Coastal wetlands are critical environments for which hydrogeologic processes are poorly appreciated. We propose fundamental hydrologic-science research to understand the functional links between coastal wetland vegetation and transport processes involving subsurface water, dissolved salt, and heat. We hope to test the hypothesis that a plant "engineers" a beneficial environment, in part by controlling local salinity and soil moisture in a positive feedback system. We propose to study a salt marsh in the Palo Alto Baylands where we can engage in a combination of intensive field monitoring, low-altitude high resolution thermal and visual remote-sensing, and quantitative analysis based on multi-scale simulation. Our investigation would include inherent matters of spatial and temporal variability. Three nested models spanning scales from a single plant to a vegetation patch to the larger salt-marsh system will aid in quantifying the physical controls on the marsh-plant system. Based on two years of original field data for calibration, our models would quantify fluxes of water, salt, and heat on multiple scales. These models will enable us to test hypothesis and identify key controls on wetland hydroecology and to evaluate potential environmental impacts.
This project will contribute to hydrologic science by providing data and multi-process models that will be broadly applicable to other salt-marsh environments. Potential contributions include understanding species-specific hydrologic response, which will improve understanding of physical controls on ecosystem function. Quantification of local gradients and their causes would provide insight into the role of near-surface hydrogeology in binding adjacent plant patches into a united ecologic community. Our salt-marsh system model can be used to explore scenarios related to restoration and environmental change. Our data and modeling results might improve quantification of near-shore submarine groundwater discharge. Outreach will be achieved via publications and presentations. The Baylands Nature Interpretive Center has agreed to provide us with a venue for public/private workshops on wetland hydroecology and the scientific basis for wetland restoration. Our proposed research cuts across disciplinary boundaries by contributing to the fields of coastal ecology and biogeochemistry by helping to better identify fluxes and hyporheic exchange of water, solute, and heat that are of interest to wetland and marine ecologists and biogeochemists. Our results are also of interest to those in the fields of landscape ecology, plant physiology, benthic zoology, near-shore marine ecology, and coastal engineering.
Coastal wetlands are critical environments for which subsurface water flow processes are poorly appreciated. The aim of this proposed research was to obtain a fundamental understanding of the relation between near-surface hydrology and the state of coastal vegetation in the salt-marsh environment. Understanding the controls on salt-marsh vegetation is critical to the design and success of marsh restoration efforts. Our work focused on a marsh in the Palo Alto Baylands in northern California, which served as an ideal field site. There we engaged in a combination of intensive field monitoring of the near-surface hydrologic system, vegetation mapping, surface geophysical measurement campaigns, and high resolution thermal remote-sensing. These data sets were analyzed quantitatively using detailed numerical simulation of the 3D coupled surface-water and groundwater system. There were five outcomes of this research that were based on our unique salt-marsh dataset, innovative implementation of field methods, and analysis using quantitative modeling. 1) For the first time, we characterized salt marsh channel temperature changes using fiber optic distributed temperature sensing (DTS), which served as a 170 m long, continuously reading thermometer. This work showed the critical interaction between seawater and groundwater in the channel, as each water source had a different temperature signature. 2) We employed a geophysical method known as surface electromagnetic induction (EMI) that measures electrical conductivity of the plant root zone of the soil. Electrical conductivity depends on both water content and soil-water salinity. We presented a new approach called time-lapse differential EMI that along with our detailed vegetation maps enabled us to discover a statistically supported, yet unappreciated, linkage between salt marsh vegetation patterning and temporal changes in soil water and salt content. 3) We developed a new thermal infrared imaging approach to develop plant water use (transpiration) and plant leaf (stomatal) resistance maps at a very fine scale. 4) Based on a combination of measurements of meteorological conditions, heat flux, hydrologic parameters, and quantitative analysis using six different types of empirical models, we discovered a threshold effect on vegetation carbon dioxide exchange with the atmosphere that was related to tidal depth and duration. This work showed that the effects of flooding on salt marsh–atmosphere exchange are temporary but strongly affect the marsh water, carbon, and energy balance despite their short duration. 5) We developed a quantitative numerical simulation model of our intensively studied salt marsh field site, integrating coupled 2D surface water and 3D groundwater flow and zonal plant water use. Through our analysis, we identified regions of distinctive root zone hydraulic conditions, caused by both vegetation and sediment spatial patterns, that we termed ‘‘ecohydrological zones.’’ We suggest that ecohydrological zones, which reflect the combined influences of topographic, sediment, and vegetation spatial variability, are the fundamental spatial habitat units comprising the salt marsh ecosystem. This body of research has been published in the journals Water Resources Research, Environmental Science and Technology, and Ecosystems, and results were presented at numerous professional society conferences and university venues. Apart from the intellectual merit of the results discussed above, our research contributed broadly to methods and concepts that can be applied to a range of field environments and problems. Our deployment of fiber optic DTS to measure channel bed temperatures and thereby deduce surface water - groundwater interactions has already been employed by researchers in other settings. We note that the method enabled identification of small 'micro-tributary' channels. These micro-channels may be hydrologically significant in a variety of settings. Our new time-lapse differential EMI data analysis approach provides a novel means to quantitatively map changes in near-surface water content and salt content. Such data are often prohibitively difficult to obtain at such high spatial resolution over comparable areas using manual soil sampling and analysis methods. Our new field results quantified tidally-induced changes in salt marsh water, heat, and carbon dioxide fluxes based on a variety of measurements and models. This effort provides new general insight into the ecological consequences of temporally variable surface water extent. The measurement of carbon dioxide flux is critical to our understanding of the influence of salt marshes on atmospheric greenhouse gasses. Our thermal infrared (TIR) remote sensing of leaf temperatures, combined with surface energy balance modeling, provides a new method to efficiently map spatially distributed evapotranspiration (ET), which is a critical and often difficult to measure component of the hydrologic cycle. Our comprehensive simulation model of surface water, groundwater, and plant interactions provide a stepping stone for continued work aimed at better understanding the nature of vegetation patterning in salt marshes and ultimately the planning of cost-effective marsh restoration practices. Finally, our new concept of ecohydrological zones, has the potential to change the way we think about the role of hydrologic processes in salt marshes and other groundwater dependent ecosystems.
