Hydrology of Acrisols beneath Dipterocarp forests and plantations in East Kalimantan, Indonesia

The hydrology of Haplic Acrisols on sandstone beneath old-growth and slightly logged Dipterocarp forests, and beneath Paraserianthes and Eucalyptus plantations in the lowlands of East Kalimantan was investigated to determine possible ecological constraints for forest and plantation management. The soils are characterized by 40–60 cm thick E horizons, texture silt clay, above clayey Bt horizons. Soil moisture (θ ), measured during a period of few weeks at the end of the rain season, was typically close to field capacity. Field-saturated hydraulic conductivities (K GP ) indicate that the E horizons are moderately permeable (medians of K GP 2.5–42.4 cm d−1 ), while the Bt horizons are virtually impermeable (medians of K GP < 0.17 cm d−1 ). Macropores generated by biota are likely the cause of the observed large within-plot variability of K GP in the E horizons and may provide pathways for preferential water flow. Soil water budgets constructed from measurements of precipitation (245 cm y−1 ), throughfall (210–230 cm y−1 ), using soil hydrological constraints obtained from K GP values measured in the Bt horizons, and assumed transpiration rates (90–110 cm y−1 ) suggest that 40–140 cm y−1 of the soil water is drained laterally in the E horizons. Accordingly, < 5–40 cm y−1 are available for deep seepage below 40–60 cm depth. Since lateral water flow is expected to increase with distance from the top of slopes, lower slopes receive large amounts of lateral water, depending on the slope length. It is concluded that the thickness, pore size distribution and permeability of the E horizons along with slope aspect are master variables for soil hydrology in the study area and determine site quality for forestry. Degradation of the E horizons due to compaction and erosion will increase surface flow, the frequency of flooding events and the impact of droughts on Dipterocarp forests and plantations of fast growing tree species in the lowlands of East Kalimantan. The conservation of topsoil physical and hydrological characteristics is therefore a prerequisite for sustainable forestry and landuse in this region.