Application of a Two-Dimensional Water Quality Model (CE-QUAL-W2) to the Thermal Impact Assessment of a Pumped-Storage Hydropower Plant Project in a Mountainous Reservoir (Matalavilla, Sil River, Spain)

Abstract

Water temperature is surveyed and simulated in a small mountainous reservoir (Matalavilla, NW Spain) in order to assess the thermal and hydrodynamic effects of a projected pumped-storage hydroelectric plant in the water body and downstream on the Sil River. Detailed field measurements were taken from June 2014 to November 2015 including meteorology, radiation balance over the water surface, water temperature of tributaries and thermal vertical profiles inside the reservoir. Such database provides information to analyse the current thermal and hydrodynamic behaviour of the system as well as to calibrate and validate the two-dimensional water quality model CE-QUAL-W2 for the simulation of the future pumped-storage plant scenario. The model satisfactorily reproduces the intensity and temporal occurrence of the thermal processes observed in the reservoir, with a mean error below 0.05 °C (root mean square error below 1.1 °C) at a daily temporal scale and 1 m vertical spatial scale. Currently, in the absence of the projected pumped-storage plant, Matalavilla Reservoir is subjected to an important flow regulation for hydroelectric generation purposes, which influences its thermal structure. When the water column is stratified (summer, according to its annual monomictic thermal cycle), it presents two well-defined thermoclines: one generated from the atmosphere–water interface, whose location is related to the water surface position and whose stability is highly sensitive to the inflows regime; and another one related to the level of the reservoir water outlet, located in depth next to the dam and fairly stable most of the year. In the presence of the projected pumped-storage hydroelectric plant, the model results predict the homogenisation and heating of intermediate depths of the water column and mixing about one month earlier. Water outflow from the reservoir to the downstream Sil River would be slightly warmer, up to a maximum temperature increase of 1.5 °C under the ordinary interbasin transfer management.