Airborne thermal remote sensing for water temperature assessment in rivers and streams

Abstract

Airborne remote sensing methods are needed to assess spatial patterns of stream temperature at scales relevant to issues in water quality and fisheries management. In this study, we developed an airborne remote sensing method to measure spatially continuous patterns of stream temperature and evaluated the physical factors that influence the accuracy of thermal remote sensing of flowing waters. The airborne thermal infrared (TIR) system incorporated an internally calibrated thermal imager (8–12 μm) aligned with a visible band camera in a vertically mounted, gimbaled pod attached to the underside of a helicopter. High-resolution imagery (0.2–0.4 m) covering the entire channel and adjacent floodplains was recorded digitally and georeferenced in-flight along 50- to 60-km river sections ranging from 2 to 110 m in width. Radiant water temperature corresponded to kinetic water temperature (5–27°C) in a range of stream environments within ±0.5°C. Longitudinal profiles of radiant water temperature from downstream to headwater reaches provided a spatial context for assessing large-scale patterns of thermal heterogeneity and fine-scale thermal features such as tributaries and groundwater inputs. Potential sources of error in remote measurements of stream temperature included reflected longwave radiation, thermal boundary layer effects at the water surface, and vertical thermal stratification. After taking into account the radiative properties of the surrounding environment and the physical qualities of the stream, thermal remote sensing proved highly effective for examining spatial patterns of stream temperature at a resolution and extent previously unattainable through conventional methods of stream temperature measurement using in-stream data recorders.