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.
