The answer to your question actually resides in the shortlived dynasty immediately preceding the Tang, which provided much of the latter's fundamentals: the Sui. Chang'an was supplied throughout the Sui and tang regimes via the completion and maintenance of the Grand Canal network that had been begun under the Sui, specifically initially under its Emperor Wen (隋文帝), to compliment (and indeed make possible) his grand reconstruction of the ancient Western Han capital and pivotal trade nexus, Chang'an. This new city, constructed alongside the ancient city was initially called Daxing. the name literally means “Great Revival”, an aspirational statement if ever there was one, but understandable given that China was only just coming out of some four centuries of devastating strife and civil war. In the words of Arthur Wright, “The building of the capital on such a scale early in the dynasty was an act of faith expressing the confidence of the Sui founder and his advisers that their dynasty would have a wider sway and a longer life than the regimes which preceded them.”

However, since it was built literally next door to the old city of Eternal Peace, it shared a very similar set of challenges as a center to the empire: The only river near the capital, the Wei, was prone to silting, which rendered it seasonally too shallow to properly navigate, and required near-constant dredging and maintenance. That was both difficult and of course quite expensive to supply and maintain. The food production capacity of the local region was already strained even before the city had been completed, and the further expansion of the capital city promised to stress the local agriculture well past its breaking point.

As such, as early as 584, Emperor Wen had commissioned the engineer Yuwen Kai to design and begin construction of a massive canal from the capital itself to Dongguan Pass, where it would connect to the confluence of the Wei and Yellow Rivers. He dubbed this network the Guangdeng Qu, the “Canal of Expanded Communications”. This project would, of course, prove backbreaking and extremely longterm… not to mention extremely expensive. Labor, for now, would prove no object, as families without property or taxable income were expected supply their duties to the government in the form of some 20 days of statute labor per year. corvée labor as a form of tax on the peasantry. From the local tribes living in the vicinity of the construction project, each household was to provide 20 days of labor on the walls each year. But the 30,000 workers of 581 was just the beginning: by 586 the ranks of forced laborers had risen to 110,000, then another 100,000 the following year. This was not just used on the canal network - which rapidly ballooned well past the initial link to capital, but all the way south to Hangzhou on the Yangtze River, and eventually all the way north past the site of modern Beijing - but also the further construction of Daxing itself, as well as the reconstruction of the ancient Great Wall network. As for the expenses incurred? Well, that would be what the taxable population was for. The Canal of Expanded Communications promised to more than pay for itself- and the toils of the populace directed to construct it – in eventual surpluses and a year-round navigable waterway from the capital that would be open to both official and private use.

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Sima, Guang. 1084. Zizhi Tongjian.

Stewart, Chris. 2015. "#77 - Sui 1: Internal Affairs" & #78 - Sui 2: Reconstruction" in The History of China Podcast

Wright, Arthur F. 1979. "The S'ui Dynasty (581-617)" in The Cambridge History of China, vol. 3: Sui and T'ang China, 589-906, Part I