Wpływ pożaru lasu na czas opóźnienia odpływu bezpośredniego wód opadowych z małej zlewni w Kalabrii

Hydrological processes in forested areas such as runoff and erosion after heavy rainfalls are sensitive to changes in land surface properties that can be significantly modified by fire. Rainfall−runoff events recorded in a small forested catchment in Calabria (southern Italy), of area of 1.36 hectare, in the period of 2014−2015, were analyzed to check differences in lag time of direct runoff, i.e. the basic characteristics in rainfall−runoff modelling, before and after the forest fire. Lag time, defined as the elapsed time between the occurrence of the centroids of the effective rainfall intensity hyetograph and the storm runoff hydrograph, was computed with the use of two methods of effective rainfall estimation, i.e. CN−SCS (Soil Conservation Service) and Duband method. In the first one effective rainfall starts after no runoff period, when rain water is used for inter− ception, filling local depressions and high infiltration. In the other one, effective rainfall starts at the beginning of rain and then increases more gentle than in the first method. The results show that in response to the forest fire, the average lag time was reduced to 28% of the average value representative for original conditions when CN−SCS method was used (from 0.530 h before the forest fire to 0.145 h after the fire) and to 38% when the other method was used (from 0.637 h to 0.243 h, respectively). Significance of the differences in lag time was confirmed statistically with Mann−Whitney U test. The results of the investigation, i.e. reduction of lag time, and in consequence higher value and earlier appearance of peak discharge, confirm the thesis that forest fire would increase the risk of flood flows.