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Abstract
In this paper, we report on the adaptation and application of a one-dimensional marsh surface elevation model, the Wetland Accretion Rate Model of Ecosystem Resilience (WARMER), to explore the conditions that lead to sustainable tidal freshwater marshes in the Sacramento–San Joaquin Delta. We defined marsh accretion parameters to encapsulate the range of observed values over historic and modern time-scales based on measurements from four marshes in high and low energy fluvial environments as well as possible future trends in sediment supply and mean sea level. A sensitivity analysis of 450 simulations was conducted encompassing a range of porosity values, initial elevations, organic and inorganic matter accumulation rates, and sea-level rise rates. For the range of inputs considered, the magnitude of SLR over the next century was the primary driver of marsh surface elevation change. Sediment supply was the secondary control. More than 84% of the scenarios resulted in sustainable marshes with 88 cm of SLR by 2100, but only 32% and 11% of the scenarios resulted in surviving marshes when SLR was increased to 133 cm and 179 cm, respectively. Marshes situated in high-energy zones were marginally more resilient than those in low-energy zones because of their higher inorganic sediment supply. Overall, the results from this modeling exercise suggest that marshes at the upstream reaches of the Delta—where SLR may be attenuated—and high energy marshes along major channels with high inorganic sediment accumulation rates will be more resilient to global SLR in excess of 88 cm over the next century than their downstream and low-energy counterparts. However, considerable uncertainties exist in the projected rates of sea-level rise and sediment availability. In addition, more research is needed to constrain future rates of aboveground and belowground plant productivity under increased CO 2 concentrations and flooding.
Highlights
Tidal freshwater marshes rely on accumulation of both inorganic sediment and organic matter on the marsh plain, which results in vertical accretion, to maintain their position in the tidal frame (Reed 2000; Neubauer 2008; Drexler et al 2009a; Drexler 2011)
We report on the application of Wetland Accretion Rate Model of Ecosystem Resilience (WARMER) to tidal freshwater marshes in the Delta in order to explore the future sustainability of these marshes under a broad range of possible conditions
The WARMER model results show that marsh surface elevations relative to mean sea-level (MSL) in the Delta will decrease for most of the 450 scenarios
Summary
Tidal freshwater marshes rely on accumulation of both inorganic sediment and organic matter on the marsh plain, which results in vertical accretion (cm per year), to maintain their position in the tidal frame (Reed 2000; Neubauer 2008; Drexler et al 2009a; Drexler 2011). There is less robust feedback between the elevation of the marsh surface and marsh accretion in tidal freshwater marshes than in salt marshes (Kirwan and Guntenspergen 2012), making them susceptible to drowning from relative sea level rise (Reed 1995). Swanson et al (2013) further adapted the Callaway et al (1996) model in the Wetland Accretion Rate Model of Ecosystem Resilience (WARMER) to incorporate the appropriate dynamic feedback among vertical accretion processes, marsh surface elevation, and temporally variable sea-level rise for San Francisco Bay tidal salt marshes, but did not adapt or apply the model to tidal freshwater marshes of the Delta. The model applied constant or temporally variable mass accumulation rates to recreate peat accretion histories, but lacked the dynamic feedback between inundation and the magnitude of organic matter accumulation that approximates the natural self-regulating process responsible for sustaining marshes. Swanson et al (2013) further adapted the Callaway et al (1996) model in the Wetland Accretion Rate Model of Ecosystem Resilience (WARMER) to incorporate the appropriate dynamic feedback among vertical accretion processes, marsh surface elevation, and temporally variable sea-level rise for San Francisco Bay tidal salt marshes, but did not adapt or apply the model to tidal freshwater marshes of the Delta. Schile et al (2014) examined the future resiliency of four marshes across the San Francisco Estuary using the Marsh Equilibrium Model coupled with digital elevation models; no tidal freshwater sites in the Delta were included
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