Evaluating irrigation water supply reliability under variable rainfall: the role of reservoir regulation and managed diversion in the Nachchaduwa sub-catchment, Sri Lanka

Abstract

Irrigation water availability in Sri Lanka’s dry zone is affected by seasonal and inter-annual rainfall variability, but reservoir storage and trans-basin diversions can buffer these fluctuations. This study evaluates the role of reservoir regulation and managed diversion in irrigation water reliability in the Nachchaduwa sub-catchment, Sri Lanka. A continuous semidistributed HEC-HMS model was developed using historical rainfall, reservoir-level and diversion data to represent rainfall–runoff processes, storage and managed water transfers. The model reproduced reservoir-level dynamics with NSE values of 0.453 and 0.626 for calibration and validation, respectively, with RSR of 0.70 and 0.60 and PBIAS of 0.36% and 0.05%. Although calibration performance was limited for short-term variability, the low PBIAS indicated close agreement in overall water balance. Seven-day cumulative rainfall showed the strongest, but weak, association with diversion, indicating that diversion is also influenced by reservoir storage and operational decisions. Despite rainfall variability, irrigated area remained comparatively stable, highlighting the buffering role of reservoir regulation and managed diversion. The results demonstrate that irrigation reliability is governed by the interaction of local rainfall, external water transfers, reservoir storage and operational management.