Effect of slope and different intensities of rainfall using rain simulator on runoff and soil loss one year after forest fire (Shenroud Forest, Guilan)

Forest fires is one of the most important environmental problems and often as an important driver in runoff generation and soil erosion in the forest. This study was conducted with the aim of investigating the mutual effect of slope and rainfall intensity on the runoff generation and soil loss in a burned forest using a rainfall simulator.

This study was conducted in series 4 of Babakoh of the 25 Forestry Zone of Shenroud, Siahkal County, Guilan Province, where about 8(ha) of the forest area of this zone suffered from a relatively severe fire in 2019. The climate of the region is semi-humid and moderate, the minimum and maximum relative humidity varies between 70 and 90 percent, respectively. The height above the sea level is 350 meters, the slope direction is southwest, the annual rainfall is 1465 mm, the initial soil moisture is 16.46%, the texture of the soil is loamy sand. Changes in parameters of runoff volume, runoff coefficient, soil loss and sediment concentration due to changes with two treatments, slopes at two levels of 15 and 30% and rainfall intensity at three levels of 8.16, 7.16, and 4.13 mm min-1 in the time of 6.26 min was investigated using a rainfall simulator. Analysis of variance of the parameters measured in the experiments was performed with three replications within 18 plots (25×25 cm) by using Komforest type rain simulator. Factorial analysis of variance was performed in the form of complete randomized design and comparison of the average interaction effects of treatments was performed with Duncan's method at the 95% confidence level.

The results of analysis of variance showed that the interaction effect of slope treatments and rainfall intensity on the studied components is significant (p≤0.05). The highest amount of runoff volume was recorded in the treatment of rainfall intensity of 16.8 mm/min and slope of 30% with a value of 2418.33 ml and a runoff coefficient of 81.53%  and the lowest one related to the treatment of rainfall intensity of 13.4 mm/min and slope of 15% was recorded with a value of 846 ml. The results of soil loss showed that the highest value was related to the rainfall intensity of 16.8 mm/min with a slope of 15% to the value of 11.06 gr and the lowest soil loss was 13.4 mm/min related to the intensity of rainfall and a slope of 30% to the value of 1.79 gr. The highest concentration of sediment was recorded at a slope of 15% and intensity of 13.4 mm/min.

This research showed that the total average volume of runoff increased by 17.61% in the period of one year after the fire in this area with the increase of slope from 15% to 30%. Also, in the rainfall intensity of 8.16 mm/min, as the maximum intensity in this research, the volume of runoff increased by 33.6% from the slope of 15 to 30%. The results of the study of soil loss showed that it had an average increase of 61.9% in the slope of 15% compared to 30%. Also, there was a 76% increase in the rainfall intensity treatment of 8.16 mm/min. These results indicate that the slope and intensity of rainfall are effective factors in the hydrological cycle and erosion in the forest area of this study after the fire.