Hydrological modeling for estimating under-construction reservoir lifespan in a tropical Indonesia mountainous watershed

Hydrological models have become essential for analyzing watershed responses and supporting watershed management, particularly in tropical regions. Bener Reservoir catchment area (BRCA) in Indonesia is a tropical mountainous watershed. Due to high annually rainfall and high extensive agricultural, sedimentation has increased, which affects the service life of the under-construction reservoir. This study analyzes erosion proneness, estimates sedimentation rate, and predicts the reservoir’s service life using the Soil and Water Assessment Tool (SWAT) and the modified Brune method for sediment trap efficiency. The service life is determined through changes in the reservoir capacity curve using the empirical area reduction method (EARM). SWAT model results are calibrated and validated with observed monthly discharge and sediment load data from 2009 to 2022. The performance of model was evaluated using Nash-Sutcliffe efficiency (NSE), coefficient of determination (R2), and percent bias (PBIAS). Validation for discharge shows NSE = 0.889, R2 = 0.890, and PBIAS = 0.86%, while sediment load validation results in NSE = 0.582, R2 = 0.730, and PBIAS = 11.67%. The estimated average erosion rate in the catchment area is 176.9 tons/hectare/year, while the sedimentation rate is 574,611 tons/year (499,662 m³/year in volume). The study predicts that the reservoir’s service life, based on full dead storage capacity, is 51 years after becoming operational—just one year longer than originally planned. These findings highlight the impact of land use on sedimentation and emphasize the need for watershed management to extend the reservoir's lifespan.