Changes in jökulhlaup sizes in Grímsvötn, Vatnajökull, Iceland, 1934-91, deduced from in-situ measurements of subglacial lake volume

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Abstract
A record of volumes of jökulhlaups from the subglacial Grímsvötn lake, Vatnajökull, Iceland, has been derived for the period 1934–91. The change in lake volume during jökulhlaups is estimated from the lake area, ice-cover thickness and the drop in lake level. The jökulhlaup volumes have decreased gradually during this period of low volcanic activity and declining geothermal power. The two jökulhlaups in the 1930s each discharged about 4.5 km3(peak discharge 25-30 × 103m3s−1). In the 1980s, jökulhlaup volumes were 0.6.-1.2 km3(peak discharge 2 × 103m3s−1). The lake level required to trigger a jökulhlaup has risen as an ice dam east of the lake has thickened. Water flow in a jökulhlaup ceases when the base of a floating ice shelf covering Grímsvötn settles to about 1160 m a.s.l. Apparently, the jökulhlaups are cut off when the base of the ice shelf collapses on to a subglacial ridge bordering the lake on its eastern side. The decline in melting rates has resulted in a positive mass balance of the 160-170 km2Grímsvötn ice-drainage basin. Comparison of maps shows that the average positive mass-balance rate was 0.12 km3a−1(25% of the total accumulation) in the period 1946-87. A gradually increasing positive mass balance has prevailed since 1954, reaching 0.23 km3a−1in 1976-86 (48% of total accumulation).