Long-term prediction of rainfall over a catchment is a challenge for hydrologists. It is required for water resources management, hydropower energy forecasting and flood risks assessment in river basins. Several large scale climate phenomena affect the occurrence of rainfall around the world i.e El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are most famous for their effect on India, North and South America and Australia. This study is motivated to evaluate the performance of Statistical Downscaling Model (SDSM) developed by annual and monthly sub models for rainfall downscaling from Global Climate Models (GCMs) over the two districts in Sarawak. It is noted that the monthly sub-models have better performance over the annual sub-models. However, both monthly and annual sub-models have poor correlation with the recorded rainfall for the calibration and validation period. Results indicate that both stations show increasing trend in the future annual rainfall under H3A2 and H3B2 scenarios of HadCM3. SDSM predict that the annual rainfall at Belaga and Limbang is expected to increase by 37.8% and 22.7% respectively by 2074. Overall the SDSM approximates the average rainfall very well during the calibration and validation period but the correlation between observed and forecasted rainfall was not so good. And there is a need to improve the statistical downscaling modelling to develop better correlation between predictand and predictors to have better model performance over the wet regions like Sarawak.
This article explores the projected changes in precipitation, maximum temperature (T max) and minimum temperature (T min) in the Malaysian state of Sarawak under Representative Concentration Pathways (RCPs) with the CanESM2 Global Circulation Model. The Statistical Downscaling Model (SDSM) was used to downscale these climate variables at three stations in Sarawak. The model performed well during the validation period and thus was used for future projections under three RCPs with the CanESM2 General Circulation Model. It is noted that the T max will increase by 1.94°C at Kuching, 0.09°C at Bintulu and 1.29°C at Limbang, when comparing the period of 2071-2100 with the baseline period of 1981-2010, under the most robust scenario of RCP8.5. T min is also expected to increase by 1.21°C at Kuching, 0.15°C at Bintulu and 2.08°C at Limbang, under the RCP 8.5 projection for the same period. The precipitation at Kuching and Bintulu is expected to increase slightly to 1.6% and 1.4% at Kuching and Bintulu respectively; however, the seasonal shift is projected as follows: lesser precipitation during the December-February period and more during the June-August season. On the other hand, precipitation is expected to increase at Limbang during all seasons, when compared with the period of 1981-2010; it is expected that under RCP4.5 the annual precipitation at Limbang will increase by 10.5% during the 2071-2100 period.
Rajang River Basin (RRB) is the largest river basin in Malaysia, in the central region of Sarawak Malaysia. There are two large dams (Murum Dam and Bakun Dam) in upper RRB and these are built as a cascade. Bakun Dam is located downstream of the Murum Dam and to assess the flood risk to the Bakun Dam, the estimation of frequent peak flood discharge (PFD) is important. Rainfall-runoff routing modelling was undertaken with RunOff Routing on Burroughs (RORB) tool to estimate PFD for 1 in 2 annual exceedance probability (AEP) up to 1 in 100 AEP. RORB tool was used to derive flood hydrograph, and a hydrological model was established for the Bakun catchment. Based on the analysis, for the 1-day storm, the 2-year and 100-year return period design rainfall are 115 mm and 206 mm, respectively. For the 3-day storm, the 2-year and 100-year return period design rainfall are 188 mm and 344 mm, respectively. The peak flood discharge for 1-day storms is higher than the 3-day storms. For the 1-day storm, the 2-year and 100-year return period, peak flood discharges are 3,867 m 3 /s and 7,043 m 3 /s, respectively. For the 3-day storm, the 2-year and 100-year return period, peak flood discharges are 3,632 m 3 /s and 6,722 m 3 /s, respectively.
Monsoonal rainfall plays an important role in the annual rainfall distribution over Bangladesh. It is generally believed that monsoon depressions and cyclonic storms significantly affect the rainfall distribution over Bangladesh during the monsoon months and their absence causes deficient rainfall during the individual monsoon months. This aspect has been examined by computing the average rainfall for 32 meteorological observatories of Bangladesh Meteorological Department during the period 1948.91 for those monsoon months which were free from depressions and cyclonic storms. It has been found that the absence of monsoon depressions and cyclonic storms is not the main factor which causes deficient rainfall and consequent drought conditions in the individual monsoon months over different stations of the country. All the stations in the country experienced normal rainfall conditions inspite of the absence of depressions and cyclonic storms in the monsoon season (June-September).
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