Monthly hydrological report July 2014

Groundwater levels below the seasonal mean, lake water temperatures exceptionally high

Water temperature in July 2014

July was warmer than normally throughout the country, exceptionally so in Lapland. Very high regional and temporal variation was recorded in precipitation, although generally the rainfall figures were below the seasonal mean. Hot weather in July decreased both lake- and groundwater levels and increased surface water temperatures very high. By the end of the month, water levels were generally low both in watercourses and in groundwater formations. Lake water temperatures were generally exceptionally high towards the end of the month.

Precipitation

Precipitation in July was generally low during an extended period of high pressure lasting almost throughout the month. Most of the precipitation came as local thunderstorms, resulting in very high regional variation. The temporal distribution of the rainfall was very uneven; in many areas the greater part of the monthly precipitation occurred during the last day of July. Rainfall along the southern coast and in northern Lapland was very low, below 40 mm. Elsewhere the precipitation was generally 40-100 mm. In most parts of Lapland, Pohjanmaa and eastern Finland the overall July precipitation was close to or above the seasonal mean.

Water level and discharge

Due to the warm and dry weather, water levels and discharges were generally decreasing in typical summer fashion during July. Scattered thunderstorms caused temporary increases in discharge in some rivers with small drainage basins. In the river Kymijoki and Vuoksi watercourses, monthly discharges were generally close to the seasonal mean, but in some other major Finnish watercourses the discharges were clearly below the mean.

By the end of the July the level of lake Saimaa was still above the seasonal mean despite the decrease occurring during the month, whereas lake Päijänne was at its seasonal mean. Lakes Oulujärvi and Inarijärvi were both below the mean. In south-western and western Finland, most lake levels were still below the mean after the dry spring.

The deviation of the level of lake Pielinen from the seasonal mean at the end of July was -23 cm, and corresponding figures in some other Finnish lakes were: Kallavesi -8 cm, Saimaa +18, Keitele -16, Päijänne +0, Pyhäjärvi, Säkylä -9, Längelmävesi -18, Näsijärvi +2, Lappajärvi +5, Lammasjärvi -21, Oulujärvi -13, Lokka -60 and Inari -17 cm. The monthly discharge of the river Pielisjoki was 80% of the mean July discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 102%, Kymijoki 99, Karjaanjoki 44, Kokemäenjoki 55, Siikajoki 48, Oulujoki 92, Iijoki 51, Kemijoki 63 and Tornionjoki 82%.

Groundwater level

In most parts of the country groundwater levels were 5-30 cm below the seasonal mean, with the exception of some areas which had received high precipitation early in the summer. In these latter areas, groundwater levels were 0-15 cm above the mean. In larger formations in western Finland, groundwater levels were generally within ±10 cm of the seasonal mean. In eastern and northern regions, the corresponding figures were +20…+30 cm.

Surface water temperature

Lake water temperatures were clearly below the seasonal mean during the first week of July. With the warming weather, the water temperatures also started to increase. Exceptionally warm weather at the end of July caused surface water temperatures to increase particularly after the 25th of the month. Temperatures of 24-26°C were recorded in many areas of southern and central Finland, and temperatures in excess of 20°C were recorded even well into Lapland. Thunderstorms during the last days of the month mixed and cooled the surface waters, but temperatures were still generally above the seasonal mean in most areas except Lapland. Water temperatures at the end of the month were 22-25°C in southern and central Finland and 14-21°C in the north.

Evaporation

As a result of the warm weather, evaporation during July was high both from watercourses and from soil layers. The monthly evaporation sum from the larger lakes on the basis of watercourse model calculations was generally 60-100 mm. The corresponding figures from soil layers were 50-100 mm.

Maps, graphs and tables

Published 2014-11-18 at 15:25, updated 2014-11-18 at 15:25