48 
3.4. Present hydrological budget for Greenland 
Assuming the Greenland Ice Sheet to be presently in balance with the accumulation rate 
distribution shown in Fig. 1 and assuming net ablation rates as the ones given in the Table 
below, the values for the various terms in the hydrological budget for Greenland can be esti- 
mated. The results are presented in the Table below, where the sectors refer to the division 
of the Greenland Ice Sheet shown in Fig. 2. The total calf-ice discharge for the various sec- 
tors can be distributed between the main outlet glaciers according to their catchment areas. 
The result of such a division is shown in Fig. 2. (Where calving rates have been observed, 
observed values are shown.) 
Sector P fic Qp AB OT @N,AB QM, AB 
cue / Ja) 
P JF P,T F Net- 
a) (km?/a) ne /a) ne /a) (km’/a) (km 
Fy cas 
m’/a) Rae /a) (m/a) 
SW 178 27 34 239 60 87 118 34 1.0 
NW 83 8 2 93 22 30 61 2 1.0 
N 47 9 32 88 26 35 21 32 0.7 
NE 74 alt 29 114 38 49 36 29 0.7 
SE 105 15 73 193 23 38 82 73 1.0 
TOTAL 487 70 170 727 169 239 318 170 
QP, ACC precipitation over accumulation area of ice sheet 
Qp,AB = precipitation over ablation area of ice sheet 
= precipitation over ice-free land and local glaciers 
Qp ToT = Qp,acc + Qp,aB +t QP, IF 
N,AB = net "palance (net ablation) over ablation zone 
QM,AB = Qp, AB + QN,AB = total melt in ablation zone 
QCALF = calf-ice discharge 
QR,IF = run-off from ice-free land and local glaciers 
4. Temporal variations of Greenland mass balance 
Analyses of ice cores from the Greenland Ice Sheet indicate (i) fairly constant accumula- 
tion rates back to 1400 B.P. (Reeh et al., 1978), (ii) up to 25% increased accumulation rates in 
the period 3000-7000 B.P. (Reeh et al., 1984, figures 7 and 8), and (iii) a more than 50% 
decrease in the accumulation rate in late Wisconsin time (Hammer et al., 1978). 
Past ablation rates and calving rates cannot be deduced by direct measurements. However, 
volume changes of the ice sheet can be estimated from the history of ice margin fluctuations 
(Weidick, 1975b), see Fig. 3a, combined with ice sheet profile theory. Fig. 3b (from Weidick, 
1975b), shows derived ice sheet volume changes since 14,000 B.P. Assuming the above men- 
tioned temporal accumulation rate changes to be representative for the entire ice sheet, and 
deriving the rate of ice sheet volume change from Fig. 3b, the time history of the total water 
loss from the Greenland Ice Sheet can be calculated by means of equation (2). The results are 
presented in the following table. 
Time Ice Sheet Accumulation Water loss Run-off from Total water loss 
(years volume loss on ice sheet from ice sheet ice-free area from Greenland 
B.P.)  (km3/a) (km3/a) (km3/a) (km3/a) (km3/a) 
12000 0 400 400 0 400 
10000 150 430 580 20 600 
8500 320 490 810 80 890 
7000 100 690 790 190 980 
5500 0 560 560 170 730 
0 0 560 560 170 730 
