Understanding Spatial and Temporal Variability of Water Balance from Tropical Peatland Landscape

Hydrology plays a pivotal role in the geomorphology and carbon balance of tropical peatlands. The alteration of the hydrological processes due to climate and/or land cover change might result in significant impacts to this ecosystem. Therefore, improved understanding of tropical peatland hydrology is critical in order to evaluate their fate under current and future climate and ultimately to develop sustainable peatland management practices. However, due to its complexity related to various flow interactions and anthropogenic interferences, comprehensive hydrological studies based on measured field data on tropical peatlands are still limited. Alternatively, hydrological models have been used to simulate the major components of the hydrological processes and to answer “what-if” questions.

In this context, a fully distributed and physically-based MIKE SHE model was used to simulate the water balance within Padang Island in the eastern coast of Sumatra, Indonesia. The island is characterized as a mosaic landscape of natural forest, forest plantation and smallholder agriculture. Comprehensive data set from field measurements including high resolution digital terrain model derived from airborne LiDAR were used for the model development. The model was calibrated and validated against observed groundwater level and stream flow data distributed across the island. The simulation was performed using current climate data that cover a distinct dry and wet year. The subsidence impacts were investigated by simulating the future projection up to 50 years. Further, additional scenario was developed to represent the pre-existing condition without agriculture and forestry practices to evaluate the land cover change impacts.

The results show that the water balance is predominantly controlled by climatic variables. The evapotranspiration accounts for the main water loss representing 50 – 80 % of the total annual rainfall. The amount of evapotranspiration remains relatively constant in the temporal basis irrespective to the rainfall, which means that the magnitude and direction of the remaining hydrological flow paths are driven by the balance between rainfall and evapotranspiration. In the dry period with a rainfall deficit, the water storage is depleted in order to meet the evapotranspiration demand. In the wet period, the excess rainfall is transformed into overland flow, base flow and positive storage change which contributes to increased inundation frequency.

The future projection indicates that there is a shift in the hydrological flow path, as the overland flow increases and the groundwater flow decreases due to the changing topography from peat subsidence. However, the hydrological flow path of the natural forest in the central part of the island remains relatively intact. The agriculture and forestry practice doesn’t significantly alter the hydrological flow path compared to the pre-existing condition. In addition, the boundary impact to the natural forest is not apparent under the wet period, while it gets more prominent in the dry period (~300 meter under current condition).

Our results, which are among the first comprehensive hydrological studies for the tropical peatlands, should help to improve the understanding of landscape scale hydrological processes in tropical peatland, which is relevant for scientists and policymakers to develop science-based peatland management practices.