*3.2. Variations in η*

In this section, we analyze the influence of the water exchanges on the global *η* of UPSH plants. Considering that the pumped volume of water was the same in all scenarios, the variation of *η* over a year was computed by comparing the total discharged water obtained from the simulations by the total discharged water under ideal conditions (i.e., without the existence of groundwater exchanges). Table 1 shows the variation in the *η* for all the simulated scenarios with respect to the ideal reference scenario. Overall, the *η* was less affected by groundwater exchanges in the MIDDLE compared with in the TOP scenarios. In the TOP scenarios, the *η* was clearly related to the value of k, decreasing up to 37.3% in scenario TOP-3 with respect to the ideal reference scenario.



The results show that the variation in the *η* was acceptable for values of *K* smaller than 10−<sup>5</sup> m/s; however, if *K* was increased above this value, the *η* decreased dramatically. The differences in *η* were much smaller in the MIDDLE scenarios, and all of them were acceptable. The *η* decreases in some scenarios were a consequence of exceptional large periods of discharge during which the underground reservoir was totally filled. In terms of *η*, a low piezometric head in the surrounding medium would be more favorable. However, additional problems could arise if the piezometric head was too low, as sometimes there would not be enough water to pump in the underground reservoir, and a portion of the excess of electricity could not be stored, decreasing the *η* of the plant.
