-and-channel configurations. That this correspondence held across marsh ages (decades to millennia) and at both high and low marsh elevations suggests the following: (1) marshes of shared physiography can exhibit highly variable ecosystem structures; (2) young marshes are not necessarily simple nor necessarily develop vegetation complexity with age and elevation; (3) Bay marshes should continue to exhibit both simple/complex configurations in the future despite a likely shift toward low marshes; (4) salt marshes may tend to occupy two alternative stable
states characterized by linked complexity in vegetation and channel organization. This final point may help fill the gap at the marsh scale between biogeomorphic models explaining marsh occurrence at larger coastal and smaller vegetation patch scales."

Moffett, K.B. and S.M. Gorelick. 2016. Alternative stable states of tidal marsh vegetation and channel pattern complexity in the San Francisco Bay estuary, California, USA, Ecohydrology, doi: 10.1002/eco.1755.

7. Conclusion

In summary, a new conceptual model of salt marsh ecohydrology is based on a definition of “ecohydrological zones” as the relevant unit of ecohydrological structure and function within the coastal salt marsh system. Distinctive ecohydrological zones are created by hydraulic interactions between surface water, groundwater, vegetation, and the atmosphere. The specific nature of each ecohydrological zone depends on both: a) the soil hydraulic properties resulting from the local geomorphological history and b) the plant water uptake and transpiration governed by each plant species’ unique physiology and zonation. The water and carbon balances of the wetland are inherently linked and are affected by periodic tidal flooding in surprising ways. We found that vegetation pattern complexity was related to marsh salinity but independent of marsh age and elevation. Channel complexity was related to marsh age but independent of salinity and elevation. Vegetation pattern complexity and channel complexity were significantly related, forming two prevalent biogeomorphic states: complex versus simple vegetation-and-channel configurations. New technologies, such as Distributed Temperature Sensing (DTS), new analyses, such as the Q-DEMI approach to mapping intertidal saturation and salinity dynamics, and new models, such as the thermal remote sensing-based stomatal resistance mapping submodel, provide valuable insight into the spatial patterns and temporal dynamics of wetland ecohydrology.