Seasonal evolution of the isotopic composition of atmospheric water vapour above a tropical lake: Deuterium excess and implication for water recycling

Abstract

We present an analysis of the rainfall–evaporation–atmospheric moisture cycle in a semi-arid tropical zone (southwestern Madagascar) to quantify the recycling and mixing processes that occur above an endorheic lake system (Lake Ihotry) during an annual cycle. The study combines an isotope mass balance with a detailed field investigation of the lake system and a previously established daily time-step lake water balance model. The mass balance and Craig-Gordon equations are used to calculate the isotopic composition of the evaporative flux from the lake surface ( δ E) and to derive a daily time series of the ambient atmospheric water vapour composition above the lake ( δ AL) during a 8-month dry season. Calculated δ AL results from a mixing between regional moisture ( δ AR) and locally evaporated water ( δ E), the latter representing 50% of δ AL at the end of the dry season. The contribution of recycled moisture to on-lake precipitation during the wet season is estimated to ⩾16%. We show that, as expected, the deuterium excess is high in recycled precipitation and low in evaporated precipitation, but also that the recycled moisture in an endorheic system may have a low deuterium excess resulting from the low deuterium excess in regional precipitation. In case of a long evaporative season, the atmospheric moisture is not in isotopic equilibrium with the annual composition of precipitation because of the contribution of the recycled vapour to the local atmospheric pool. Our approach demonstrates the importance of water recycling on the atmospheric moisture cycle and precipitation in a tropical semi-arid system, and can be applied to other natural systems, enlarging the potential range of investigation of the atmospheric vapour cycle and rainfall sources in tropical lands. It may also represent a valuable complement to direct water vapour sampling, in yielding the long-term evolution of the atmospheric vapour composition with spatially averaged values and smoothed temporal variations.