Surface and Englacial Drainage of Glaciers and Ice Sheets

Supraglacial drainage occurs wherever snowpack, firn, or ice at the glacier surface is at the pressure melting point and supplied with additional energy, thereby generating melt water. Energy sources vary, but net radiation is usually the dominant source, although inputs of rainwater can also provide large volumes of surface runoff. The surface melt water is routed through the snowpack, firn, and across the glacier surface according to the local hydraulic gradient. In general, routing of water through snow and firn is slow (10−4–10−5 m s−1), contributing to a significant lag between melt water production and runoff at the glacier snout, whereas flow across exposed ice surfaces is typically 3–5 orders of magnitude faster. Under certain conditions where parts of the snowpack, firn, and/or ice surface are below the pressure melting point, the melt water will refreeze. Otherwise, the melt water will be routed supraglacially to the glacier margin unless intersected by a pathway from the glacier surface to the glacier interior. Such pathways include crevasses and moulins, and in temperate glaciers, microscale englacial veins. Flow rates through the englacial system vary considerably according to the hydraulic efficiency of the route taken. The routing of melt waters through both supraglacial and englacial drainage systems therefore affects the runoff response of an ice mass to rain and melt water inputs. During the course of a melt season, the efficiency of routing through both systems evolves, thereby altering the runoff response time to input variations. Keywords: supraglacial drainage; englacial drainage; water storage; firn; supraglacial lakes; boreholes