Dry and wet periods in north-central Poland in the years 1952–2020

Abstract

The study investigates correlations between atmospheric circulation, precipitation, potential evaporation, Climatic Water Balance (CWB), and river runoff in north-central Poland and quantifies the occurrence of dry and wet periods. The analysis covers the years 1952–2020. Dry and wet periods were identified using monthly Standardised Streamflow Indices (SSI) calculated for moving windows of 1, 3, 6, and 12 months (SSI-1, SSI-3, SSI-6, SSI-12). The study also employs a unique calendar of circulation types over north-central Poland, from which atmospheric circulation indices were calculated: zonal (W), meridional (S), and cyclonicity (C). The results show that, in north-central Poland during 1952–2020, the annual totals of potential evaporation and Climatic Water Balance (CWB) both exhibited statistically significant trends. The rate of change was + 27.8 mm per decade for potential evaporation and − 29.6 mm per decade for CWB. The annual precipitation totals did not display a statistically significant trend. The analysis revealed a significant correlation between the cyclonicity index (C) and the Standardised Streamflow Index (SSI) for all the analysed rivers and moving windows. Dry periods occurred under strong anticyclonic circulation, whereas wet periods were associated with the dominance of cyclonic circulation. Additionally, the correlation coefficients between precipitation and SSI, potential evaporation and SSI, and CWB and SSI were statistically significant across all moving windows. The value of the correlation coefficient was lowest for SSI-1 and highest for SSI-12, reflecting the influence of seasonal variability in the annual cycles of the analysed climatic and hydrological elements within the study area.