Evaluation of time-variant travel time distribution and storage selection function in Lake Taihu

Abstract

Lake Taihu is a large shallow lake with a complex river network suffering from a severe eutrophication problem for a long time. Harmful algae blooms are frequently observed in Lake Taihu due to the excess nutrients and pollutants. Understanding water cycle dynamics is an essential prerequisite for investigating biogeochemical and ecological processes in Lake Taihu. The time-variant travel time distribution (TTD), residence time distribution (RTD) and StorAge Selection (SAS) function which describe how the water is stored, mixed and released in the lake provide fundamental information on the timescale of water transport, storage dynamics and variation of flow regimes. In this study, long-term deuterium isotope data in Lake Taihu and surrounding rivers are used to estimate the time-variant TTD and RTD of lake water. Three models are established based on the two-year time-series data including river flow rate, precipitation, lake volume, wind speed, lake temperature, humidity, deuterium isotope compositions in precipitation, in rivers and in the lake. The mass balance model for lake water ensures the mass balance of lake water; the age master equation model tracks the volume of water parcels of different ages; and the isotope mass balance model is employed to calibrate the model parameters used in the age master equation model. The estimated travel time of lake water ranges from 4-8 months in dry season and 2-5 months in wet season. The time-variant RTD shows that most lake water is younger than 8 months. Lake Taihu shows an inverse storage effect, i.e., more young water is released when lake volume is high, and more old water is released when lake volume is low. Temporal variation of spatial distribution of deuterium isotope content indicates storage selection preference of lake water of different ages is controlled by the variation of flow paths and velocities of lake water.