The Effect of Deforestation on Catchment Response in the Tropical Climate Region: Case Study for Sungai Padas Catchment
Deforestation activities have been widely known as one of the devastating factor to the river system and ecological system in a catchment. Deforestation activities at some areas within the Sungai Padas catchment has resulted in the changes of land cover particularly in Tambunan, Sook and Sipitang catchment area. The main objective of this study is to investigate the direct runoff hydrograph in Sungai Padas, due to the changes of land cover caused by deforestation activities using HEC-HMS (version 2.2.2). The design direct runoff hydrograph resulting from 2, 5, 10, 25, 50 and 100-yr ARI for 72 hours rainfall duration were applied to evaluate the effect of deforestation on catchment response. The results of the study indicated that the increase of deforestation approximately 20% has led to the increase of the runoff hydrograph peak and volume between the ranges of 3.34% to 5.71%, and 3.30% to 5.93%, respectively, based on 2, 5, 10, 25, 50 and 100-yr ARI for 72-hours rainfall duration. In addition to the evaluation of runoff hydrograph due to the land cover changes from forest area into logged forest area (disturbed forest), the model (HEC-HMS model) has also been used to evaluate the generation of runoff hydrograph resulted from the changes of land uses in the future when logged forest converted into large scale agriculture (e.g. rubber or oil palm). The result from the study indicated that the cultivation of large scale agriculture activity causes the additional increase in runoff hydrograph peak and volume compared to hydrograph resulted from the logged forest area. The overall results of the study summarized that the increase of runoff hydrograph peak at Beaufort discharge station is ranging from 3% to 6% due to 20% increase of deforestation area without cultivation, and 19% to 30% due to 20% of the total area converted from deforested area into large scale agriculture such as palm oil and rubber trees.