**2. Materials and Methods**

Our fundamental premise is that we can determine the potential of remote sensing to be useful for ambient monitoring and synoptic data collection in a given waterbody by mapping this potential at the USGS' Hydrologic Unit Code 12 (HUC-12) subwatershed scale. The HUC-12 subwatershed classification allows us to analyze watersheds at the 1:24,000 scale. At this scale, these subwatersheds typically contain four stream segments, except in a few larger subwatersheds in the Northeast and the Great Lakes. We feel that this spatial scale is sufficient to identify the potential for remote sensing, as the streams encompassed within these subwatersheds are usually higher order distributaries or small areal bodies with similar climate, land use and terrain characteristics. In total, there are 102,973 HUC-12 subwatersheds in the conterminous United States. We discuss our approach in Section 2.1 below.

We used numerous Geographical Information System (GIS) resources in this study to map access, acquisition, constraints and risks of impairment. All GIS operations and visualizations were performed in QGIS [26], while the model development was performed in MATLAB [27]. In determining the potential for remote sensing and its applications, we used existing accessibility and image acquisition conditions, and did not account for the effects of land use and land cover change or climate change. After determining the potential of remote sensing as a data source within each subwatershed, we subsequently subset subwatersheds on the basis of applicability and resource constraints as described in Section 2.2 below. We have listed all the datasets we used in these analyses in Table 2.


**Table 1.** Data sources used in this study.

