The structure of precipitation and rainfall erosivity in the upper Parsęta catchment (Drawskie Lakeland, NW Poland) in 1987-2020 as an indicator of climate change

The impact of climate change observed in recent decades can be noticed in the structure of precipitation. The increasing amount of periods without rainfall, decreasing annual snowfall totals, and shortening the duration of snow cover significantly affect water resources and the intensity of a number of environmental processes, such as soil erosion by water.

The main aim of this paper is to determine the structure of rainfall in years 1987-2020, based on series of meteorological measurements in the Parsęta Base Station of Integrated Monitoring of Natural Environment at Storkowo in Drawskie Lakeland (NW Poland). The analyzes included precipitation amounts, number of days with precipitation, rainfall intensity, kinetic energy and erosivity and several rainfall indices. During this period, there is observed a significant increase of air temperature, which equals 0.47°C for 10 years. In the case of precipitation, a small increasing trend is marked statistically insignificant. The average annual precipitation was 698.6 mm, whilst precipitation in the winter half-year equalled 41.2% of total and 58.8% in the summer half-year. The annual rainfall erosivity, calculated according to Wishmeier and Smith’s formula, changed from 144.7 to 782.1 MJmm/ha/h, while Modified Fournier Index (MFI) ranged from 53.8 mm to 119.0 mm and was not statistically significant.

The analysis of precipitation with different daily totals did not show a significant increase in the share of precipitation with higher values. The relative precipitation index (RPI) showed no increase in the number of dry months of a year. Moreover, the analysis of occurrence of periods of light droughts, dry spells and droughts does not indicate any significant increase in the number and frequency of such events. On the other hand, a similar analysis of vegetation period (April-September) shows statistically insignificant trend of decrease in the number and frequency of precipitation less series. Another indicator important for the assessment of water conditions, the Sielianinov hydrothermal coefficient was calculated for period April-October, and showed lack of long-term trend changes in the observed period.

The water shortages in the upper Parsęta catchment observed in recent years are probably the result of decrease in the contribution of snow in the precipitation structure and a significant reduction in the number of days with snow cover. This limits the underground retention and surface outflow and has an impact on the functioning of biotic environment and agriculture.