Master of Science in Water Resources Engineering & Management

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Browsing Master of Science in Water Resources Engineering & Management by Author "Bamunawala RMJ"

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    item: Thesis-Abstract
    Drought assessment of Kirindi oya and Kelani river basins in Sri Lanka under climate change impacts
    (2022) Azmi F; Bamunawala RMJ; Wijayaratna TMN
    Drought is a natural phenomenon that occurs because of climate change. Droughts are localized events influenced by climatic variables such as precipitation, evapotranspiration, and temperature. As a result, the characteristics and implications of drought differ depending on the climatic administrations in various regions around the world. Drought is one of the maximum significant intervals in Sri Lanka. Sri Lanka is very sensitive to the effects of climate change. Drought is an extremely considerable interval in Sri Lanka in terms of people concerned and helps provided, and the country also serves as a recent example for drought interval and risk assessment in tropical regions. This research investigates the probable use of drought indices at Kirindi Oya and Kelani River basins and provides drought assessment for future climatic scenarios. This research was directed to perceive the changes in drought, their consistencies according to seasonal analysis in the Kirindi Oya and Kelani River basin in Sri Lanka using normalized difference vegetation index (NDVI), standardized precipitation index (SPI), and streamflow drought index (SDI) for future climate change RCP 8.5 which is one of the worst scenarios according to 5 th assessment report of the intergovernmental panel on climate change (IPCC). The drought assessment has been divided into three-time intervals such as observed period (1985-2015), mid-century (2040-2059), and end-century (2080-2099). Further, future climate rainfall data has been forecasted by bias correction monthly factor of historical climate rainfall and observed rainfall data using linear scaling. The NDVI has been calculated by using Landsat images near-infrared (NIR) and RED bands in GIS 10.3. Initially, SPI and SDI have been calculated for observed rainfall and streamflow data respectively. Hydrological model HEC-HMS was set up and calibrated (2002-2006) with a root mean square error standard deviation ratio (RMSE std dev) value of 0.6, nash sutcliffe (NSE) value of 0.59, and percent bias (PBIAS) of 7.63%. The model was validated from 2010 to 2014 with an RMSE std dev value of 0.7, NSE value of 0.51, and PBIAS of 3.22% for Kirindi Oya basin. Further, for the Kelani basin. the HEC-HMS was set up and calibrated (1990-1995) with an RMSE std dev value of 0.6, NSE value of 0.64, and PBIAS of 0.64% and validated (2007-2011) with RMSE std dev value of 0.7, NSE value of 0.56 and Percent Bias of -3.27% for Kelani basin. Thereafter, mid and end-century SPI and SDI have been calculated for future bias-corrected rainfall data and future simulated streamflow, respectively. To achieve the objectives of this research work, The rate of recurrence of drought occurrences was determined using a combined SPI and SDI evaluation which identified 1989, 1990, 1992, 2001, and 2004 as a severe drought-affected year in the Kirindi Oya river basin in this observed interval. For the Kelani River basin, severe drought has been identified during 1990, 2001, 2012, 2013, and 2014 in the observed interval. According to seasonal analysis, the probability of occurrence of extreme drought according to SPI values in Kirindi Oya basin is decreasing 25% for mid and 50% end-century, in the Kelani basin 93.75% for mid and 68.75% in end-century. According to SDI values in the Kirindi Oya basin is decreasing 25% for mid and 25% end-century, in the Kelani basin 93.75% for mid and 50% in end-century. First inter monsoon has been found more severe to drought for both SPI and SDI combination in Kirindi Oya river basin, the northeast monsoon period is the driest season for the Kelani River basin which is situated in wet zone in Sri Lanka.
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    item: Thesis-Abstract
    Impact of climate change on droughts in Maduru Oya river basin in Sri Lanka over the 21st century
    (2022) Kour G; Bamunawala RMJ; Wijayaratna TMN
    Drought is an creeping hazard that is least understood and the most complex of all-natural hazards. The drought study requires large historical climatological and meteorological datasets and their complex inter-relationships. Its impacts are prominently observed on a local scale only when the severity becomes high, and the coherent onset and persistence of mild droughts may go unnoticed. The current study investigates the existing drought conditions and future drought risk in the Maduru Oya River Basin over the 21 st -century in terms of meteorological and hydrological drought indices (i.e., SPI, SPEI, RDI, EDI and SRI). The future hydrology over the basin is simulated for this research, using bias-corrected precipitation and potential evapotranspiration outputs under RCP 4.5 and 8.5 of the MPI-M-MPI-ESM-MR. The relevant drought-related indices were computed in monthly and seasonal timescales over the 19512099 period. The time series have been classified for drought characterization, including drought frequency, severity, trend, and probability computation. Further, to assess the impact of these droughts on the basin's response, a hydrological model (i.e., HEC-HMS) was developed to simulate the discharge at the Padiyathalawa outlet considering 2008-2012 as validation period. The results of the monthly timescale for SPI (approximately similar drought frequency and severity by RDI and EDI) depicted that the severe and extreme droughts (45) occurred in March (8), August (5), September (4), October (6) and November (9) in the historical period. Severe and extreme droughts (110 under RCP 4.5,104 under RCP 8.5) are projected in January (17), February (12), April (10), May (12) and December (13) under RCP 4.5 and January (12), February (10), April (13), June (13) and August (10) under RCP 8.5 over the 21 -century. The SPEI at monthly timescale identified highest number of severe and extreme drought (67) events in the historical period and projected highest severe and extreme drought (128 under RCP 4.5,122 under RCP 8.5) events over the 21 st -century in the study area. The hydrological drought index, SRI projected severe and extreme droughts under RCP 4.5 (65) and RCP 8.5 (62) over the 21 st -century that is about 50 % frequency of the meteorological drought indices. The Northeast Monsoon season had the least drought episodes (~20) in the historical period, and on a seasonal time scale, high drought frequency (~30 using meteorological drought indices and ~20 using SRI under RCP 4.5 and RCP 8.5) and severity(severe and extreme droughts) are projected in the Northeast Monsoon. It is also observed that there is a consistent mild drought throughout the mid (~70) and end (~65) century for a maximum duration compared to the historical (~50) period. The accuracy of results obtained from the continuous HEC-HMS model (NSE, RMSE Std. Dev, and R 2 st values of 0.59, 0.72, and 0.60 achieved in validation)highlights the efficient way to simulate a basin's hydrological parameters. The model can project the future variation of streamflow of the Maduru Oya River Basin under varied climatic conditions. The discharge is projected to have a decreasing trend (Sen’s slope=0.008) for future years, identified as droughts. It can be concluded that the impact of climate change on meteorological drought will affect the discharge of the basin. Moreover, due to time lag between meteorological and hydrological drought, about 50 % of meteorological droughts may lead to a severe and extreme hydrological drought in the Maduru Oya River Basin over the mid-century (14) under RCP 8.5 and end-century (13) under RCP 4.5 scenarios. This study will begin with quantitative investigations including streamflow variability and climatology over the basin incorporating the application of regional circulation models.