In the hydrological years 2000-2004, studies were conducted in the Mała Wełna river catchment (up to the Kiszkowo gauging section) having a high lake density index. The water balance was used to establish the changes in water retention in the catchment. Then, relationships were established between the retention changes and the precipitation totals and the changes in groundwater levels. It was found that the monthly and yearly retention changes were positively correlated with the monthly and yearly precipitation totals and groundwater level changes in two observation wells situated at a distance of 135 and 250 m from a girdling ditch. Therefore, the changes in groundwater levels in these wells may provide a basis for estimating the retention changes in the catchment.

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