Groundwater Fluctuation of a Meliorated Forest Catchment in Connection with the Climate and the Growth of Forest Stands—30 Years of Monitoring

Hydromelioration networks aim to expand the vadose zones of waterlogged and peaty sites, which is expected to provide more soil space for trees’ roots, thus improving forest stand stability and production. However, the recent climate is manifested by increasing air temperatures, changed distribution of precipitation and frequent droughts. This raises the issue of the suitability of such measures. Additionally, the impacts on the ground water table (GWT) level, drainage, and forest health are disputed. This study deals with the behaviour of the shallow-aquifer GWT level in a formerly deforested headwater catchment named U Dvou louček (UDL) which had been monitored for 30 years and placed at a mountain site in north-eastern Bohemia, Czech Republic. GWT (84 m long transect with 22 probes), precipitation, throughfall, discharge, air temperatures and stand parameters were measured. Young Norway spruce stands now dominate within the UDL area. Average precipitation of the open area reached 1285 mm, and precipitation in years 2003, 2015 and 2018 was minimal (910 to 950 mm). Calibration of the GWT measurement took place in 1992–1995. After a 5-year stabilisation period in 1996–2001 with gradually falling GWT levels, following the digging of new ditches, the GWT no longer fluctuated significantly. The discharge and GWT change responses to stand growth were modest, and the impact of forest tending was limited. Lower GWTs in dry years were temporary. The inter-annual water levels in the immediate vicinity of the ditches fluctuated the least. The small-scale technical hydromelioration intervention neither had a negative impact on the GWT level, nor threatened the water supply of growing forest stands even in dry years. The results confirmed the long-term potential of such measures to improve the water regime of reforested clear cuts on waterlogged forest soils in mountains with a minimal risk of threat in warming environments.