Water storage and runoff processes in plinthic soils under forest and pasture in Eastern Amazonia

Extensive areas of the Amazon River basin are underlain by soils with shallow impeding horizons. To evaluate how the distinctive hydraulic properties of soil with a plinthic horizon under forest and pasture affect water storage and runoff process, two first-order catchments drained by ephemeral streams were instrumental in eastern Amazonia. Field measurements showed the presence of a strong vertical gradient of saturated hydraulic conductivity, which declines to extremely low values (median <1 rum h(-1)) at the plinthite layer, limiting both vertical and lateral flow, and keeping the soil water content close to saturation throughout most of the wet season. This scenario led to the frequent occurrence of saturation overland flow (SOF) under both land covers and very small amounts of shallow sub-surface flow (SSF). The annual flow in the exit channels was 3.2% of throughfall (2.7% of annual rainfall) under forest and 17% of annual rainfall for pasture, while the frequency of days with overland flow (OVF) was about 60% of the days for both catchments during the wet season. In the forest, all OVF originated from saturated areas, while in the pasture, infiltration-excess OVF accounted for 40% of the runoff and SOF accounted for 55% of runoff. The higher flow generation in the pasture could be explained by the higher water storage compared to the forest, promoting more frequent SOF, and additionally by the lower hydraulic conductivity near the surface favouring the occurrence of Horton overland flow (HOF). Copyright (C) 2006 John Wiley & Sons, Ltd.