Master of Science in Water Resources Engineering & Management

Permanent URI for this collectionhttp://192.248.9.226/handle/123/40

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item: Thesis-Full-text
Analysis of hydroclimatic variability and adequacy of channel flows in an arid zone of Pakistan
(2019) Rashid, S; Rajapakse, RLHL
Water is becoming progressively scarce and effective usage of accessible supplies is of major concern. Globally, 15% ~ 21% of the water allocated for irrigation is lost due to poor management and non-optimized conveyance practices. Pakistan is an agricultural country which hosts one of the world’s largest irrigation networks, Indus Basin Irrigation System (IBIS). The system has been found to operate with an irrigation efficiency of a mere 35% ~ 50% which is abysmally low. It is thus vital to oversee the proper management of this scarce resource while limiting the losses within the system. The selected Hakra canal covers an irrigated area of 2031 km2 with a 92 km of total length and lies in the semi-arid region in Punjab, Pakistan. The aim of the present study is to evaluate the competence of the available irrigation channel flows to meet actual Crop Water Requirement (CWR) and variations in availability with climatic irregularities. For the detailed analysis of hydroclimatic variability and channel flow adequacy, data of daily channel flows and climatic parameters were obtained for the period of 2010~2017 while monthly rainfall data from 1978~2017 was used for long-term trend analysis. The CWR was estimated using CROPWAT 8.0. Observed deficit in supply is provided by groundwater abstraction and was estimated using root zone water balance approach. Mann-Kendall and the Sen's slope tests were used to detect the possible trend and its magnitude. An upstream rainfed basin is selected and used for the verification of observed climatic variations. Trend analysis depicted an increase in annual rainfall from 1978~2017 over the region with the estimated contribution of 13% to irrigation supply. Irrigation supplies are the dominating source of water and highly fluctuating. The seasonal shortfall has shown a variation of 7%~26% in Rabi season and 71% ~78% in Kharif season. Further analysis of data revealed an increasing trend in the maximum and minimum temperature values especially in the months where rainfall has also shown an increase i.e. June and September. The observed climatic variability in the downstream of IBIS is highly reliant on hydrological behaviour of upstream catchments. Four parameter ‘abcd’ lumped model with incorporated snow parameter ‘m’ for icy catchment is used to sensibly screen and verify the reaction of a catchment under the climate change scenario by evaluating the changes in hydrological processes. The better understanding of meteorological and hydrological conditions of the study area helped proper investigation and imitation of the actual situation. Unreliable supply of water in the irrigation system along with variability in climatic factors i.e. precipitation and temperature would disturb the dynamics of hydrological water cycle hampering crop yield. It would elevate the maximum soil moisture deficit that results in crop failure or low yield.
item: Thesis-Full-text
Analysis of precipitation trend and streamflow sensitivity to precipitation in Maduru oya river basin with HEC-HMS model simulations
(2020) Kirupacaran S; Rajapakse RLHL
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.
item: Thesis-Full-text
Analysis of rainfall trend and ITS impact on future hydropower generation - case study on victoria reservoir.
(2021) Lakmali JWR; Rajapakse RLHL
Mahaweli river basin is the major river basin for hydropower generation in Sri Lanka and it supplies about 1800 GWh annually to the national grid, but the expected generation is about 2400 GWh (2019). The annual hydropower generation in Sri Lanka is decreasing and the contribution of other nonrenewable sources are continuously increasing accordingly. There are eight reservoirs in the upper catchment of Mahaweli Basin which generate hydropower under the Mahaweli Complex. These reservoirs experience both drought periods and high flood periods as well throughout the year. As hydropower generation totally relies on the rainfall amount of the sub-catchment of the reservoirs, the planned hydropower generation cannot be achieved during the drought periods due to the failure in receiving expected rainfall to the sub-catchments of reservoirs. Hence, identifying the rainfall pattern, its peaks and troughs, and possible trend in future rainfall are crucial for managing and optimizing the reservoir operations such that hydropower generation can be maintained at the maximum possible capacity This study is focused on the analysis of rainfall trends in the upper catchment of Mahaweli Basin and its impact on hydropower generation in Victoria reservoir according to the possible variations in future rainfall. The rainfall trend was analyzed for the Mahaweli Upper catchment considering rainfall data of seven rainfall stations with 30 years of monthly rainfall data. The base period for rainfall trend analysis was selected from the year 1981 to 2010 as per World Meteorological Organization (WMO) guideline. The missing rainfall data in selected rainfall stations were filled