A thousand years ago, severe droughts and floods swept South India, prompting the Chalukyas of Kalyana, the ruling dynasty, to build sophisticated rainwater harvesting systems. This adaptation was a pivotal advancement, a study¹ using stalagmite isotopic data and archaeological records reveals.

Extreme monsoon variations — far more intense than today — profoundly impacted several other ancient communities and kingdoms in the Deccan across three millennia. In response, they pioneered advanced irrigation technologies and diverted river waters through canals for rainwater harvesting, ensuring water security and resilience, according to a team of scientists at the Indian Institute of Tropical Meteorology (IITM), Pune, and archaeologists at the Yogi Vemana University, Kadapa in Andhra Pradesh.

“While present-day hydrological technology, such as major dams, is crucial for advancing agriculture, small-scale irrigation features like tanks and wells, known to be adopted by medieval kingdom rulers, can significantly mitigate severe drought or famine,” the study added.

Paleoclimate scientist, Atreyee Bhattacharya at the University of Colorado, Boulder, in the United States, says such deep time studies of climate-society linkages help secure some large-scale connections between climate and society that may not be captured in models that address shorter timescales. “Statistical modeling is where such high resolution data can be very powerful,” she says.

Mineral mounds tell time in the past

As rainwater trickled into the Kadapa caves of the parched Rayalaseema region in Andhra, dissolved minerals accumulated steadily, a centimetre every century— forming icicle-like, seashell-hued stalagmites rising from the cave floors. The minerals in the stalagmite growth rings locked in chemical signatures of that time's environmental and climatic conditions as rainwater fell on the surface, serving as timekeepers. The isotopic composition of oxygen, a chemical signature, within these growth zones – a hundred times thinner than a tree ring – preserves a record of local rainfall patterns.

Using the isotopes, IITM’s Naveen Gandhi and colleagues reconstructed changes in the Indian summer monsoon rainfall across 3200 years. “Just 18 cm of the stalagmite yielded 1,800 samples with data points detailed enough to track changes almost year by year from 1200 BC,” says Gandhi.

The rainfall evidence in the dripstones synchronized with archaeological records of settlement patterns from the historic Brahmagiri, Maski, and Tungabhadra corridor sites along the Tungabhadra river in the Krishna basin straddling Karnataka, Telangana, Andhra Pradesh and Maharashtra — states in the Deccan embroiled in water disputes.

Mapping rainfall extremes against archaeological records

For instance, settlements farther from water bodies or rivers, such as Sanganakallu and Maski (in Karnataka) were deserted during droughts lasting 40 to 50 years. In contrast, during the Iron Age, people in settlements near rivers and bodies of water, such as Brahmagiri and Kadebakele, moved from hilltops to riverbanks or next to water, and continued inhabiting these areas. Residents deserted low-lying or cyclone-prone areas on India’s eastern coast during high rainfall periods.

Successors built on knowledge, improving water management. From the third century BCE to the fifth century CE, the Pallavas, Mauryas, and Vishnukundinas established tanks to ensure water supply during drought.