Jökull - 01.01.2005, Page 134

Sverrir Guðmundsson et al.
Figure 13. Six-day averages of the glacial runoff (DM ) as estimated from energy budget maps (from Figure
12a) compared to the runoff DDDM (a), DTIM (b) and DEEB (c) calculated by Eq. (3) with as from Eqs.
(4-6), respectively. – Afrennsli frá jökli, metið út frá orkubúskap og reynslubundnum líkönum.
where S is the glaciated area of the water drainage-
basin of the river (Figure 1). The glacial melting
(as) was derived by Eq. (2) with Mc from the en-
ergy budget maps and Eq. (3) used to compute the
glacial runoff (DM ). Results were compared with
the observed daily river discharge (DJ ) and average
weather components and surface characteristics on
the glacier (Figure 12). Typical delay time between
the daily glacial melting peak in ablation areas and
the corresponding discharge peak of the outlet glacier
have been estimated to be ∼4 hours (e.g. Rist, 1983).
During the ablation season, the main contribution to
glacial runoff was due to net radiation that followed
the incoming solar radiation and albedo (Figure 12).
During the first part of the ablation season, the con-
tribution from the turbulent heat fluxes was close to
zero, but glacial runoff became more influenced by
these heat fluxes during a warm and windy period
from mid-July to mid-August. Of special interest is
the period from July 28 to August 14 (before and dur-
ing the floods in Jökla), which we separate into three
phases:
1. A period of exceptionally strong winds from
July 28 to August 1 with high turbulent heat
fluxes, and low net radiation due to low solar
radiation (Figures 12a-c and 3e). The average
134 JÖKULL No. 55