Visitors to early (second century bce–fifth century ce) Buddhist monastic sites across South Asia encountered prominent figural images of nāgas, serpent-like beings who were believed to be closely connected to water and rainfall. Such images are commonly identified as guardians and occasionally have been linked to nearby water systems, such as ponds, tanks, and rivers. Yet, these images have not been studied as an aspect of water regulation within the monasteries themselves. This paper will first consider the water-related challenges that confronted the monks and architects at rock-cut monasteries. Then methods of hydraulic engineering designed to regulate the flow of water at the sites will be considered in relation to the role of nāga imagery. Their proximity to gutters and tanks reveals the Buddhist reliance on supernatural forces as an aspect of water control. The highly visible nature of this arrangement helps to explain the emergence of ritual texts, primarily after the fourth century ce, in which Buddhist ritualists adopt the role of rainmakers. These elaborate ceremonies promise to bring rain or end flooding for the benefit of the saṃgha and the wider community. The ritualists invariably invoke a special relationship with the nāgas, whom they enjoin to rectify undesirable conditions. This connection between image and text reveals a centuries-long process by which the monastic community developed an association with weather regulation that was contingent on a cultivated and highly public relationship with Buddhist-friendly nāgas.