Climate Change Impact on Surface Water Availability in an Ungauged Basin in Northern Sri Lanka

Abstract

Reliable assessment of water availability in ungauged catchments remains a major challenge in regions where hydrological observations are sparse. This study investigates the challenges associated with modelling the ungauged Kanakarayan Aru basin in northern Sri Lanka and evaluates potential changes in surface-water availability under a future climate. A conceptual rainfall–runoff modelling approach was applied using parameters transferred from the hydrologically comparable Kala Oya basin. Historical meteorological data, including rainfall, temperature, and evaporation, were compiled and quality-controlled where possible. Future climate conditions were represented using PRECIS regional climate-model simulations driven by the HadCM3 global climate model under the A1B emissions scenario for 1960–2099. PRECIS rainfall was adjusted using a change-factor bias-correction procedure before being used as input to the rainfall–runoff model, while potential evaporation was estimated from temperature using the Thornthwaite method. The model estimated mean annual surface-water availability in the Kanakarayan Aru basin at approximately 289 million m³, equivalent to 28% of the estimated mean annual precipitation of 1034 mm. Approximately 77% of the annual water yield occurred between November and January, emphasizing the strong seasonal concentration of runoff. The climate simulations indicated a substantial increase in temperature toward the end of the century, whereas changes in precipitation were comparatively uncertain. Modelled annual water availability decreased from 276 million m³ during 1960–1990 to approximately 149 million m³ during 2070–2099. The reduction occurred despite a small increase in projected annual precipitation and was primarily associated with increasing temperature and consequent increases in evaporative demand. The findings demonstrate both the potential vulnerability of water resources in the basin and the considerable uncertainty associated with hydrological prediction in ungauged catchments. Improved hydrometeorological monitoring, direct streamflow observations, improved spatial representation of rainfall, and multi-scenario climate modelling are therefore required to support robust long-term water-resources planning.