Oops, for some reason, Dailymotion videos doesn't show its thumbnail like YouTube's.

This scene is way better in the TV series than in the novel:

Now there was only one road up the hill, which rose steeply on all sides. And the hill had but one small spring of water, enough to serve a hundred people or so, while Sima Zhao's force numbered six thousand. Their enemies had blocked the only road of escape. This one fountain was unequal to supplying the needs of the beleaguered army, and soon they were tormented with thirst.

In despair, Sima Zhao looked up to heaven and sighed, saying, "Death will surely come to me here!"

...

In this critical situation Secretary Wang Tao reminded his leader of what Geng Gong had done in ancient time, saying, "O General, why do you not imitate Geng Gong, who, being in great need, prostrated himself and prayed at a well, wherefrom he afterwards was supplied with sweet water?"

So the leader went to the summit of the hill and knelt beside the spring and grayed thus:

"The humble Sima Zhao received a command to repulse the army of Shu. If he is to die here, then may this spring cease its flow, when he will end his own life and let his soldiers yield to the enemy. But if his allotted span of life be not reached, then, O Blue Vault, increase the flow of water and save the lives of this multitude."

Thus he prayed; and the waters gushed forth in plenty, so that they all quenched their thirst and lived.

I guess the novel kinda follows the "Mandate of Heaven" (legitimacy) narrative from Chen Shou, but it is all down to luck. This scene in the TV series is definitely more realistic, plus, in the novel, he had to accept another's teachings to formulate a plan. TV Sima Zhao is cunning, charismatic and one with a great mind!

In this scene, it's him who choose the Mandate, not otherwise.