THE STORMFLOW CHEMISTRY AT LA-CUENCA, WESTERN AMAZONIA

The stormflow chemistry and its link to hydrological flowpaths is poor ly documented for Amazonian rainforest streams. This paper presents a case from the so far undocumented Ultisol region of Wester Amazonia. W ater samples were taken frequently during thirty events over more than two years and analyzed for major cations and silica. Bulk overland fl ow samples were collected for the same events. Throughfall and pre-eve nt soil water, hillslope and floodplain ground-water were collected fo r some of these events. The stormflow response pattern of calcium, mag nesium and silica was a consistent dilution: the extent of which was l arger in the dry season. A small pH depression was recorded for all ev ents regardless of season. The potassium storm-flow chemographs are co ntrolled by antecedent moisture conditions, they range from flat, disc harge-independent during the peak of the rainy season to spike-like, d ischarge-dependent in the dry season. The dilution effect for silica a nd the concentration effect for potassium are generally accounted for by fast flowpaths, as exemplified by overland flow. This hydrochemical approach underlines the importance of overland flow in this tropical rainforest catchment.