with the linear regression method. Rainfall trend was analyzed with the Mann Kendall test and the magnitude of the trend was estimated by Sen’s Slope method which were performed using RStudio Software. According to the trend analysis, the rainfall trend is negative in dry periods and a positive trend is observed in rainy seasons and the negative trend is higher than the positive trend. It could be expected that dryer periods getting dryer with a high degree of variation and rainy periods getting even more rainfall to a lesser degree. This implies that overall annual rainfall has a negative trend in the study area. The future rainfall was estimated for further 30 years from 2020 to 2050 as monthly data with parameters obtained from Sens’ slope method and Mann Kendall test. The average annual rainfall was about 2,390 mm in the study area for the selected base period and the estimated future mean annual rainfall for next 30 years will be around 1973 mm with a decrease of 18% compared to the last 30 years. The catchment runoff was calibrated for Victoria reservoir with HEC HMS model for the five years from 2001 to 2005 and the model was validated for the period 2006-2010. The future inflows were predicted for the period 2021 - 2025 with generated monthly future rainfall data. The future annual inflow of Victoria reservoir in next 5 years will be reduced by 10% compared to recent 5 years of inflows of Victoria reservoir. The HEC ResSim model was developed and applied for Victoria reservoir to obtain the potential power generation and the analysis of reservoir operations of Victoria reservoir. HEC ResSim model was calibrated with reservoir operational data in the year 2015 and validated with reservoir operational data in the year 2016. Future power generation was obtained for the time period of 2021 - 2025. It was found that the future annual power generation of the Victoria power plant will be reduced by 23% compared to the last five years due to the predicted decrease in rainfall. This future scenario was analyzed based on monthly data, hence the peak events were not taken into account. Since the hydropower generation in the Victoria reservoir is decreased yearly, optimization of reservoir operations is necessary considering the variation of future rainfall trends.
item: Thesis-Abstract
Analysis of the effect of climate change impacts on floods in Kelani river basin, Sri Lanka
(2023) Kulathunga, SPSP; Rajapakse, RLHL
Sri Lanka is highly vulnerable to climate change impacts, including rising land and sea temperatures, changing precipitation patterns, more extreme weather events, and sea-level rise. Notably, climate change has been observed to increase flood frequency, expand flood areas, and intensify flood damages. Previous research in Sri Lanka has mainly focused on rainfall estimation using weather models and examining climate change scenarios. This study aims to improve flood forecasting by analyzing climate change-induced changes in rainfall depths from Intensity-Duration-Frequency (IDF) curves and considering design sea levels. The objective is to gain insights into future flood characteristics, specifically the projected increases in discharges and water levels. The HEC-HMS Hydrological modelling tool was selected for the hydrological modelling of the entire Kelani Basin, while the HEC-RAS model was used for flood modelling in the Lower Kelani Basin which is downstream from Glencourse. HEC-HMS simulating discharges from rainfall inputs that served as boundary conditions for the HEC-RAS model. The verified models are utilized to simulate the 50-year design rainfall dataset lasting 3 days, incorporating published IDF equations from selected rain gauge locations along with the calibrated models. Rainfall depth multipliers of 1.100, 1.122, and 1.140 were applied to the design rainfall dataset for the RCP4.5, RCP6.0, and RCP8.5 projections, respectively. Simulations also considered sea-level rise values of 0.47 m, 0.48 m, and 0.63 m corresponding to the respective climate change projection scenarios. Calibration and validation of the three HEC-HMS models (Kelani Upper, Kelani Middle, and Kelani Lower) and the HEC-RAS Flood model for Lower Kelani (downstream to Glencourse) Basin were successfully calibrated using 2016 May and validated using 2017 May flood event data. The Nash Efficiency values during calibration were 0.79, 0.95, and 0.85 for the Kelani Upper, Kelani Middle, and Kelani Lower models, respectively. During validation, the Nash Efficiency values were 0.87, 0.85, and 0.25, respectively. The calibration Nash Efficiency values for the HEC-RAS model were 0.57, 0.56, and 0.52, and the validation Nash Efficiency values were 0.80, 0.57, and 0.53 for the respective models considering Hanwella Discharges, Hanwella Water Levels and Nagalagama Street Water levels, respectively. The research concluded that, under climate change projections, the Glencourse Peak Discharge is projected to increase by approximately 13.3% to 16.2%. Similarly, at Hanwella, the peak discharge is expected to increase by approximately 6.4% to 8.8%, while the maximum water level is anticipated to rise by approximately 3.1% to 4.2%. Moreover, the maximum water level at Nagalagama Street is likely to experience an increase of around 16.2% to 21.7% under climate change projections.
