Abstract:
Climate change has influenced long-term rainfall patterns, with wet zones getting wetter and
the inverse occurring in the dry zone, posing severe implications in water resource analyses
and management. This has become a major issue in dry zone watersheds since baseflow is
crucial in maintaining dry period streamflow. The main objective of this research is to identify
tools and applications for assessing climate change vulnerability in the Kalu river basin at
Ellagawa from the wet zone and the Kirindi Oya river basin at Thanamalwila from the dry
zone, as well as to assess the climate change impacts on surface water-groundwater interaction
in these zones. Before selecting any modelling tool, a comprehensive study on tools and
indicators that are used for climate change vulnerability assessment was carried out. The
accessibility, validity, and adaptability of multiple hydrological models were identified, and
considering them, HEC-HMS was selected as the runoff modelling tool.
After considering the benefits and drawbacks of several objective functions in runoff
modelling, the mean ratio of absolute error (MRAE), R-squared correlation (R2), and Nash
Sutcliff objective function (NASH) were chosen as objective functions. For the Ellagawa subbasin,
the NASH value, R2 and MRAE were 0.63, 0.85, and 0.18, respectively while for the
Thanamalwila sub-basin, the goodness of fit were 0.77, 0.87, and 0.45, respectively in
calibration period. In the validation period, for the Ellagawa sub-basin, the NASH value, R2
and MRAE were 0.67, 0.83, and 0.49, respectively and for the Thanamalwila sub-basin, the
values were 0.60, 0.69, and 4.47, respectively. Therefore, objective function values
demonstrate that the models perform well in both watersheds. According to the sensitivity
analysis of model parameters, impervious percentage, recession constant and ratio to the peak
can be considered the most sensitive parameters. When impervious percentage, recession
constant and ratio to peak change from -50% to 50%, the MRAE value shows the highest
percentages changes as 119%, 131%, and 160%, respectively and the NASH value shows the
highest percentages changes as 24%, 21%, and 69%, respectively. Time of concentration and
soil percolation can be considered the least sensitive parameters.
To assess the climate change vulnerability of the basins, synthetic climatic change scenarios
were considered. Considering all the climate change combinations, change in mean annual
streamflow will vary between -42% to 30% and -36% to 32% for the Ellagawa watershed and
Thanamalwila watershed, respectively, and mean annual groundwater flow will vary between
-56% to 27% and -32% to 62% for Ellagawa watershed and Thanamalwila watershed,
respectively for the two selected scenarios. From the results obtained, it can be identified that
the wet zone basin is more vulnerable in terms of streamflow changes and less vulnerable in
terms of groundwater flow changes than the dry zone basin. For future studies, HEC-HMS can
be recommended as a feasible and less complicated modelling tool for runoff simulation.
