Changes in groundwater spring yield patterns in Czechia

Springs are crucial for maintaining surface-water resources and healthy ecosystems and provide a valuable source of information about groundwater. Monthly, seasonal and annual trends in spring yield of 138 springs in Czechia over the period 1971–2020 were evaluated. Furthermore, field significant trends in spring yields in 18 hydrogeological regions were identified. Monotonic deterministic trends were detected by the trend-free pre-whitening modification of the Mann–Kendall test using mean monthly values. Most of the series (70%) did not exhibit any long-term trends in annual data. However, considering statistically significant changes, the decrease prevailed over the increase. A reduction in the yield was observed at 28% of the springs, while an increase only at 2% of them. In the seasonal cycle, the yield decreased mostly in summer (32% of springs), in other seasons the yield decreased at 23–28% of the springs. Most often, the yield decreased in May (42% of springs), June (38%), and July (36%), least often in March (14%) and February (18%). Monthly long-term yield increases were observed only at a maximum of 5% of the springs in any of the months. In past years, a spring maximum shift from April to March or even to February was observed (compared to the 1981–2010 period). However, the increase in March and February spring yield expected due to this shift, probably caused by the air temperature rise and earlier snowmelt, has not been detected yet. Annual time series at 7 of 18 hydrogeological regions exhibited a significant decreasing trend (p < 0.05). No increasing trend was detected in any of the regions when analyzing annual data. The decrease prevailed in crystalline bedrock hydrogeological regions (4 of the 7 regions). However, a significant spring yield decrease in the region of the North Bohemian Cretaceous Basin (area between the Jizera and the Lower Elbe) is worth noting since this area represents a protected area of natural water accumulation. Similar to individual site trends, the most field significant trends occurred in summer (11 of 18 regions) and the least in winter (5 of 18 regions).