Monthly hydrological report May 2011

Low water levels in many large lakes, groundwaters also low

The weather during May was variable, although the monthly means were close to normal. After a dry start to the month several low pressure fronts reached Finland and the monthly precipitation was close to the mean. Snow melted even in Lapland, apart from some of the tundra regions and some bare north-facing slopes. Groundfrost persisted rather generally in parts of Lapland, but melted in other areas at the latest in the beginning of the month. Break-up of ice occurred in the lakes of central Finland at the beginning of May and in the former county of Oulu and in southern Lapland by the middle of the month. By the end of May ice covers persisted only in the lakes of the northern tundra zone, although they were so weak that measurements could no longer be made. Surface water temperatures were close to the seasonal mean at the beginning and the end of May; in the middle of the month they were somewhat above the mean.

Precipitation

The beginning of May was very dry but heavy rain occurred towards the end of the month. The monthly precipitation was generally somewhat above the seasonal mean, but remained below the mean in many parts of southern Finland and in Pohjanmaa. In the former county of Oulu and in southern Lapland the precipitation varied from around normal to almost double the seasonal mean.

Snow

In northern Lapland the water equivalent of the snow cover increased slightly during the first days of the month, but further south melting of snow continued. After the fifth of the month, rapid melting also began in northern Lapland. On May 10th the snow-free zone extended as far north as Sodankylä, and during the next two weeks snow melted from the remainder of Lapland with the exception of the tundra zone and some bare north-facing slopes.

Water level and discharge

In many of the rivers of Lapland the spring flood was divided into several periods during May because of changes in the prevailing temperature. As a result of the rather thin original snow cover and the discontinuous melting, spring floods were rather weak especially in southern Lapland; in some cases even record-breaking low floods were recorded. The rather heavy spring flood in the river Tenojoki and the normal flood in the river Tornionjoki occurred during the last week of May. In the Lokka reservoir the water level was lower than ever recorded during the period 1980-2010.

South of Lapland both water levels and discharges were generally decreasing during May, although precipitation at the end of the month did result in slight increases in discharges in many areas. In large and regulated lakes the increase in water levels continued. By the end of the month water levels were generally close to their seasonal means, although in the larger lakes of eastern Finland, e.g. lake Saimaa the levels were clearly below the mean. The discharges of the major watercourses during May were well below the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of May was -29 cm, and corresponding figures in some other Finnish lakes were: Kallavesi -15 cm, Saimaa -33 cm, Keitele -8 cm, Päijänne +4 cm, Pyhäjärvi, Säkylä +2 cm, Längelmävesi -23 cm, Näsijärvi -12 cm, Lappajärvi +8 cm, Lammasjärvi -30 cm, Oulujärvi -10 cm, Lokka -161 cm and Inari -3 cm. The monthly discharge of the river Pielisjoki was 86% of the mean monthly discharge for May during the reference period 1971-2000, and corresponding figures in some other rivers were Vuoksi 79%, Kymijoki 69%, Karjaanjoki 48%, Kokemäenjoki 62%, Kalajoki 39%, Oulujoki 33%, Iijoki 44%, Kemijoki 40% and Tornionjoki 68%.

Groundwater level

Meltwaters from snow did not significantly replenish groundwater reserves in the spring of this year. As the spring season was also characterized by rather low precipitation, groundwater levels were generally 0–30 cm below the seasonal mean throughout the country. In parts of eastern Finland the measured deficit was as much as 50 cm. In small groundwater formations, reacting rapidly to fluctuating weather conditions, the levels were decreasing.

Groundrost

The groundfrost layer melted at the beginning of May in all areas except northern Finland. In Lapland the thin snow cover had resulted in a deep layer of groundfrost, which persisted during May in some areas.

Evaporation

In the beginning of May evaporation was above seasonal mean as a result of the dry and warm weather, but it decreased in the middle of the month when rainy weather started. In May the evaporation sum in Ostrobothnia was below the seasonal mean, but in Lapland it was above it. The monthly evaporation sum from Class A pans varied from 70 mm in southern Lapland to over 110 mm in southwestern Finland. Based on the watercourse models the evaporation of  the great lakes of eastern and central Finland was 60–90 mm at the most.

Ice cover

Break-up of ice occurred in the lakes of central Finland at the beginning of May and in the former county of Oulu and in southern Lapland in the middle of the month. Many lakes were freed of their ice covers some days earlier than the long-term seasonal mean. By the end of the month, ice covers persisted only in the lakes of the tundra zone. Lake Kilpisjärvi retained its ice cover but due to its poor strength, measurements of ice thickness could no longer be made.

Surface water temperature

At the beginning of the month surface water temperatures were close to their long-term seasonal mean values. Warm weather in mid-May increased temperatures as much as 2-6°C above the seasonal mean. Later in the month the weather was cooler and by the end of May the temperatures were again close to or even slightly below the seasonal mean. In southern and central Finland the water temperatures were 10–15°C and in Lapland 5–12°C.

Maps, graphs and tables

ISSN-L 0358-6367

ISSN 1799-6899

Published 2011-08-15 at 13:13, updated 2013-07-01 at 12:24