Thermal video recording while walking: A simple method for mapping groundwater discharge points along forested headwater streams

Groundwater discharge along channels can affect stream runoff, chemistry, and ecological communities. Although the spatial distribution of groundwater springs can be investigated by areal thermal remote sensing of wide rivers, this technique is difficult to apply to forested headwater streams because their channels are often covered by riparian trees. We present a method of mapping groundwater springs for forested headwater streams based on recording thermal video while walking along channels. We applied this method at two sites in Hokkaido, Japan. At one site, groundwater springs were spaced every ~100 m, and their distribution did not follow the topography at a 1.3-km-long reach underlain by Pleistocene andesite lava. Here, almost all of the springs were colder than stream water and had similar chemistries to each other. At the second site, cold and warm springs were underlain by Holocene volcanic ash. The cold springs mainly seeped from the streambed at the downstream part of the site while warm springs were located at the footslope. Some cold springs had much higher solute concentrations than the stream and warm springs, which suggests that the groundwater discharge to the stream had multiple sources. We also used our method to map the spatial heterogeneity of the stream temperature associated with groundwater inputs. This method is suitable for wide-area surveys because it can easily map the spatial distribution of the surface water temperature and the groundwater discharge along headwater streams.