PEATLAND HYDROLOGY IN TROPICAL INDONESIA

Most global peat deposits are located within three bands on the earth, centered near the equator and 50 degrees north and south latitudes, where precipitation generally exceeds evapotranspiration on an annual basis. Within these bands, where the land surface is nearly flat, excess water input maintains an oxygen-deficient, waterlogged environment where decay of vegetation is incomplete and peat may accumulate. Peat hydrology influences both peat preservation (enhanced at pH 3-4) and methane production (enhanced at pH 5-6). In equatorial Indonesia, domed (convex upper surface), ombrogenous (rainfall dominated), oligotrophic (low nutrients and dissolved solids) peat deposits predominate and their hydrology is described.

Prior to recent anthropogenic disturbance, peat covered approximately 232,000 km² in Indonesia and Malaysia. Most of the peat is on coastal lowlands (<20 m elevation) up to 100 km wide. Individual peat deposits cover interfluve areas, are bypassed by rivers draining interior mountains, and are not influenced by marine water. Vegetation on domed peat is dominated by trees 20-40 m tall. From the surface to the base of the peat deposits, pH increases from 3-4 to 4-6, total dissolved major cations (TDS) increase from ~5 to ~17-110 ppm, and hydraulic conductivity (K) decreases from >10^-2 to ~10^-3 cm/s. Pore waters in sediments below the peat have a pH of 5-7, TDS ~83 to 260 ppm, and K ~10^-4 cm/s.

Cross sections in thick (>5 m) peat deposits with only minor disturbance show the majority of peat has pore water chemistry and hydraulic conductivity similar to surface peat. In parts of each peat deposit, thin layers near the base of the peat are influenced by ground water flow of pore water from sediments below the peat. Literature review suggests that the proportion of peat that is rainfall influenced with low pH and low TDS is probably greater in tropical ombrogenous peat deposits than in temperate peatlands. Any such latitudinal differences in peat hydrology must be considered in global models that incorporate peatland processes, for example, the role of peat in the global carbon cycle.