item: Thesis-Full-text
Analysis of the effect of loss and baseflow methods and catchment scale on performance of HEC-HMS model for Kelani river basin, Sri Lanka
(2018) Ud Din, AM; Rajapakse, RLHL
Hydrological models have become an indispensable tool for efficient water resource management which requires proper estimation of runoff in basins and recognition of appropriate catchment scale. The HEC-HMS (Hydrologic Engineering Center's Hydraulic Modeling System) is a reliable and freely available model. Different loss and baseflow estimation methods available in HEC-HMS have their own pros and cons. Lumping of model parameters over a large area reduces the model performance. In order to find the best loss and baseflow methods for simulating rainfall runoff and to check the possibility of further improvement in model performance by moving toward distributed modeling, Glencorse watershed in Kelani river basin of Sri Lanka was selected as the project area. Daily rainfall data from 2006/2007 to 2008/2009 and 2010/2011 to 2013/2014 for four rainfall stations in Glencorse watershed with daily stream flow data of Glencorse gauging station for the same duration were used for this study. Two different combinations of baseflow and loss methods for simulation of runoff were considered while Clark unit hydrograph method was used as transform model. In the First Option, the Deficit and Constant Method and Recession Method were used as loss and baseflow methods, respectively, while for the Second Option, the Soil Moisture Accounting (SMA) and Linear reservoir methods were used for continuous simulation. Glencorse watershed was divided into 3, 6, 9 and 16 sub divisions to assess the improvement in model performance by shifting toward distributed modelling. Manual calibration approach was used for with Mean Ratio of Absolute Error (MRAE) as the main objective function while another two statistical goodness of fit measures, Nash–Sutcliffe model efficiency coefficient (NASH) and percent error in volume were also checked as an additional observation. Soil Moisture Accounting as loss model and linear reservoir model as baseflow model simulated runoff more efficiently as compared to the other combination. Evaluation showed value of MRAE and NASH for Option 1 were 0.38 and 0.67 for calibration and 0.40 and 0.42 for verification, respectively. Option 2 evaluation showed MRAE and NASH as 0.31 and 0.70 for calibration and 0.34 and 0.57during verification, respectively. Soil Moisture Accounting and Linear Reservoir method used for distributed model showed improvement in model performance up to 6 sub-divisions after which the model performance started declining. Selection of appropriate method among different methods available in HEC-HMS should be in accordance with overall objective of study as it plays an important role in accurate estimation of runoff. Moving toward distributed modelling improves model performance but high resolution data and machine power is required..
item: Thesis-Abstract
Applicability of a two parameter water balance model to simulate daily rainfall runoff – case study of kalu and gin river basins in sri lanka
Dissanayake, PKM; Wijesekera, NTS
Most of hydrological models are complex, data intensive and require optimization of many model parameters. Due to prohibitively high institutional pricing and access constraints associated with data, water research even at daily time scale is a challenge. In this aspect monthly data can be treated as better. Lack of a simple and reliable rainfall runoff model to simulate daily rainfall runoff with an indication for soil moisture is a concern when field applications are carried out. In this backdrop the present work investigated the applicability of a monthly model in the daily time scale. The two-parameter monthly water balance model (Xiong and Guo,1999) performed well in two Sri Lankan watersheds was selected. This model after an initial evaluation was calibrated with monthly data. Daily streamflow estimations were done for Ellagawa (1372 km2) and Thawalama (364 km2) watersheds for the respective durations 2006-2014 and 2000-2015. Estimations were compared using MRAE as the objective function, hydrographs, duration curves and water balance. Nash-Sutclifff was used to observe the goodness of fit in the high flow estimates. Initial evaluations with the previously calibrated dataset showed satisfactory results with the recent data used for the present work but were inferior to the previous outputs probably due to temporal setting or other data quality issues. The two parameter model calibrated and verified for the recent data showed very good results for the Tawalama watershed and good results for the Ellagawa watershed with different degrees of overestimation. Daily flow estimations agreed reasonably well with the Thiessen averaged rainfall and observed streamflow patterns but demonstrated an overestimation with a noticeable pattern. After observing monthly and daily outputs in both catchments, the model concept was modified to incorporate a third parameter called AF (Adjustment Factor) to arrest over estimation which may have caused due to the need to incorporate watershed effects arising from variations in slope, land cover, detention and soils. This Three Parameter Monthly model showed excellent results with the matching of outflow hydrographs, duration curve and water balance for water resources management. In case of Tawalama watershed, the average MRAE values for the two parameter and Three Parameter Models were 0.2061 and 0.1657 respectively. In Ellagawa watershed average MRAE values for the same were 0.7668 and 0.3135 respectively. Respective c and Sc values for the Two Parameter Model were 0.89 and 1,288.63 for Tawalama watershed while the same were 1.29 and 829.84 for Ellagawa. Respective c, Sc and AF values for the Three Parameter Model were 1.02, 1,292 and 0.83 for Tawalama watershed while the same were 0.52, 975.2 and 0.46 for Ellagawa. IV Conceptualization extended in the three parameter model demonstrates the potential of successful catchment process conceptualization within the monthly and daily temporal resolutions. Present work concluded that in case of two case study watersheds, the three parameter monthly model concept is applicable for both monthly and daily time scales. Therefore this model is recommended for water resources planning and identification of climate change impacts in similar watersheds.
