Popular Science: Forest, tell me how high will the spring flood be?
The rate of snow cover accumulation and melting depends on several factors. In addition to the meteorological and topographic characteristics of the area, the vegetation type and condition also play a major role. Within the Czech Republic, snow accumulates primarily in mountainous areas, which are more or less covered with forests in various states of health, which is why scientists focused on evaluating energy balance at three sites with different canopy structure: healthy dense spruce forest, disturbed spruce forest affected by the bark beetle and open meadow. In order to avoid misleading results, all three sites were located in the same catchment area – the Ptačí Brook catchment in the Bohemian Forest – at the same altitude and topography.
The snowmelt rates and the snowpack energy balance were measured over three consecutive cold seasons between 2016 and 2018. The canopy structure was described by the leaf area index (LAI), which was calculated from hemispherical photographs of the sky and canopy. In short, this index is defined as the projected area of leaves over a unit of area. Scientists used the amount of precipitation, snow and air temperature, snow depth, snow water equivalent (SWE) and both incoming and net shortwave and longwave radiation, which were measured with CNR4 Net radiometers. The measurement results represented the input data for calculating the snow energy balance and snow cover rate. A coniferous forest generally modifies the snowpack energy balance by reducing the total amount of shortwave solar radiation and enhancing the role of longwave radiation emitted by trees.
The study results showed that the net shortwave radiation at a healthy spruce forest site represented only 7% of the amount observed at an open site. This was mainly due to the shading effect of the trees. In contrast, the net longwave radiation emitted by trees was the most important energy source for snowmelt. Overall, however, snowmelt rates at a healthy spruce forest site were significantly lower compared to the open site. The results from a disturbed spruce forest site affected by the bark beetle were also very interesting, as scientists had detected progressive decay of the forest over the years, thus noting changes in both leaf area index and shortwave and longwave radiation. The longwave radiation emitted by trees decreased significantly during the study period, from −3.1 W/m2 to −12.9 W/m2 and, conversely, received shortwave radiation increased from 31.6 W/m2 to 96.2 W/m2. Such changes in the energy balance at a disturbed spruce forest site affected by the bark beetle caused an increase in modeled snowmelt rates by 50% compared with a healthy spruce forest during the observed period.
Research into energy flows and snowmelt rates is an essential part of forecasting spring floods and is important for a better understanding of the water cycle. This is especially true today when, in addition to the impact of climate change, landcover in mountain areas is changing rapidly due to forest dieback. The current study shows that, as a result of the large-scale forest dieback in mountain areas, we can expect faster and more severe spring floods. In addition, faster runoff from the basin can also cause a worsening of summer droughts.