Research this aim for analysis transformation rainfall rain become debit with tank model approach in the Tallo River watershed (DAS) and evaluate water availability in the region mentioned. Tank model implemented for simulate flow surface based on rainfall data rain and evapotranspiration during period 2014–2023, and calibration as well as validation done with using observation discharge data in the reservoir Nipa-Nipa. Results simulation show daily discharge variation with minimum value of 3.88 m3/s and maximum discharge that varies every year, reaching up to 143.54 m3/s at 2019. Average annual debit ranges from 6.09  to 7.87 m3/s, shows trend fluctuations that are influenced by pattern rainfall rain annual. Calibration of the model with 2021 data yields mark coefficient determination (R2) of 0.80, while validation using 2022 and 2023 data results in R2 value of each of 0.85 and 0.98, which indicates level high accuracy in discharge simulation. Results this show that tank model enough reliable in estimate water availability and can used as tool planning source water resources in the Tallo River watershed.