Abstract
This paper describes a method of mapping riparian vegetation in semi-arid to arid environments using the Landsat normalized difference vegetation index (NDVI). The method successfully identified a range of riparian community types across the entire state of Nevada, USA, which spans 7 degrees of latitude and almost 4000 m of elevation. The landscape was stratified into units of similar elevation and solar exposure, and riparian areas were identified as having anomalously high NDVI within a local neighborhood. Thousands of calibration points were used in a simplex optimization to select the spatial neighborhood, the elevation and insolation strata, the minimum NDVI to be considered as potentially riparian, the number of standard deviations from the mean for an anomaly to be classified as riparian, and a limit on upslope position. Mapping of subpixel riparian corridors was improved by applying a directional high-pass filter to the NDVI data. Irrigated areas in agricultural and urban areas were removed based on land ownership maps and manual editing. The final map was tested with 400 independent test points: producer’s accuracy was 84.6% and user’s accuracy was 93.5%. This method should be broadly useful for mapping riparian features across large and complex regions with arid to semi-arid environments.
Highlights
Riparian ecosystems provide critical resources for arid to semi-arid regions [1], enhancing local plant and animal diversity [2] and supporting migratory bird populations [3]. maps derived from satellite imagery are an important tool for monitoring and managing these important ecosystems [4]
Results from the analysis of normalized difference vegetation index (NDVI), the spatially filtered NDVI, and the perennial stream line buffer were merged into one map and were masked by a map of irrigated lands in urban and agricultural areas
Agricultural land use in riparian zones that appeared to be based on available surface water, and for which NDVI values were comparable to neighboring unmodified riparian areas, was not masked out
Summary
Riparian ecosystems provide critical resources for arid to semi-arid regions [1], enhancing local plant and animal diversity [2] and supporting migratory bird populations [3]. maps derived from satellite imagery are an important tool for monitoring and managing these important ecosystems [4]. Many riparian mapping studies have been performed by pragmatically generating a spatial buffer around streamlines in digital hydrographic datasets [5,6], or by more realistically applying a flooding algorithm to a digital elevation model (DEM) [7,8]. The level of detail and accuracy of available hydrographic datasets is quite variable in remote areas of Nevada, the width of riparian corridors changes dramatically over short distances, and small but important riparian patches exist that are disconnected from mapped flow lines. Simple distance buffers that would be reasonable for a small study area are not effective for this large region of complex terrain. In the arid to semi-arid climate of Nevada, riparian vegetation may be distributed discontinuously within an arbitrary buffer zone or digitally flooded model
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