item: Thesis-Full-text
Applicability of ABCD water balance model for the assessment of water resources in Kelani basin, Sri Lanka
(2018) Wangchuk, U; Rajapakse, RLHL
Water resources management in watersheds has become increasingly important due to rapid expansion of human settlements while pollution caused by industrial development has led to the part of the available precious water resources unusable for consumption, thus aggravating scarcity of fresh water resources. The impacts are further exacerbated due to global warming. The use of the multi-parameter, distributed hydrologic models for water resources assessment in the local basins are hindered due to scarcity of data and other resources. The lumped parameter rainfall runoff hydrologic models are widely applied to predict watershed response of small watersheds by simulating rainfall runoff generation and thus useful in water resource management in ungauged basins. This study aims at identifying distinct characteristics of one such widely used model, ABCD Water Balance Model, and studying its applicability to a selected sub basin in Kelani River Basin for simulating catchment response in terms of rainfall runoff. The model was subsequently applied to analyze surface and groundwater resources available in the basin, targeting effective and sustainable water resources development and management. The data required for the ABCD water balance model were precipitation, evapotranspiration, average temperature and minimum and maximum temperatures. The model was developed in Excel spread-sheet format focusing on the data period from 1994~2011 in the Kelani basin. For model calibration, precipitation and potential evapotranspiration data during the period 1994 to 2001 were used. The generated model streamflow was compared with observed streamflow at Glencorse station for the same period. For the validation of the model, the precipitation and potential evapotranspiration data in the latter 10-year period were used. For estimating the goodness-of-fit, Nash-Sutcliff efficiency coefficient method was used, while model response to four distinct parameters were assessed based on sensitivity analysis and parameter optimization. The calibrated model has shown that the model is less sensitive to parameters a (0.9) and b (20) while on the other hand, the model was highly sensitive to parameter c (0.68) and d (0.01). It was noted that even with the lesser amount of moisture infiltration from the upper soil zone, the aquifer was able to produce runoff. Hence, it proved that in the wet zone, the propensity of the area to produce runoff was largely independent of rainfall intensity. For the model calibration runs, the correlation or coefficient of determination (R2) between model flow and observed flow was 0.77 with NASH coefficient value of 0.71 and MRAE of 0.27. The model produced a better response to medium flows between 5% ~ 82% with NASH value of 0.78 and good response for high flows below 5% of percent exceedance with acceptable results (NASH = 0.62). The model could not response well for low flows (NASH = 0.45). This model with four parameters could adequately simulate the rainfall runoff response of the selected sub-watershed area in Kelani Basin (at Glencorse). Hence, this lumped parameter model was deemed suitable for streamflow forecasting and water resources assessment in Kelani basin and it can also be applied in areas elsewhere with similar hydrological characteristics.
