Mass balance measurements provide information on the gain or loss in glacier mass. It is typically observed once a year (towards the end of the melt period) by vertical measurements of both the (annual) ice loss at ablation stakes on the glacier tongue and the (annual) snow gain in the upper part of the glacier. In order to compare the two types of observations, the vertical point measurements are corrected for corresponding snow and ice densities, spatially extrapolated to the entire glacier, and reported as (annual) mass balance in the unit millimeter water equivalent [mm w.e.].
This corresponds to the (annual) average thickness change of the glacier in water column per square meter with positive and negative values indicating a mass gain and loss, respectively. Glacier melt multiplied by the glacier surface area, hence, corresponds to the glacier contribution to runoff.

Figure 1. Glacier as input-output systems (Winkler et al. 2010, after Winkler 2009)..

Although great efforts have been made to determine the mass balance indirectly with less effort (i.e., remote sensing, modelling, etc.), field measurements still provide the most reliable results and ensure a high temporal resolution as well as a separation of the relevant processes (accumulation, melting). However, it is necessary to check these from time to time using an independent method in order to recognize and reduce systematic errors in the measurements.

Figure 2. In the accumulation area, the mass gain is measured by digging a snow pit until the last summer horizon. In the ablation area, the melt can be measured using ablation stakes installed in the previous year. Source: SCNAT Wissen (https://naturwissenschaften.ch/snow-glaciers-permafrost-explained/glaciers/mass_balance).

References

Winkler, S., Chinn, T., Gärtner-Roer, I., Nussbaumer, S. U., Zemp, M., & Zumbühl, H. J. (2010). An introduction to mountain glaciers as climate indicators with spatial and temporal diversity. Erdkunde, 97-118.

Winkler, S. (2009): Gletscher und ihre Landschaften – eine illustrierte Einführung. Darmstadt.