Monthly hydrological report in April 2004
Dry in the south, rising water levels in the north
April was warm and dry throughout the country, with record-breaking low precipitation in some areas. Evaporation in southern and western Finland was considerably greater than precipitation. Snow melted rapidly due to a warm period starting in
mid-April. Due to the very low precipitation the spring floods in the southern coastal region and in Pohjanmaa were mainly the result only of snowmelt and were generally rather low. Water levels and discharges began to increase towards the end of the
month. In southern Finland lake ice broke up about one week ahead of schedule.
Precipitation during April was very low, setting new low records in some areas. In most of southern and western Finland the precipitation was below 10 mm, less than one fifth of the mean. In the east and north the corresponding figures were 10-30 mm,
in many places only about one third of the mean.
Snow melted rapidly during a warm period starting in mid-April. At the end of the month the snow cover was still intact mainly to the north of a line from Savonlinna through Kuopio to Oulu. The water equivalent of the snow cover at the end of the month
was >130 kg/m2 (mm) in north-eastern Lapland. In Kainuu, parts of Koillismaa and near the source waters of the rivers Kemijoki and Paatsjoki the snow cover was 40-130 kg/m2, elsewhere generally below 40 kg/mm2.
Water level and discharge
Due to the almost zero precipitation the spring discharge and level maxima in rivers of southern Finland and Pohjanmaa were caused generally only by water from snowmelt and were lower than normally. Both water levels and discharges decreased to very
low levels for the time of year in small coastal rivers towards the end of April. In the east, lake levels were generally increasing but in the south and west they even decreased in some areas. Lake levels in eastern Finland were either close to or above
the seasonal mean, in central and western Finland they were below the mean in some regions. In Lapland, water levels and discharges increased very rapidly at the end of the month due to melting of snow.
The deviation of lake Pielinen from the long-term seasonal mean at the end of April was +26 cm and corresponding figures in some other Finnish lakes were: Kallavesi +2 cm, Saimaa +15, Keitele +8, Päijänne +10, Pyhäjärvi, Säkylä –8,Längelmävesi
–5, Näsijärvi +1, Lappajärvi +21, Lammasjärvi –10, Oulujärvi +43, Lokka –130 and Inari +13 cm. The discharge of the river Pielisjoki was 100% of the mean April discharge during the reference period 1971-2000 and corresponding figures in
some other rivers were: Vuoksi 98%, Kymijoki 96, Paimionjoki 85, Kokemäenjoki 62, Kalajoki 109, Oulujoki 97, Iijoki 94, Kemijoki 72 and Tornionjoki 59%.
Evaporation from soil in southern and western Finland during a warm, dry period during the second half of April was clearly greater than the precipitation for the whole month. In south-western Finland the soil moisture deficiency was >40 mm,
elsewhere it was smaller.
Groundwater levels increased during April in the moraine soils of southern and western Finland. In the east the surface began to increase at around the middle of the month, in the north at the end of the month. The levels were in many places close to
the seasonal mean, with the exception of the counties of Oulu and Lapland and along the coastal zone, where the groundwater level was 20-50 cm below the seasonal mean. In the largest groundwater formations the water levels remained below the seasonal mean
almost throughout the country.
Groundfrost melted in open areas of southern Finland during the first half of the month, in eastern areas at the end of the month after melting of the snow cover. Night frosts and dry weather then slowed down the eventual melting of the groundfrost
layer. By the end of the month groundfrost layers had melted to some extent from below, but there was still an appreciable layer of frozen ground.
Lake ice broke up in southern Finland after the 20th day of the month, about one week ahead of schedule. At the end of the month ice thickness could generally be measured only in northern Finland. In the north the ice thickness was generally 50-60 cm,
with 80 cm over lake Kilpisjärvi. These ice thicknesses were generally about 10 cm less than the seasonal mean.
Tables and graphs