item: Thesis-Full-text
Applicability of model parameter transferability of tank model in streamflow simulation in gin river basin
(2020) Boralugoda BPD; Wijesekera NTS
Amidst of the population growth and increased demand due to rising level of living standard, stress on the water resources has been increased rapidly in recent years. Water practitioners, researchers have been stressing on the need of development of water resources in integrated and cohesive manner. Hydrological modelling has become the essential tool for planning and designing of water resources development as it gives the quantity of water available. Many modelers face the problem of developing solutions at ungauged basins. Typically, hydrological models are developed at gauged locations and whenever necessary, modelers tend to use the same model structure with verified parameters. This is a gray area in hydrological society as the model transferability is yet to be convinced. The need of more researches is essential in this regard for increase confidence of use of model parameter transferability. This study developed a lumped conceptual tank model with four tanks for simulating streamflow in Gin Ganga basin at Tawalama and Baddegama and appraise the effectiveness of the model parameter transferability in ungauged basins of Gin basin. Model is developed in MS Excel and multi-start GRG-nonlinear search engine is used as parameter optimizing method while employing Mean Ratio of Absolute Error (MRAE) as the objective function to evaluate goodness of fit of the optimized parameters. Daily precipitation and evaporation data from water years 2008/09 to 2017/18 is used for the modeling. Model was warmed up using five water years to stabilize soil moisture in each calibration and validation. Calibration for each catchment was done using first five years of data and validation was done using remaining portion of data. Thereafter, optimized parameters were transferred under spatiotemporal, spatial and temporal approaches to simulate the flow of each catchment. Then model performance was evaluated in each scenario by comparing goodness of fit, annual water balances, flow hydrographs and flow duration curves for low, high, and medium. The models were calibrated at Baddegama and Tawalama with MRAE value of 0.233 and 0.246 respectively for daily streamflow simulation. Then both models were validated for the two location with MRAE of 0.298 and 0.346 respectively. Better matching in high and medium flow is observed while average annual water balance error varying from 1.7% to 19% on average. All three transferability methods showed adequate results while maintaining accuracy ranges from 59% to 72% in daily streamflow simulation and model predicted average annual and average monthly flow estimations with an accuracy of 81% and 77% respectively under any transferability approach. Among the three approaches spatial transferability is selected as the best since it shows streamflow simulation accuracy over 66% and annual water balance errors varying with 1.7% to 3.4% on average. Further, spatiotemporal transfer method shows accuracy over 56% and temporal transfer has showed accuracy over 69% in daily streamflow simulation. In all modelling effort it was observed that accuracy of monthly flow estimations was over 77% and accuracy of annual water balance was over 81% on average. Finally, the model could be used to predict daily streamflow with an accuracy of 68% and monthly scale flow estimations with an accuracy of 89% by applying either set of optimized parameters, indicating the model suitability for parameter transferability & water management in ungauged catchments in Gin Ganga basin.
item: Thesis-Abstract
Applicability of, reference crop evapotranspiration from measured temperature (hargreaves method - 1982) to Sri-Lanka
Seneviratne, LKP; Kariyawasam, HC
Considering the problems associated with the availability and reliability of climatic data in developing countries and the possible errors in the more sophisticated methods for estimating crop water requirements, it has become necessary to develop a computational procedure which requires least and widely available data. The method of estimating reference crop evapotranspiration (ETO) from measured temperature introduced. by professor Hargreaves of Utah State University in 1982 has given satisfactory results in many regions of the world. The study of its applicability to Sri- Lanka could do much to improve irrigation and agricultural sectors. Three stations from different climatic regions of Sri-Lanka Here selected with the intention of studying the applicability of this new method. ETG estimates from Modified Penman, Hargreaves and Pan evaporation methods were computed for seven years and the average values Here compared. The results indicated that the Hargreaves method provides, satisfactory’ results for Yala seasons in dry zone. The application of Hargreaves method can be extended even to Maha seasons In dry coastal regions. The ETO estimates from Penman method have given over predictions specially for high wind velocities. Two average years were selected for station Mahailluppallama and Batticaloa. ETO estimates from Modified Penman were computed for varying wind speeds. The results have shown that a local calibration is required for the aerodynamic term of Modified Penman. A relationship has been develop between maximum and minimum temperatures, relative humadities, wind speed and the deviation of ETO values of Hargreaves method from pan evaporation method. This indicated that for given temperatures and relative humidity, Hargreaves method provide better results only for a particular range of wind speed. Estimated crop evapotranspiration (ET crop) for soya beans from Hargreaves and Pan evaporation methods were compared with lysimeter measurements for Agricultural Research Station Mahailluppallama. The total estimated ET crop from Hargreaves method has only varied 5.6 % from the measured lysimeter values.
