Monthly hydrological report in July 1999
The period of exceptionally warm weather continued into July. Parts of northern and central Finland received high amounts of water in the form of heavy showers. In the south and west of the country, rainfall was very low and a severe drought occurred.
The levels of both surface- and groundwater formations were exceptionally low in southern and western Finland. Water temperatures remained several degrees above the seasonal mean until about the 25 of the month.
Precipitation in Lapland during July was very high in some areas. Precipitation figures in central Finland were 50-120 cm and in the south and west 20-60 mm. In some parts of the southwestern archipelago, the total rainfall for July was only a few
millimetres. In parts of southern Finland the overall precipitation for the three months May-July was as low as a few tens of millimetres.
Evaporation from watercourses during July was over 100 mm, in southern regions even 150 mm. In many land areas the evaporation was effectively limited by drying out of the soil and withering of vegetation.
Water levels of larger lakes in northern Finland remained above the seasonal mean or even increased. In southern and central Finland the levels of both lakes and rivers continued to decrease. The level of lake Saimaa decreased almost to its seasonal
mean. In smaller watercourses of southern Finland the water levels were very low. The deviation of the level of lake Pielinen from the long-term seasonal mean at the end of the month was -28 cm and corresponding figures in some other Finnish lakes were:
Kallavesi -8 cm, Saimaa +6, Keitele -24, Päijänne -11, Pyhäjärvi, Säkylä -15, Vanajavesi -2, Sääksjärvi -39, Pyhäjärvi, Pyhäjoki -26, Lentua -1, Oulujärvi 0, Lokka +40 and Inari +7 cm.
Discharges in many rivers of southern and western Finland decreased to exceptionally low levels at the middle of the month, whereas approximately normal discharges were recorded in the larger rivers of northern Finland. The discharge of the river
Pielisjoki was 80% of the mean July discharge for the period 1961-1990 and corresponding figures in some other Finnish rivers were: Vuoksi 111 cm, Kymijoki 73, Paimionjoki 23, Kokemäenjoki 47, Kalajoki 55, Oulujoki 78, Iijoki 82, Kemijoki 92 and
Groundwater levels of most formations in southern and south-western Finland and in Ostrobothnia at the end of July were several tens of centimetres below the seasonal mean and were still decreasing. The situation resembled that during the record dry
summer of 1997. To the north-east of a line approximately from Kokkola through Mikkeli to Imatra, groundwater levels varied on both sides of the mean. Soil layers were extremely dry in many areas of southern, south-western and western Finland with very
Surface water temperatures remained severeal degrees above the seasonal mean almost throughout the month in southern and central Finland. The onset of a period of cooler weather from about the twenty-fifth of the month caused rapid cooling of the water
layers, but an increase in temperature again occurred at the very end of the month. The following temperatures and deviations from the seasonal mean (°C) were recorded on August 1 in some major Finnish lakes: Tuusulanjärvi 22.7 (+3.0), Saimaa, Lauritsala
20.5 (+0.5), Kallavesi, Kuopio 18.1 (-0.5), Pyhäjärvi, Säkylä 19.8 (+1.0), Lappajärvi 18.0 (-1.0), Oulujärvi, Manamansalo 15.7 (-3.0) and Inari, Nellimvuono 14.4 (-1.5).