Changes in runoff in two neighbouring catchments in the Bohemian Forest related to climate and land cover changes

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 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.

Numerical experiments to explain multiscale hydrological responses to mountain pine beetle tree mortality in a headwater watershed

“…[, ] found significant stand‐level changes in intercepted snow under MPB‐affected stands; however, their larger‐scale synoptic surveys and paired watershed approach did not consistently find changes in peak SWE. Statistical analysis across impacted watersheds in Colorado also identified no change in peak runoff with MPB infestation [ Biederman et al ., ], in contrast to earlier studies of the hydrologic effects of MPB outbreak in the Rocky Mountains [e.g., Bethlahmy , ; Potts , ] and more recent work in the Bohemian Forest of Central Europe [ Bernsteinová et al ., ]. Recent watershed modeling efforts have also identified the potential for increased streamflow when only land surface processes are robustly considered [ Chen et al ., ], but also highlight the importance of regenerating vegetation in mitigating the streamflow response [ Livneh et al ., ].…”

“…The tree‐scale loss of transpiration [ Collins et al ., ; Hubbard et al ., ] and canopy cover [ Pugh and Gordon , ] combine to produce inconsistent changes in evapotranspiration (ET) and snow processes at the forest‐stand or hillslope scale. While some studies report decreases in modeled [ Mikkelson et al ., ] or observed [ Frank et al ., ; Bernsteinová et al ., ] total ET, leading to increased water yield at the hillslope or flux tower scale, others show mitigating changes that minimize the net effect [ Biederman et al ., ; Brown et al ., ]. The loss of transpiration often leads to a temporary increase in soil moisture under affected trees [ Clow et al ., ; Bearup et al ., ; Norton et al ., ] that may feedback to increased ground evaporation, which is also enhanced through increased exposure with canopy loss.…”

“…Similar to the inconsistent response in ET, peak snow water equivalent (SWE) has been observed to either increase [ Boon , ; Pugh and Small , ; Perrot et al ., ; Winkler et al ., ; Bernsteinová et al . ] or to be unaffected [ Biederman et al ., ] in MPB‐affected forests. Increases in exposure with canopy loss may lead to compensating sublimation rates [ Biederman et al ., ] and an advancement in the timing of snowmelt [ Boon , ; Pugh and Small , ; Biederman et al ., ].…”

Are bark beetles responsible for droughts in the Šumava Mts.? A mini-review

“…Both the cooling effect of green forest and the decrease in evapotranspiration when green forest dies occur, but this is also true for clear-cut areas to an even greater extent. However, vegetation loss does not necessarilly mean an increase in water run-off, which is supported by the long-term measurments recorded for Modravský potok (Hruška et al 2016) and upper Vydra and Große Ohe (Bernstein et al 2015).…”

Forest Disturbance Feedbacks From Bedrock to Atmosphere Using Coupled Hydrometeorological Simulations Over the Rocky Mountain Headwaters

“…In response to lowered transpirative demand after initial outbreak, soil moisture increases under individual MPB‐infested trees (Clow et al, ; Mikkelson et al, ). Conceptual models predicted that this heightened moisture, paired with changes to vegetative surface roughness, would increase streamflow in affected watersheds (Adams et al, ; Pugh & Gordon, ), a hypothesis which some stream gauging, paired watershed studies, and timber harvest experiments appeared to support (Bernsteinová et al, ; Potts, ; Stednick, ; Zhang & Wei, ). Recent hydrologic research, however, has shown no identifiable change in water yield in response to infestations (Biederman et al, ).…”