Physical changes within a large tropical hydroelectric reservoir induced by wintertime cold front activity

M P Curtarelli,E H Alcântara,C D Rennó,J L Stech

doi:10.5194/hess-18-3079-2014

Abstract

Abstract. We investigated the influence of wintertime cold front activity on the physical processes within a large tropical reservoir located in Brazil. The period chosen for this study consisted of 49 days between 28 April 2010 and 15 July 2010. This period was defined based on information from the Brazilian Center for Weather Forecasting and Climate Studies (CPTEC), data collected in situ and the interpretation of remotely sensed images. To better understand the governing processes that drive changes in the heat balance, differential cooling and mixing dynamics, a simulation was performed that utilized a three-dimensional hydrodynamic model enforced with in situ and remote sensing data. The results showed that during a cold front passage over the reservoir, the sensible and latent heat fluxes were enhanced by approximately 77 and 16%, respectively. The reservoir's daily averaged heat loss was up to 167% higher on the days with cold front activity than on the days without activity. The cold front passage also intensified the differential cooling process; in some cases the difference between the water temperature of the littoral and pelagic zones reached up to 8 °C. The occurrence of cold front passages impacted the diurnal mixed layer (DML), by increasing the turbulent energy input (∼54%) and the DML depth (∼41%). Our results indicate that the cold front events are one of the main meteorological disturbances driving the physical processes within hydroelectric reservoirs located in tropical South America during the wintertime. Hence, cold front activity over these aquatic systems has several implications for water quality and reservoir management in Brazil.

Highlights

Physical processes that occur within reservoirs, such as mixing and differential cooling, are mainly controlled by momentum and energy exchanges with the atmosphere, inflowing rivers and offtake discharges (Imberger, 1985; Imberger and Patterson, 1989)
Several studies describe the physical dynamics and main driving forces in lakes and reservoirs (Imberger and Patterson, 1989; MacIntyre et al, 2002; Wüest and Lorke, 2003), few studies have addressed this issue for tropical South America reservoirs, which are subject to local weather conditions that vary throughout the year and frequent meteorological disturbances, such as cold front incursions
We investigated the effects of winter cold front passages on the heat balance, differential cooling and diurnal mixed layer (DML) dynamics of the Itumbiara Hydroelectric Reservoir (IHR)

Summary

Introduction

Physical processes that occur within reservoirs, such as mixing and differential cooling, are mainly controlled by momentum and energy exchanges with the atmosphere, inflowing rivers and offtake discharges (Imberger, 1985; Imberger and Patterson, 1989). Cold front incursions are one of the most recurrent synoptic patterns over South America; they greatly affect atmospheric circulation, precipitation and temperature regimes (Garreaud, 2000). These events occur year-round and reach the Brazilian territory every 1–2 weeks; they are more frequent during the austral winter (Stech and Lorenzzetti, 1992). Winter cold fronts are more severe and can cause freezing temperatures in southern and southeastern Brazil (Marengo et al, 2002) The majority of these fronts reach the hinterlands of São Paulo, Minas Gerais and Goiás where many reservoirs are located; most of these reservoirs are used for generating energy

Results

Discussion

Conclusion