Circulation patterns in a shallow tropical reservoir: Observations and modeling

Abstract

The diurnal circulation pattern is deduced in Kranji Reservoir, a shallow, elongated tropical reservoir in Singapore via field observations and results from the 3-D Estuary, Lake, and Coastal Ocean Model (ELCOM) model for the month of July 2012 which is during Singapore’s Southwest Monsoon season. Detailed comparisons of the modeled temperature and velocity fields with field measurements during a dry period of July 21–26 2012 without rainfall or inflow events are presented in terms of point comparisons, longitudinal profiles and cross-reservoir transverse velocity profile, thus providing a good representation of the flow field. Both observation and modeling show that the diurnal circulation pattern is driven by wind and by temperature gradients due to upwelling which depend on the time of the day. There is a daytime circulation pattern characterized by one large loop along the main channel, and a nighttime circulation characterized by two loops: one large loop between the reservoir dam and a location K1 (2500 m upstream from the dam where inflow tributary flows merge), and one small loop between K1 and the tributaries. These circulation patterns are consistent with the observed horizontal distribution of chlorophyll a during the Southwest Monsoon months over years 2008–2012. High chlorophyll a concentration occurs near the dam as expected due to downwelling within the large circulation loop and also close to the tributaries due to the nutrient from the catchments being trapped within the smaller loop. These results demonstrate the contributing effect of hydrodynamic flow on the biomass distribution that can occur in shallow tropical systems and which affects the diurnal and seasonal variation of biomass for such systems.