Firn layer impact on glacial runoff: a case study at Hofsjökull, Iceland

Abstract

A mass balance–runoff model is applied to Hofsjökull, an 880 km² ice cap in Iceland, in order to assess the importance of the firn layer for glacial runoff. The model is forced by daily temperature and precipitation data from a nearby meteorological station. Water is routed through the glacier using a linear reservoir model assuming different storage constants for firn, snow and ice. The model is calibrated and validated using mass balance data and satellite-derived snow facies maps. Simulated mass balances and snowline retreats are generally in good agreement with observations. Modelled cumulative mass balance for the entire ice cap over the period 1987/1988 to 2003/2004 is − 7·3 m, with uninterrupted negative mass balances since 1993/1994. Perturbing the model with a uniform temperature (+1 K) and precipitation (+10%) increase yields static mass balance sensitivities of − 0·95 m a⁻¹ and +0·23 m a⁻¹ respectively. Removing the firn layer under otherwise likewise conditions results in almost unchanged total runoff volumes, but yields a redistribution of discharge within the year. Early summer discharge (June to mid August) is amplified by roughly 5–10%, whereas late-summer/autumn discharge (mid August to November) is reduced by 15–20% as a result of accelerated water flow through the glacial hydrological system. In comparison, applying a climate-model-based temperature and precipitation scenario for Iceland until 2050 results in higher runoff throughout the year, increasing total runoff by roughly one-third. The results emphasize the role of the firn layer in delaying water flow through glaciers, and the influence on discharge seasonality when firn areas shrink in response to climate-change-induced glacier wastage. Copyright © 2006 John Wiley & Sons, Ltd.