Sokolak is a Polish demo area with the general aim to demonstrate the possibility of retaining drainage water from agricultural areas in mid-forest pond and to assess the effects of this solution for trees and the quality of groundwater around the reservoir.
Besides of the need to reduce the pollution load entering the Baltic, a significant problem of water management in Poland is a decrease of water resources which has been observed for about 30 years.
Mean annual temperature: 7,5°C
Number of day degrees:
Growing season days: 210 days
Annual precipitation is low and ranges from 450 to 550 millimetres, and in the growing season 160-200 millimetres. There are also wet years, such as in 2017, when the annual rainfall amounted to 718 millimetres.
Middle boreal forest vegetation zone:
The Sokolak reservoir is located at the Pułtusk Forest Inspectorate. It is surrounded by fresh forests with mostly pine stands. Within a radius of about 600 metres from the reservoir, the age of stands is between 4 and 60 years, and the direct adjacent is 28 years old. Near the reservoir there are permeable soils: medium and fine sand, loamy sand and sandy loam. In this area until the 1970’s there was a reservoir of water and it was surrounded by wetlands. The main source of water supply was the surface runoff from agricultural land adjacent to the forest.
After the drainage system was installed in agricultural areas and the water from the drains was directed to the river, the mid-forest reservoir was systematically drying up, until it completely disappeared. The new reservoir was made in 2008 (area of the pond 1,25 hectare, pond volume 29,600 m3 and maximum pond depth: 4,3 metres). Water from a drainage system which was made on arable land is collected in the well (S1) and flows by a 225 millimetres diameter PVC pipeline into the reservoir.
Conducted work and time plan for future activities
The scientific question was whether nitrogen and phosphorus content will cause harmful changes in the forest ecosystem. In order to verify this hypothesis, an investigation of the quality of drainage outflow from agricultural land retained in the reservoir located in the forest as well as the quality of groundwater in the vicinity of the reservoir were carried out. The study was performed also in order to clarify, what amount of nitrogen and phosphorus compounds retained in the forest and does not pollute the Baltic Sea. Water samples for physicochemical analyzes were taken from wells once a month since December 2016.
Installations of infrastructure
A flowmeter was installed in the well S1 for measuring the amount of water flowing into the reservoir. Water gauge was mounted in the pond. Two transects of eight observation wells with electronic devices were installed for determination of an impact of the pond on the changes in groundwater level and their physicochemical parameters (P1-P8).
Continuation of water quality control in the reservoir and groundwater as well as dendrological research in the objectives of the assessment of the drainage water impact on tree increments in the region of the reservoir.