Jökulhlaups in Skaftá: A study of a jökul- hlaup from the Western Skaftá cauldron in the Vatnajökull ice cap, Iceland Bergur Einarsson, Veðurstofu Íslands Skýrsla VÍ 2009-006 Jökulhlaups in Skaftá: A study of jökul- hlaup from the Western Skaftá cauldron in the Vatnajökull ice cap, Iceland Bergur Einarsson Skýrsla Veðurstofa Íslands +354 522 60 00 VÍ 2009-006 Bústaðavegur 9 +354 522 60 06 ISSN 1670-8261 150 Reykjavík
[email protected] Abstract Fast-rising jökulhlaups from the geothermal subglacial lakes below the Skaftá caul- drons in Vatnajökull emerge in the Skaftá river approximately every year with 45 jökulhlaups recorded since 1955. The accumulated volume of flood water was used to estimate the average rate of water accumulation in the subglacial lakes during the last decade as 6 Gl (6·106 m3) per month for the lake below the western cauldron and 9 Gl per month for the eastern caul- dron. Data on water accumulation and lake water composition in the western cauldron were used to estimate the power of the underlying geothermal area as ∼550 MW. For a jökulhlaup from the Western Skaftá cauldron in September 2006, the low- ering of the ice cover overlying the subglacial lake, the discharge in Skaftá and the temperature of the flood water close to the glacier margin were measured. The dis- charge from the subglacial lake during the jökulhlaup was calculated using a hypso- metric curve for the subglacial lake, estimated from the form of the surface cauldron after jökulhlaups. The maximum outflow from the lake during the jökulhlaup is esti- mated as 123 m3 s−1 while the maximum discharge of jökulhlaup water at the glacier terminus is estimated as 97 m3 s−1.