Extending the Ability of Near-Infrared Images to Monitor Small River Discharge on the Northeastern Tibetan Plateau

River discharge gauging is scarce in high mountainous regions, especially on the Tibetan Plateau, where rivers are widely distributed. Although remote sensing is an important mean of monitoring river discharge, previous methods are most suitable for large rivers, and the ability to monitor small rivers (with widths less than 100 m) is limited. To resolve this issue, a multiple pixel ratio (MPR) method is presented for monitoring the discharge of small rivers based on the relationship between river discharge and the near-infrared reflectivity. Utilizing 281 Landsat images (1990-2015), we monitored river discharges in two subbasins in the upstream region of the Heihe River located on the northeastern Tibetan Plateau. Our results indicate that the performance of the MPR method is more stable than the previous calibration/measurement method. The monitoring accuracy was correlated with the length and location of the selected inundated river channel. Using selected inundated river channel lengths between 300 and 600m can provide better monitoring accuracy. The Nash-Sutcliffe efficiency of monitoring results (2013-2014) of Qilian station was 0.82; the Zhamashike station (2015) was 0.45; the two stations multiyears (1990-2013) monitoring results were 0.32 and 0.41; the ungauged basin was 0.45. Our results suggest that the MPR method can expand the ability of remote sensing to monitor discharge in small rivers with widths greater than 30 m on the Tibetan Plateau. In addition, the new method also has the potential to monitor the discharge of ungauged small rivers (with widths greater than 30 m).