Management of available water resources has been practiced throughout human history. We examine here some aspects of water resource management in the ancient (6th to 13th century) town of Si Mahosot (Thailand). Based on isotopic measurements (2H, 18O, and 222Rn) of both surface water and groundwater samples, we confirmed that groundwater is the main source of water that feeds the town's canal network. Based on a series of shallow wells in the area, we estimated a radon endmember activity of 14,200 Bq/m3 for the groundwater that feeds the canals. We used a radon mass balance model that considers radon inputs via groundwater and losses via decay and atmospheric evasion. Applying our estimated endmember activity, we calculated a water flux into the canal domain studied of 145 ± 36 m3/day, enough to replace the water within this section of the canal within 16–26 days. A separate independent approach using a18O mass balance resulted in a very similar discharge estimate of 154 ± 39 m3/day into the studied canal. The flux may have been higher when the former town was occupied as there are indications that groundwater levels were higher here in the past.