Infiltration Model of Former Skid Road

(Case Study in PT. Musi Hutan Persada Industrial Plantation Forest Concession Area II Benakat, South Sumatera)

Tri Padukan Purba, Ujang Suwarna, Elias

ABSTRACT

Introduction. Timber harvest is one of the activities involved in forest management. In its operation, timber harvest causes damage to the environtment, such as changes in hydrology and its functions.

This research aims to find out changes in infiltration rates in former skid roads, create an infiltration model to predict change of infiltration rate in former skid roads, determine the best infiltration model to calculate changes in infiltration rate in former skid roads, and study the relationship between soil density and change in infiltration rate in former skid roads.

Methods. Measurements of infiltration rate and soil sampling were conducted in former skid roads with cutting plot/setting tebang (setting 18, 183, 183b, and former skid road in landing site), former skid roads not passed by forwarder (tidak dilalui forwarder or TDF), and in Acacia mangium stand as control site.

Results and Discussion. Setting 183b had the greatest soil bulk density of 1.437 gr/cm3 while the least bulk density of 0.992 gr/cm3 was obtained at control side. Average bulk density in former roads passed by forwarder (settings 18, 183, 183b, and landing) is 1.304 gr/cm3, which is the greater compared to TDF (1.247 gr/cm3) and control (0.992 gr/cm3). Greatest water content was found in landing site with value of 38.499%, while setting 18 had the least water content of 20.544%. From regression analysis between water content and bulk density the formula Y = 1.35 - 0.0034 X was obtained with R2 of 0.21. This shows a low correlation between water content and bulk density. Soil bulk density therefore is not determined by amount of water contained. In this research, soil bulk density is more affected by forces acting upon the soil surface. Greatest and least soil porosity was obtained at control site and setting 183b with 62.566% and 45.774%, respectively. Changes in soil porosity occur because of pressures on the soil surface. Average infiltration rate of control site had largest value of 2.294 cm/minute, and lowest value was obtained at setting 183b with 0.006 cm/minute. Average infiltration rates of former skid roads passed by forwarder (setting 18, 183, 183b, and landing) were categorized as slow to medium, while infiltration rates at TDF and control were categorized as fast. Decrease of infiltration rates in former skid roads ranged from 96% to 99.73% with decrease rates of 132.142 cm/hour - 137.281 cm/hour. At TDF, decrease of infiltration rate value is 71.51% with decrease rate of 98.438 cm/hour. Regression analysis between infiltration rate and soil bulk density shows a correlation between the two values, where infiltration rate decreases while soil bulk density increases. Regression analysis between infiltration rate and soil water content shows low correlation. In this study, infiltration rate is more affected by soil bulk density and porosity. From calculation of square differences, Horton's model is the best model for all seven locations sampled.

Conclusion. Changes in infiltration in former skid roads occur due to decrease in infiltration rate. Decrease of infiltration rate ranges from 96% to 99.73%. Decrease in infiltration rate in former skid roads is the result of increase in soil bulk density due to forces acting upon soil surface, in this case the forwarder. The best infiltration model to calculate infiltration rate in former skid road is Horton's model. Increase in soil bulk density is caused by pressure of forwarder on soil surface.