Abstract:
Water resources management in a basin needs an intensive analysis of historical data in
terms of different climate elements and streamflow. Several researchers have examined the
influences of climate change over several main basins during the past years. However, no
studies have been performed in the Maduru Oya basin and associated sub-catchments.
Hence, the main objective of this study was to identify rainfall trends and then to analyze the
streamflow elasticity to the climate in the Maduru Oya basin. Widely used non-parametric
trend tests such as Mann-Kendall (MK) test, Modified Mann-Kendall (MMK) test and Sen’s
slope estimator were adopted to perform the trend analysis in annual, seasonal and monthly
scales. The results displayed by all three tests were in very good agreement except for very
few cases. On an average, a positive trend of annual rainfall was experienced in Maduru Oya
basin with 1.05 and 1.103 trends, respectively from MK and MMK tests with the yearly
increment of 12.52 mm/year. During cropping seasons, Maha season predominantly
exhibited positive trends where Yala season was witnessed mostly with negative trends.
Likewise, during rainfall seasons, except for SWM season, remaining FIM, NEM and SIM
seasons displayed positive trends. The monthly analysis found out that November and
December experienced strong positive trends whereas the highest negative trends were
revealed in September.
Further, for Padiyathalawa sub-basin located in the upstream of Maduru Oya river basin,
analysis of streamflow elasticity to precipitation, defined as the proportional change in mean
annual streamflow divided by the proportional change in mean annual rainfall, was
performed on historical data. This part of the study was carried out using a non-parametric
estimator and a method proposed by finding the slope of the graph plotted between the
proportional variation of annual streamflow and proportional variation of annual
precipitation. Both results indicated that the variations in rainfall are magnified in
streamflow. The non-parametric method and the graphical method revealed that a 1% change
in mean annual rainfall would respectively result in 1.12% and 1.92% change in mean annual
streamflow. Moreover, in an attempt to incorporate the impacts of climate change in
streamflow variability due to variation in climate elements, a HEC-HMS hydrological model
was developed, calibrated and verified for this sub-basin. The model performance was good
in both calibration and verification periods with MRAE and Nash-Sutcliffe Efficiency values
of 0.433 and 0.665 and 0.559 and 0.642 respectively. Hypothetical climate change scenarios
were predicted as future climate change scenarios by modifying the input rainfall and
evapotranspiration data. The results indicated that the relationship between rainfall and
streamflow is stronger than that between evapotranspiration and streamflow as an increase of
10% in rainfall without any change in evapotranspiration results in 20.42% increase in
streamflow while the same amount of increase in evapotranspiration with no variation in
rainfall results 6.30% decrease in streamflow.
In conclusion, the analyses revealed positive trends of rainfall in annual scale for the entire
Maduru Oya river basin as well as for Padiyathalawa sub-basin while the streamflow
elasticity for the sub-basin using the non-parametric estimator was found out to be 1.12 for
the data periods considered.