# The Definition of Perennial Streams Based on a Wet Channel Network Extracted from LiDAR Data
## 2 Data and Methodology  
A total of 30 study watersheds, located across ten states in the United States, were selected based on the availability of both LiDAR data and streamflow observations, as shown in Figure 1. LiDAR data were obtained through the USGS Center for LiDAR Information, Coordination and Knowledge (CLICK) website ([[http://lidar.cr.usgs.gov](http://lidar.cr.usgs.gov)]([http://lidar.cr.usgs.gov](http://lidar.cr.usgs.gov)), accessed on 20 August 2015), and the data quality varied for each LiDAR dataset. The LiDAR acquisition dates for each watershed and the corresponding average daily streamflow, from the USGS streampage at the downstream outlet of each watershed, during the acquisition period are listed in Table 1. The LiDAR survey years for the study watersheds range from 2009 to 2012. As the LiDAR datasets used in this study were acquired before the 3D Elevation Program was established, the minimum specification and five quality levels were not indicated [31]. The intensity map and the land surface topography were derived from the LiDAR point cloud data using the QCoherent LP360 toolbox for ArcGIS ([[https://www.lp360.com/](https://www.lp360.com/)]([https://www.lp360.com/](https://www.lp360.com/)), accessed on 20 August 2015). The drainage area, climate aridity index (\(E_{P}/P\)), streamflow variation, and streamflow exceedance probability (\(E_{Q}\)) during the LiDAR acquisition day are summarized in Table 1, and the watersheds are arranged by the climate aridity index from the smallest (0.3) to the largest (2.7). Streamflow observations were acquired from the USGS National Water Information System ([[http://waterdata.usgs.gov/nwis](http://waterdata.usgs.gov/nwis)]([http://waterdata.usgs.gov/nwis](http://waterdata.usgs.gov/nwis)), accessed on 3 January 2023). \(E_{Q}\) was defined as the probability that a specific streamflow will be achieved and exceeded for each watershed by generating a flow duration curve based on the daily streamflow records from USGS pages. The \(E_{Q}\) for the study watersheds ranged from 7% (high flow) to 98% (low flow). For example, the \(E_{Q}\) of Shafer Creek and General Creek were 98% and 95%, respectively, indicating a low flow condition when extensive dry channels were expected. The climate aridity index, defined as the ratio of annual potential evaporation to precipitation (\(E_{P}/P\)), was used as a numerical indicator of the climate in a watershed [30; 32]. Except for Tucca Creek (0.3), Schafer Creek (0.4), and Pine Creek (2.7), the climate aridity index for the study watersheds varied from 0.8 to 2.1. Perennial streams were obtained from the National Hydrography Dataset (NHD) [33]. The flowing channel networks corresponding to certain streamflow exceedance probabilities were compared with NHD perennial streams to evaluate the determination of perennial stream. The key component of the LiDAR data for identifying wet channels was the signals' intensity of ground returns. The signals' intensity is a relative measurement of the return strength of a laser pulse received by a LiDAR sensor. LiDAR systems operate in the near-infrared (NIR) range, and the absorption of NIR signals by water is significantly higher than that of dry land [34]. This characteristic of NIR leads to the fact that LiDAR return intensities over water surfaces are relatively low compared with those of dry lands such as dry channels and hillslopes. Figure 1: Location of study sites and available LiDAR data. The red dots indicate a total of 30 study watersheds across the ten states and the blue dots means the availability of LiDAR raw point data in the United States as of 2015. Figure 2 shows the intensity map for a headwater catchment in Ward Creek. Intensity is encoded as a digital number (DN), which is the raw values recorded by the sensor. Wet channels show consistently lower intensities and continuous patterns compared to other locations, such as hill slopes and dry channels. Wet channel heads, which are the starting points of wet channels, are visually detected and displayed by a blue dot.