Full parameterization of coupled numerical energy/mass balance and flow models is based on detailed observations for improved process understanding, sensitivity experiments and extrapolation to areas with less comprehensive measurements. The Glacier Mass Balance Bulletin (published by the WGMS) provides extensive information on about 10 to 15 sites where the corresponding information is being collected. There are numerous sites to reflect regional patterns of glacier mass/volume change within major mountain systems, but they are not optimally distributed. Observations with a limited number of strategically selected index stakes (annual resolution) combined with precision mapping at about decadal intervals (volume change of entire glaciers) for smaller ice bodies or with laser altimetry/kinematic GPS for large glaciers constitute optimal possibilities for extending the information into remote areas of difficult access. Repeated mapping and altimetry provide important data at lower time resolution (decades). Long-term changes of glacier length within various mountain ranges are measured either in situ or with remote sensing techniques at annual to multi-annual frequencies. Locations have usually been historically chosen but in future should be more based on statistical considerations concerning climate characteristics, size effects and dynamics (e.g., normal flow versus effects from calving, surge, debris cover). More…