Comparison of historic to future without action (FWOA) land change

Abstract

One of the primary purposes of the 2023 Coastal Master Plan Integrated Compartment Model (ICM) is to forecast potential changes in coastal wetland area under varying environmental and restoration scenarios. To validate the model performance, historical analyses of observed wetland changes are needed for comparison to hindcast model runs. To generate these historical analyses, this effort has analyzed satellite imagery from 1985 through 2020 to form a basis of comparison for the hindcast model results. Wetland change is a dynamic process which varies in time and space as a result of multiple compounding and interacting stressors. Wetland area is a fluid concept which can vary depending upon the definition of “wetland” and environmental conditions at the time of acquisition of imagery utilized to estimate land area. Estimates of wetland area are known to vary by more than +/- 5% as a result of nothing more than water level variability. Often, simplistic wetland change analyses are conducted using simple pre- and post- assessments. These assessments create two wetland classifications at two points in time, difference the two, and assume any change between the two assessments is wetland change. These types of assessments ignore parameters like water level variability, lack temporal resolution to account for said variability, and as such, are often prone to misinterpreting normal environmental variability as wetland change. The wetland area change analyses presented here take a multitemporal approach to assessing wetland area change. Annual classifications were created from 1985-2020, and wetland area trends were fit statistically with lines which generalize trends through time. Additionally, the statistical methods utilized here allow for the calculation of confidence intervals with regard to wetland area, which are vital for the validation of model output. The analyses presented here provide a comprehensive historical analysis of wetland change by hydrologic compartment from 1985 through 2020. More importantly, these analyses provide a basis by which to assess hindcasting scenarios of the master plan model as it relates to wetland area change. The results from this effort can be utilized to assess model performance and quantify the confidence which may be placed in outputs of projected wetland area.