fulltext - DiVA

Features of land and climatelltheir highest and their west-exposed parts, to judgefrom run-off determinations.The maximum of precipitation is nearly alwaysin late summer, but in the west there is also muchrain or snow in autumn and early winter; elsewherewinter is drier, and late winter and spring precipitationis low in all parts. Therefore, even in thenorth most precipitation falls as rain. The periodof snow cover is extremely variable; because thesnow is nearly always unreliable in the extremesouth and on the south-west coast, there this periodis only about 40-50 days, but it rapidly increasesin length inland. The average duration of snownear Stockholm is about 3 months but there is avariation from a few weeks to about four monthsbetween various years. Four to six months of snoware typical of much of northern Sweden, but in theextreme north the time with snow exceeds sevenmonths. In the Scandes the cover is too unevento allow an estimation of duration; this would varyfrom almost none on windy ridges to all year roundin permanent snow-beds at high altitude.The depth of snow is equally variable but itsmaximum (exceeding l m on an average andoccasionally as much as 1.8 m, according to AGER1964) is in the mountain valleys of southern Lapplandand not in the extreme north; again, there is· great local variation, especially above the timberlineof conifers (the birchwoods above the latterdo not check the snow-drift to the same extent asdoes coniferous forest).Precipitation in excess of evapotranspiration(i.e. run-off per unit area) is regarded by 0. TAMM(l959b) as the best expression of humidity (of theground), giving a picture superior to, for instance,that of DE MARTONNE's index (HESSELMAN 1932) .0. TAMM developed an empirical formula for afairly accurate calculation of run-off from precipitationand temperature data, although only validfor the forested parts of Sweden. The distributionof surplus precipitation is similar to that of totalprecipitation although the diminishing due to evapotranspirationis greater in the south; this, in turn,gives a better correlation with distribution andstrength of podsolization, tendency to paludificationand types of peatland. But it should be rememberedthat most of the paludification took place5 8°Fig. 12. Average number of days with snow cover. FromAngstrom 1958.at a time long ago when distribution of surplusprecipitation was not necessarily equiformal to thepresent one, and that mire types are stronglydependent on the type of terrain and not only onthe climate.Except in the extreme south, the maximum runoffcomes after the snow has melted. In riverscoming from the mountains, this period is prolongedand there may even be a second maximum in earlysummer. In late summer and autumn water-levelstend to be sinking except in periods with greatActa Phytogeogr. Suec. 50