Abstract
Abstract There is public concern that large-scale disturbances to forest cover caused by insects and storm winds in the Bohemian Forest could intensify high water flows and enhance the expected flooding risks predicted in current regional climate change scenarios. We analysed stream discharge in Upper Vydra and Große Ohe, neighbouring catchments in the Bohemian Forest, the largest contiguous forested area in Central Europe. Upper Vydra, in the Šumava National Park, and Große Ohe (including the Upper Große Ohe headwater catchment in the Bavarian Forest National Park) are similar in size, but differ in land use cover and the extent of disturbed Norway spruce stands. Publicly available runoff and meteorological data (1978–2011) were examined using non-parametric trend and breakpoint analysis. Together with mapped vegetation cover changes, the results were used to address the following questions: 1) are there significant changes in the hydrological cycle and, if so, do these changes relate to 2) the extent and expansion of disturbance in forests stands and/or 3) altered precipitation dynamics and thermal conditions? We found no marked overall change in annual runoff or in annual or seasonal precipitation, but a significant increase in high flows in March. This overall trend related to the marked warming in late winter and early spring (+~4 K in April, p < 0.01), irrespective of altitude and slope position. It significantly shifted the end of the snow cover period by more than three weeks to the beginning/middle of April depending on altitude, and intensified snow melt. In the Upper Große Ohe catchment, a significant decrease in catchment balance, the difference between the long term precipitation and runoff (–72 mm, 11%) was found when the loss of tree cover reached 30% of catchment area. Diminished evapotranspiration losses from severely disturbed stands increased groundwater recharge during summer and caused a significant rise in low flows in autumn. However, observed increases in late winter high flows were due to warming only. They could be further intensified by the increasing winter precipitation predicted under present climate change scenarios, and would therefore increase the risk of flooding at lower elevations.
Highlights
There is only a moderate degree of certainty that precipitation has increased in the mid-latitudes of the northern hemisphere through human influence (IPCC 2013)
Over the study period from 1978 to 2011 Annual runoff yields changed by +2.2 mm yr–1 (+8%) in Upper Große Ohe catchment (UGO) and –1.5 mm yr–1 in Große Ohe (GO) (–8%)
These changes were caused by significant quantitative changes in runoff in March in UGO (+2.2 mm yr–1, +118%) and May in GO (–1.5 mm yr–1, –46%)
Summary
There is only a moderate degree of certainty that precipitation has increased in the mid-latitudes of the northern hemisphere through human influence (IPCC 2013). Changes in the hydrological cycle at the earth’s surface are directly related to warming, which, depending on the season in which warming has occurred, alters the solid-liquid partitioning of precipitation, snow cover dynamics and evapotranspiration rate (Hidalgo et al, 2009). After tree die-off, evapotranspiration from overstorey vegetation declines, while energy and water flow to the understory and on the soil surface increases (Adams et al, 2012). Water fluxes into and through the soil are altered, resulting in changes in the generation of discharge (i.e. contributions of groundwater and subsurface flows to runoff) and in the runoff yield itself when catchment area affected is large (Beudert et al, 2007)
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