Moderate resolution satellite data for mapping salt marshes

Abstract

Biophysical characteristics of salt marshes are primary indicators of their photosynthetic capacity, nitrogen content, and physiological status. These include the four main characteristics of canopy chlorophyll content: green leaf area index (ratio of green foliage area to ground area), green vegetation fraction (percent green canopy cover), green biomass (which also act as proxies for estimating net primary productivity —NPP), and carbon sequestration potential. Monitoring these characteristics through remotely sensed data can help infer the overall condition and productivity of these valuable natural resources on a larger scale. This would allow effective management strategies in high priority areas. Our research aims to enhance the marsh monitoring practices by including biophysical characteristics (the true early indicators of marsh health status) derived accurately from information such as NASA’s moderate resolution imaging spectroradiometer (MODIS) 250m and 500m data. Our objectives were to calibrate and test a suite of MODISbased vegetation indices (VIs) and develop prototype weekly composites of the salt marsh biophysical characteristics for the entire US Gulf Coast from 2000 to 2010. We then planned to perform a time-series analysis to study the overall trend of salt marsh productivity during the last decade. These VIs (normal difference, wide dynamic range, enhanced, soil-adjusted, etc.) have been widely used and tested for monitoring terrestrial vegetation, but not for salt marsh ecosystems. Our VI calibration and tuning came from establishing relationships between MODIS 250m or 500m data and field data in order to predict the four marsh biophysical characteristics mentioned above. We used atmospherically corrected MODIS 8-day surface reflectance products (250m and 500m), and in situ measurements of biophysical data from 182 locations covering four states (LA, MS, AL, FL). Preliminary analysis revealed that the accuracy of the MODIS-based salt marsh VI calibration depends primarily on three factors: the accuracy of the field data collection, the Figure 1. (a) The dual-headed OceanOptics sensor mounted on a 5m (16ft) high frame. (b, c) Instantaneous field of view (spatial resolution) of the sensor (diameter: 2.2m). (d) In situ sensor calibration.