A Freshwater Budget of the Gulf of Boothia, Northwest Territories

Résumé

A freshwater budget for the Gulf of Boothia (here considered to include Prince Regent Inlet) was estimated from oceanographic data available for the Canadian Arctic using a formula derived by Tully: integral of S(z) dz between limits L and 0 divided by S*L where C is the fraction of freshwater to the depth L, L is the depth at which the salinity attained S* or the depth to the bottom if the salinity there was less than S*, S(z) is the salinity at depth z and S* is the base salinity, in this budget taken to be 33.8 per mil. For each station occupied in 1961 and 1962 the amount of freshwater in metres, CL, was obtained. The result for 1962 is shown in Fig. 1. Sources of freshwater include direct precipitation, runoff, advection of less saline water and of ice, and condensation at the surface. It may be removed by evaporation and advection. We assumed that condensation is so small it may be neglected and that the rates of precipitation and evaporation are the same over the land as over the water. The mean precipitation in the region of the Gulf of Boothia is usually less than 20 cm/year and in nearby Barrow Strait it was estimated that the evaporation was about 8 cm/year in 1962. If the latter value is representative of the Gulf of Boothia and its drainage basin, the excess of precipitation over evaporation is about 12 cm/year. The estimate is supported by 1965 streamflow measurements of the Back River; for this basin the excess of precipitation over evaporation is about 17 cm/year. It seems that the water surplus for the Gulf of Boothia drainage basin is somewhat less so the estimate of 12 cm/year appears reasonable. As the area of the drainage basin is one and one quarter that of the gulf, runoff would add 15 cm/year to the freshwater of the gulf. Combining the gains and losses we estimate the net annual input at the surface to be 27 cm/year. If the 8 metres of freshwater estimated to occur in the gulf (Fig. 1) were due to the annual gain, it would represent an accumulation of close to 30 years. As free exchange with Lancaster Sound and beyond is possible, such a long period of accumulation seems unrealistic. There is no reason to believe that the accumulation is related to an excess of imported over exported ice, indeed export may exceed import, but data which would permit a quantitative assessment are not available. Examination of ice reconnaissance data in each year, for example that for 1967, indicates that, while movement of ice occurs from northern parts of the gulf to the south, the ice conditions observed within the system could be due entirely to ice formed there. It seems likely therefore that the accumulation is due to advection of low salinity water and that a direct relation exists between the depth of freshwater in the gulf and that calculated to occur in the archipelago to the west and in the Arctic Ocean (Fig. 1). It is generally considered that a net surface movement from west to east occurs through the archipelago; the freshwater budget provides support for this view.