Abstract. Since 2012, seven perennial snow patches in the northern Japanese Alps have been determined to be very small glaciers (VSGs: <0.5 km2). However, it had
not been determined how such glaciers could be maintained in such a warm
climate. In this study, we calculate the annual mass balance, accumulation
depth, and ablation depth of five of these VSGs, covering 2015–2019 for
four of them (2017–2019 for the fifth) using multi-period digital surface
models (DSMs) based on structure-from-motion–multi-view-stereo (SfM–MVS)
technology and images taken from a small airplane. The results indicate that, due to snow acquired from avalanches and
snowdrifts, these VSGs are maintained by an accumulation in winter that is
more than double that from the snowfall, thereby exceeding the ablation in
summer. Therefore, we classify them as topographically controlled VSGs. We
find very small yearly fluctuations in their ablation depth; however, their
annual mass balance and accumulation depth have large yearly fluctuations.
The annual mass balance, which mainly depends on the accumulation depth,
showed accumulation throughout each glacier during heavy snow years and
ablation throughout each glacier during light snow years. This
characteristic differs from the upper accumulation area and lower ablation
area that exists on most glaciers. These VSGs lack a positive annual mass
balance gradient, which suggests that they are not divided by a distinct
glacier equilibrium line
altitude (ELA) into an upstream accumulation area and a downstream ablation
area. Moreover, compared to other glaciers worldwide, we find the mass
balance amplitude of VSGs in the northern Japanese Alps to be the
highest measured to date.