Spatial Regularities of Changes in the Duration of Low River Flows in Poland Under Climate Warming Conditions

On the basis of daily discharges recorded in 140 water gauges located on 96 Polish rivers, the long-term changes of runoff and the number of days with low flows (NDLF) in relation to selected meteorological variables were studied. The analyses were performed for the entire multi-annual period 1951–2020 and two sub-periods: 1951–1988 and 1988–2020 that are before and after climate change. The average values of these hydro-meteorological variables in the two sub-periods were then compared. It was found that after 1988, a statistically significant (p < 0.001) increase in the average air temperatures, ranging from 0.9 to over 1.3 °C, occurred. Similarly, statistically significant changes were determined for evaporation, which increased by about 10–25%. Precipitation did not show such changes—a statistically significant decrease in precipitation (by over 5%) was recorded only in the southern part of the Odra River basin, and in most stations, statistically insignificant increases were recorded. The most complex changes took place in river runoff. After 1988, in most gauges, a decrease in runoff by about 5–15% was detected; in some cases, these decreases were statistically significant. In the south-eastern part of the country, primarily in the catchments of the right tributaries of the Vistula River, an increase in runoff by about 5–10% was detected. However, only in the case of one gauge, these tendencies were statistically significant. Next, in order to determine spatial regularities in long-term changes in the NDLF, the cluster analysis method was used, and the gauges were grouped according to the values of 70 annual NDLF. This resulted in separating three relatively homogenous territorially groups of rivers, demonstrating a clear regional differentiation of NDLF. It was concluded that separation of these three groups of rivers in terms of different long-term changes in NDLF was mainly influenced by climatic conditions.