Seepage exchange in an Amazon floodplain lake

Abstract

Exchange of water between the lake and the surrounding groundwater system was computed for a permanent lake on the central Amazon floodplain (Lake Calado) from an empirical seepage model incorporating 240 direct measurements of seepage from the lakebed and seasonal changes in lake stage and morphometry. Seepage rates varied with the daily rate of change in stage height and inversely with depth of the lakebed from the water surface. Since the hypsometric characteristics of the lake are well known, overall seepage for the 1983–1984 water year was estimated by integrating the product of seepage rate per unit area for a given rate of stage change and depth of lakebed with the representative area of lakebed at a given depth as each varied over the annual cycle, then integrating the daily rates of seepage over time. The volume of seepage outflow over the rising‐water period (4 × 106 m3) is plausible if the rate of groundwater flow is constrained by the hydraulic conductivities expected for the soils. The 10‐m annual change in water level and the potential movement of floodwaters into deeply weathered tropical soils, beyond the silt and clay deposits immediately adjacent to the main‐stem river channel, represent conditions that are particularly conducive to seepage exchange and are representative of a significant number of lakes on the Amazon floodplain.