The Eco-hydrology of Tropical Peatlands Associated with Land Cover Changes

Tropical peatlands play an important role in global climate system by storing an immense of carbon that had been accumulated over thousands of years. Peatlands provide another important ecosystem service by regulating the hydrology. It is believed that peatlands act like a giant sponge by absorbing substantial amounts of water in wet season and gradually releasing the water in the following dry season. Nonetheless, there is a lack of information about the hydrological processes that occur in tropical peatlands, especially the effects of land cover change on peat and peat hydraulic properties. In this study, I conducted field surveys to evaluate two main peat hydraulic properties: saturated hydraulic conductivity (Ks) and moisture retention characteristics at different land cover types in tropical peatlands of West Kalimantan, Indonesia. I also explored the potential of ground penetrating radar (GPR) to determine peat properties in tropical peatlands. Across all sites, Ks varied over four orders of magnitude with depth (ca 0.001 – 13.9 m d-1). The saturated hydraulic conductivity in forested sites at the depth of 50-100 cm (1.08 ± 0.39 m day-1) was significantly higher than Ks at deeper layers. In addition, Ks at the upper layer of forested sites was significantly higher than Ks at the same depth in other land cover types, i.e., recently burned forests, seral community, and oil palm plantation. The best-approximating hierarchical model for estimating Ks included depth, forest cover, a depth and forest cover interaction, and the von Post degree of decomposition. There was no evidence that Ks was related to other peat physical and chemical properties.The peat moisture retention characteristics presented in the van Genuchten (VG) model indicated that bulk density was strongly and negatively related to the α parameter and there was no evidence that peat properties were strongly related to the m shape parameter of VG model. The proportion of macro-porosity in the drained sites with the distance < 50 m from canal was less than those the drained- seral sites > 50 m from canal and forested sites. Peat pore distribution (i.e., the proportion of macro-, meso- and micro-porosity) also was strongly related to bulk density.The GPR results indicated that dielectric varied from 5.8 to 84.9 across all sites and were significantly lower at the 50-100 cm depth than those measured at 300- 400 cm and 500-600 cm. Parameter estimates from hierarchical models indicated that ash content and carbon concentration were strongly positively related to dielectric and the relationship varied among sites.My results suggest that tropical peatlands provide essential environmental services by storing huge amounts of water. We estimated that the potential amount of water that can be stored by undrained peat swamp forests in Borneo, Sumatra, and Peninsular Malaysia was about 51.1 – 52.5 km3 of freshwater. However, the tropical peatlands release water relatively easily when the water table is lowered since it is mostly composed by macro-porosity. Therefore, maintaining the water table close to the peat surface is crucial to prevent water loss from peat. My results also suggest that GPR can be useful for mapping peatland distribution, providing estimates of peat depth and insights into its properties. However, the manual coring is still needed to improve the accuracy and quality of peat property measurement data.

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