Seasonal Evolution of Thermal Stratification of Two High Mountain Tropical Reservoirs

Abstract:

A research was conducted to identify the dominant basin scale and season evolution of the physical processes in Riogrande II and La Fe, two high mountain Andean tropical reservoirs (>2000 masl), of different size and form, located in the northwestern of Colombia, Southamerica. Eight field campaigns were conducted in each reservoir between 2010 and 2012. Temperature, conductivity and turbidity profiles were measured along the longitudinal axes with a CTD and inflow temperature was recorded continuously with thermistors. In addition, thermistor chains were deployed on the deepest zone of each reservoir, in 2011 in La Fe and in 2013 in Riogrande II. The heat surface fluxes were calculated based on weather measurements, using heat bulk-formulations. It was found that the seasonal variability of the thermal structure in both reservoirs was dominated mainly by changes in the inflows temperature, related to the hydrological cycle, and not by the solar radiation variability. The atmospheric net heat flux revealed low seasonal changes, with the larger variability due to cloud cover and wind speed variability associated to the passage of the Intertropical Convergence Zone. The effect of the net atmospheric flux was confined to the surface mixed layer, which thickness varied between 2 and 4 m by the effect of short wave radiation heating during the day and strong heat loss starting at mid afternoon and remaining through the night. The inflow temperature was inversely correlated to the discharge, so large inflows are also colder and denser than small inflows. The plumes from small inflows are intrusive and create an intermediate layer of young water while those of large inflows remain attached to the bottom and fill the reservoir from the bottom. This resulted in the thermal structure of both reservoirs developing a bimodal annual cycle that follows the bimodal distribution of the rainfall and river discharge. Due to the discharge related changing level of the intrusion of the plumes, the vertical distribution of dissolved oxygen and nutrients also followed very closely the hydrologic variability.