Long term changes of river flow from a small agricultural catchment in the center of Poland

Hydrological monitoring of daily discharge since 1966 in a small agricultural catchment of the Mławka River of 66.2 km², located in the macro-region of the North Mazovian Lowland, which is central part of the Vistula River basin, have shown a progressive decrease in renewable water resources in the considered multiannual periods of 1966–1990 and 1991–2020. With a similar mean annual precipitation of about 565 mm in both multiannual periods, the mean annual discharge decreased by 15.6%, i.e. from 171 mm to 144 mm, respectively in 1966–1990 and 1991–2020. This corresponds to mean flow (SSQ) in the multiannual periods of 358 and 0.302 m³·s^–1, respectively. The runoff coefficient decreased from 0.303 to 0.265. Recent data, of the hydrological years 2021-2024, confirm the decrease in renewable water  resources.

This may be attributed to the increased evapotranspiration caused both by an increase in the mean annual air temperature (by about 0.30°C per 10 years in 1951–2020) in the region and an increase in the proportion of forested land in the catchment by about 10% (from 23% to 33%) at the expense of arable land.

The low flows in the considered multiannual periods, characterised by the mean flow from annual minimums (SNQ), decreased even more significantly by 29.1%, i.e. from 0.147 to 0.104 m³ s^–1.