Passage: While Jayavarman II was credited for the founding of the Khmer Empire ca. 800 AD, Indravarman I was credited for an extensive building program. He set the foundations for the future Angkorian kings to follow. The king's  first act was to perform a public service for his subjects by building an irrigation network for the rice fields. The goal was usually achieved by constructing a large reservoir to retain water during the monsoon season and then release it during the dry season through a network of canals and channels. And in Hindu mythology the reservoir also represents an ocean and the temple-mountain represents Mount Meru, the home of the gods. The king and his Brahman advisors performed many rituals throughout the year to reinforce this belief; for example, the ritual of rain-making performed before the rice planting season. Immediately, after Indravarman I acceded, he declared in his Práḥ Kô inscription: "In five days from today I shall begin digging, etc." Dig he did with a reservoir of an immense size: the Indratāṭaka was the biggest reservoir ever built before his time being 3.8 kilometres (2.4 mi) long by 800 metres (2,600 ft) wide. However, later rulers out-built him and made his reservoir look small. Now dry, it could have held about 7.5 million cubic metres of water during the monsoon season.

Question: During what season was a large reservoir constructed to hold back water?
monsoon