Characteristics of soil and hillslope runoff in a humid tropical forest and in Sumatra, Indonesia: the impact of land use change

Deforestation and plantation may exert serious impacts on soil properties and hydrological processes. In the temperate regions, increase of floods after land use change has been widely discussed. However, field investigations of subsurface runoff process are limited in a thick soil layer in a humid tropical forest and therefore the effect of land use change in those regions is still uncertain.

This study conducted monitoring of groundwater and soil moisture to understand the hillslope runoff process and the impact of the land use change in humid tropical forest in Sumatra, Indonesia. Soil hydraulic properties in the tropical and oil palm land were also examined. Unsaturated vertical infiltration was simulated using the soil properties obtained from the experiments.

The field monitoring showed that there was steady shallow groundwater table across the depth of 2 to 3 m within a 4 to 5 m soil layer. The groundwater table responded quickly and largely to rainfall events even in the thick loamy soil layer. This is due to aggregate structure of the soil, which comprises a highly permeable and saturated soil layer. The modelling showed the notable contribution of vertical infiltration to the groundwater recharge.

For the simulation of land use change, the groundwater recharge increased by 26 % in the young oil palm setting compared to the forest setting; the recharge was facilitated by the wetter soil condition due to smaller transpiration in the young oil palm setting. Furthermore, even with 40 times smaller infiltration capacity in the top soil layer in oil palm land, only little overland flow was generated.

Overall, we highlight the importance both of vertical infiltration and subsurface flow in thick soil layer in humid tropics and land use change impacts on these processes.