6. Glacier Mass Balance Regimes

The two parts of mass balance, inputs and outputs, are now considered together. Budget estimates are provided for Biafo Glacier and from partial data for Batura, Baltoro and some others. Overall quantities reflect the greater moisture availability and mass inputs at high elevations in the Karakoram. From glacier basins of the Central Karakoram, the total annual water yields appear to be of the order of 700–900 mm; less than maximum snowfalls measured in glacier source zones but three or more times greater than precipitation reported from valley weather stations. The mass balance regime is characterised as an all-year accumulation and summer ablation type, a distinctive form of Sub- or Outer Tropics regime. The vast areas of rock wall in source zones and predominance of avalanche nourishment and the significant ‘inputs’ in conventional ablation zones create problems largely ignored in mass balance assessments anywhere. Wind-fed and wind-stripped areas high up and disconnected tributaries above the snowlines add to the unique problems of input estimation. Wind redistribution and avalanches generate complicated patterns not clearly separated into accumulation and ablation zones or according to linear change with elevation. Mass balance gradients depart widely from those described elsewhere and point to distinctive constraints. An S-shaped vertical profile arises even in Alpine-type Karakoram glaciers reflecting reduced losses under debris-covered ice low down and input concentration where maximum snowfall occurs and especially through avalanche deposition. Further complications arise in profiles of Turkestan-type glaciers, where all positive as well as negative components for main ice masses are located in the conventional ablation zones. For such reasons, the notions widely employed in mass balance work are compromised. Equilibrium line altitudes (ELAs), thresholds between positive and negative mass balance, mostly occur below where firn limits or climatic snowlines are expected. These are, however, among the distinguishing features of the region that arise from the regional climatic regime, the exceptional extent of high elevations and verticality relations.