Twenty year trends in precipitation and surface water chemistry of two LTER small forest catchments in Wallonia, Belgium

The ca. 80 ha forested catchments Robinette and Waroneu are located in the state forest of Hertogenwald, Belgium (50°33′ N, 6°04′ E). Waroneu was covered in the early 1980s, with hardwoods (40%) and Norway spruce (Picea abies (L.) KARST.) (60%). By 2011, the proportions had changed to 38% spruce, 45% deciduous and 17% open areas. Waroneu was limed in 1992 with 3 T/ha dolomite lime and 200 kg/ha potassium sulphate. La Robinette was initially covered with Norway spruce, but following windthrow in the 1990s, and a clear-cut in 1996, it was partially replanted with deciduous species (1998). The geological substratum of both catchments consists of quartzites, quartzo-phyllades and Revinian phyllads, covered with acidic brown soils. Both catchments have been monitored for rainfall, throughfall, soil solution and runoff chemistry since 1992 within the frame of several projects, assessing the effects of liming, clearcut and reforestation with deciduous species. Data are also used within several regional and European assessments (State of the Walloon Environment, calculation of critical loads, effect of air pollutants on ecosystems). Since 2013, 12 intense monitoring plots were installed within the catchments to represent spatial variability and to follow soil quality (Ratcliffe et al., 2018).
Here we present temporal 1992-2014 trends of monthly rainfall, throughfall (under Norway spruce) and bi-monthly runoff data. Data were analyzed with the non-parametric seasonal Mann-Kendall test, adapted for serial dependence, if necessary. Slopes were estimated with the seasonal Kendall slope estimator. The time and number of breakpoints within the time series was estimated with the BFAST algorithm (Breaks For Additive Season and Trend).
No significant global trends were detected in rainfall, throughfall and runoff water volumes in both catchments. One breakpoint for runoff at Robinette, likely related to the clearcut, was detected. A general decrease in acidifying and eutrophying compounds was observed in both catchments, concurrent with an increase in ANC and pH, indicating a progressive recovery from acidification.

Acknowledgement: This study was financed by the ‘Service Public de Wallonie’, DGO3-DGARNE.