The voltage-sag is one of the crucial measures of power quality of electric distribution networks. Among the causes of voltage sag is simultaneously starting of water-pumping motors (WPMs). The key contributions of the present study are optimal sizing and control parameters of the supercapacitor energy storage (SCES) scheme to mitigate the voltage-sag caused by simultaneous start-up of WPMs fed by a real Karot distribution feeder (KDF) based on a recently-developed Walrus Optimizer (WO). The KDF is located in Upper Egypt to supply sixteen 30-hp induction-driven WPMs along with domestic loads. It is considered a case study to demonstrate the success of the proposed SCES in minimizing the KDF's voltage-sag. The WO is assigned to evaluate the optimal size of SCES as determined by its capacitance and voltage. The performance of the WO is compared with that of the particle swarm optimization (PSO) algorithm. For evaluating the effectiveness of the proposed SCES in minimizing the voltage-sag problem in the KDF, a comparison is made against the superconducting magnetic energy storage (SMES). The proposed SCES with capacity of 0.1 MJ and capital cost of 55.4 $ successfully reduced the voltage-sag to reach allowable limits against 0.625 MJ and 1736 $ on using the SMES.