The effects of spatial resolution on impervious cover classifications in watersheds and riparian zones in Vermont

Impervious cover (roads, rooftops, etc.) is a known stressor on stream biota and habitat and is often used as an indicator for assessing the effects of urbanization on stream health. Understanding how spatial data resolution impacts estimates of impervious cover is important for effective modeling and management of water resources at multiple scales. However, broad scale classifications of high spatial resolution data can be time consuming and expensive. High spatial resolution data classifications in Vermont, USA were compared to nationally available impervious cover classifications in order to understand the impact of scale on impervious cover estimates. I used National Agriculture Imagery Program (NAIP) imagery, a Normalized Difference Vegetation Index, ancillary road data, and a supervised evolutionary algorithm classification program to extract and quantify impervious cover for 888 catchments in Vermont, USA. Post-classification accuracy assessments were conducted to quantify the accuracy of the data set. Impervious cover characterized from NAIP imagery ranged from 1.83-48.31% in the study catchments. Overall accuracy for the NAIP impervious cover classifications was consistently high, ranging from 93-99%. National Land Cover Database (NLCD) data showed a bias towards overestimating impervious cover in more developed catchments and underestimating impervious cover in less developed catchments. The high spatial resolution dataset characterized from NAIP data was used to develop a Bayesian classification and regression tree model to predict where the NLCD may be adequate for classifying impervious cover and where higher spatial resolution data may be needed. Data inputs included NLCD land use/land cover classifications and U.S. Census Bureau housing data. High spatial resolution impervious cover was best predicted in catchments with less than 55-65% NLCD forested land cover. For catchments with greater than 55-65% NLCD forested land cover, impervious cover was best predicted where higher levels of NLCD open space development land cover existed. In areas where a full watershed analysis may not be feasible, the condition of the riparian zone along stream channels can provide information on water quality. Impervious cover in the riparian zone was calculated using both fixed-width and elevation based buffer metrics. Percent impervious cover was obtained from the National Land Cover Database (NLCD) and compared to the high resolution imagery analysis from National Agriculture Imagery Program data within the buffer zones. Percent impervious cover ranged from 1.58-8.67% within both types of buffers. The spatial resolution of impervious cover data had less of an effect in the riparian zone than in the full catchments within the same HUC 10 and HUC 12 units. Buffer type had minimal impact on percent impervious cover, except in areas of unconfined valleys, where there were notable differences between fixed-width and elevation based buffers. The results suggest that although there is a trend toward the NLCD underestimating impervious cover at lower levels of development, it may be adequate for mapping impervious cover in the riparian zone, depending on the land use/land cover characteristics of the catchments being studied.