Analys och modellering av effekter på förändrad vattenföring i brandskadad och avverkad skog
Sammanfattning: A forest fire started in the county of Västmanland on the 31st of July in the summer of 2014. As a result of a dry summer and hard winds, the fire rapidly increased in size and came to be the largest fire in modern Swedish history, covering a massive area of 14 000 hectares. Forest fires have been common historically and have occurred on a frequent basis in boreal woods, but due to more effective fire fighting over the years it has been possible to extinguish the majority of the fires before they have been able to increase in size. However, expected changes in the climate such as higher temperatures and less precipitation extend the fire season and large fires may therefore occur more frequently in the future. The aim of this study is to analyse how the discharge alter as result of forest fires and trees felled due to fire damage. SMHI’s model named HYPE was used in order to identify any possible differences in discharge. The model has been developed by SMHI to enable the calculation of discharge in areas where there are no available monitoring stations. It has however never been determined how fire damaged areas should be represented in this model. It was during this project therefore required to calibrate those parameters in the model that supposedly are correlated to the effects of fires. Discharge has been registered by SMHI between 1979 and 1998 in Valsjöbäcken, which is one of the fire-damaged watersheds in 2014. The data was used to calibrate the model for the period before the fire to eliminate any deviations that may have caused existent differences. The results were then evaluated both visually and by utilising the Nash Sutcliffe’s efficiency coefficient. After the fire, the Department of Aquatic Sciences and Assessment at SLU placed a monitor at Valsjöbäcken to register the water depth. Discharge measurements were also performed and used to create a rating curve to convert registered water levels to discharge. As a result of running simulations in HYPE, it was shown that significant alterations had occurred in terms of hydrological attributes in the affected area. The largest differences were noticed during spring flood and wet periods in the late summer and autumn period. It was also shown that the discharge registered after the fire was greater than the discharge calculated by the model. The high discharge during springtime is explained by the fact that the snow melted much faster. Also, the loss of vegetation allowed more precipitation to reach ground level and evaporation was decreased, which led to more water available for discharge in the area. After the fire, the increase in higher water levels in combination with a decrease of the capability for the ground to hold water resulted in a higher discharge. By calibrating the affected parameters, it was possible to see an increase in the Nash Sutcliffe’s efficiency coefficient. The coefficient reached 0.830 compared to the previous 0.567, which confirms the observed hydrological changes.
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