Environmental and anthropogenic conditions of changes in hydro-meteorological elements in the Warta River catchment in 1961–2022

The Warta River catchment (WRC), Poland The aim of this study is to detect which factors most likely determine changes in river runoff in the WRC — an area with the most unfavorable water balance structure in Poland, yet strongly influenced by human activity, including agriculture and open-pit lignite mining. Based on daily discharges from 41 water gauges located on 26 rivers, the long-term runoff changes were studied in relation to selected meteorological variables, and the role of the human impact was analyzed. Climate change and human activity often overlap, leading to alterations in the water cycle. While changes in runoff are relatively well understood, it is still difficult to assess the degree of influence of these two factors on these changes. After 1988, runoff from the studied sub-catchments decreased significantly, from less than 10 % (the Gwda River) to almost 50 % (the upper Noteć River); only in one sub-catchment runoff increased by 24 % (the Kiełbaska River). Precipitation did not change statistically significantly during this period. However, significant increase in evaporation was detected, with the smallest increase (up to 20 %) in the sub-catchments of the upper WRC and the Kiełbaska River, and the largest in the sub-catchments of the middle WRC (about 25 %). It was concluded that anthropogenic activity, in particular the open-pit lignite mining was the main factor contributing to the reduction of runoff. • River runoff in the WRC is altered both by climate change and human activity. • Evaporation increased significantly, while precipitation sums did not change. • Lignite open-pit mines have the biggest influence on water resources in the WRC. • It is a first such a comprehensive approach to explain causes of river runoff changes in the WRC.