Effects of Lake–Reservoir Pumped-Storage Operations on Temperature and Water Quality

Abstract

Pumped-storage (PS) hydropower plants are expected to make an important contribution to energy storage in the next decades with growing market shares of new renewable electricity. PS operations affect the water quality of the connected water bodies by exchanging water between them but also by deep water withdrawal from the upper water body. Here, we assess the importance of these two processes in the context of recommissioning a PS hydropower plant by simulating different scenarios with the numerical hydrodynamic and water quality model CE-QUAL-W2. For extended PS operations, the results show significant impacts of the water exchange between the two water bodies on the seasonal dynamics of temperatures, stratification, nutrients, and ice cover, especially in the smaller upper reservoir. Deep water withdrawal was shown to strongly decrease the strength of summer stratification in the upper reservoir, shortening its duration by~1.5 months, consequently improving oxygen availability, and reducing the accumulation of nutrients in the hypolimnion. These findings highlight the importance of assessing the effects of different options for water withdrawal depths in the design of PS hydropower plants, as well as the relevance of defining a reference state when a PS facility is to be recommissioned.

Ensemble modelling of ice cover for a reservoir affected by pumped‐storage operation and climate change

“…This reservoir, which was inundated in the 1930s, is on average 9 m deep, has a surface area of ~11.3 km 2 and a volume of 96.1 × 10 6 m 3 . Its water level varies between 889.34 and 876.84 m a.s.l., thus, the storage capacity amounts to 89.4 × 10 6 m 3 (Kobler et al, ). It is dimictic, with temperatures up to 25°C and 15°C in the epilimnion and the hypolimnion during summer stratification, and it is ice‐covered in winter.…”

“…It is dimictic, with temperatures up to 25°C and 15°C in the epilimnion and the hypolimnion during summer stratification, and it is ice‐covered in winter. Detailed information on the thermal structure and the water quality of this reservoir are given in Kobler et al (). Under present PS operations, ~10% of the total inflows to Sihlsee are pumped from Upper Lake Zurich.…”

“…However, they are also known to affect abiotic and biotic components of the connected water bodies (Bonalumi, Anselmetti, Kägi, & Wüest, ). The duration and thickness of ice covers are important drivers for lake ecology (Powers & Hampton, ; Salonen, Leppäranta, Viljanen, & Gulati, ); yet, they have been shown to be modified by PS operation (Bonalumi, Anselmetti, Wüest, & Schmid, ; Kobler, Wüest, & Schmid, ).…”

“…Here, we investigate the combined effects of PS operations and climate change on ice cover in Sihlsee, the upper reservoir of a PS scheme in Switzerland. These effects were previously assessed using the laterally averaged two‐dimensional model CE‐QUAL‐W2 both for different PS scenarios (Kobler et al, ) and interactions with projected future climate (Kobler, Wüest, & Schmid, ). However, the modifications of the ice module of CE‐QUAL‐W2 by Kobler et al () were not tested comprehensively, and therefore, uncertainties about its applicability remained.…”

“…These effects were previously assessed using the laterally averaged two‐dimensional model CE‐QUAL‐W2 both for different PS scenarios (Kobler et al, ) and interactions with projected future climate (Kobler, Wüest, & Schmid, ). However, the modifications of the ice module of CE‐QUAL‐W2 by Kobler et al () were not tested comprehensively, and therefore, uncertainties about its applicability remained. Furthermore, the ice module of CE‐QUAL‐W2 does not consider effects of snow cover or the formation of snow ice.…”