item: Thesis-Full-text
Application of 'abcd' monthly water balance model for Kalu Ganga and Gin Ganga basins and its application potential for water resources investigation
Gunasekara, DN; Rajapakse, RLHL
Only a limited number of mathematical models have been developed currently in Sri Lanka for water resources management purposes in Kalu and Gin River basins which predominantly provide water for the water supply schemes, irrigation and mini hydropower schemes. The developed models contain either a large number of parameters which increase the model complexity or less number of parameters which increase the amount of details in a parameter thus compromising the simulation accuracy. Based on available case studies, it is sufficient to have three to five parameters to reproduce most of the information in a hydrological record in monthly models for humid regions. Therefore, the “abcd” model which is a monthly lump hydrological model with four parameters was selected for the present research for the investigation of water resources in Kalu and Gin river basins considering Ellagawa and Thawalama sub catchments. For the corresponding watersheds, precipitation, streamflow and evaporation data were collected for the past 30 years and checked by visual comparison, single and double mass curve analysis and annual water balance budget to ensure data reliability, consistency and to identify suitable data periods for model calibration and validation. For Gin River, a 25 years data period was used, while 20 years of data were selected for Kalu River basin. For the model evaluation, Mean Ratio of Absolute Error (MRAE) was used as the objective function while Nash Sutcliff Efficiency coefficient was used for the comparison purposes. In addition, visual inspection of flow simulation with respect to the observed flow, annual water balance and flow duration curves were used for the model performance evaluation. The optimized a, b, c, and d parameters for Thawalama and Ellagawa watersheds are 0.961, 1066, 0.003, 0.813 and 0.998, 1644, 0.013, 0.741, respectively. The MRAE for the calibration of Thawalama and Ellagawa watersheds are 0.21 and 0.26, respectively while obtaining 0.23 and 0.43 for the validation which show satisfactory results. In both watersheds, low flows have been slightly over estimated while very high flows have been underestimated. But a balanced distribution of simulated flow results can be observed in intermediate flows. Comparatively high dispersion of simulation results can be observed in Ellagawa watershed than Thawalama watershed. In case of parameter sensitivity, parameter “a” and “b” are the most sensitive while parameter “d” is having the lowest sensitivity. As model outputs, monthly and annual variation of groundwater discharge, direct runoff, soil moisture storage and groundwater storage of the watersheds were obtained. For the overall discharge of both watersheds, the contribution from groundwater is very low. Therefore, the “abcd” hydrologic model can be recommended to use for streamflow simulations and water resources investigations in monthly temporal resolution for the watersheds which are having similar characteristics with parameter values in the ranges of a (0.961-0.998), b (1066-1644), c (0.003-0.013) and d (0.813-0.741). Key words: ‘abcd’ model, monthly water
item: Thesis-Abstract
Aspect of habitat and disturbance effects on tropical reef-fish communities
Ohman, MC
Reef-fishes live in close association with the reef-habitat and should therefore be expected to respond to various aspects of habitat structure. In this thesis the influence of habitat structure on reef-fish communities is examined. Also, the effects of human disturbance is investigated with special attention given to the consequences of habitat destruction. The results presented were obtained by observational studies in the field as well as experimental work in vitro using aquaria. The field investigations were conducted in Sri Lanka which is situated in an area where no earlier quantitative studies in reef-fish ecology have been carried out. In view of the fact that most research in tropical reef-fish ecology has focused on coral reefs this study also considered other reef types including sandstone and rock reefs. Results demonstrated that the fish communities had clear patterns in their distribution; most strongly among reef types but also among the different large-scale habitats they contained. These habitats had unique fish assemblages and most species were restricted in their distribution. Different families, as well as trophic groups, also showed distinct preferences. Reef-patches, which were disturbed by destructive fishing were dominated by coral rubble and these were compared with non-disturbed patches with a high degree of live coral cover. Disturbed patches had lower fish densities, lower species diversity and a more heterogeneous fish assemblage composition than nondisturbed patch reefs. These differences were interpreted as being caused by resource impoverishment as a consequence of habitat deformation. Habitat effects on the fish fauna were confirmed when comparing habitat and fish related variables using both univariate and multivariate methods. However, many correlations were reef-type specific. For example, structural complexity appeared to increase fish diversity on the sandstone reefs but not on the coral reefs. In general, correlations suggested that food and shelter availability were important factors influencing distribution patterns. For example, species that used branching corals as a refuge clearly correlated with Acropora colonies. Species within one family, the Chaetodontidae, received special attention as they have been suggested to be indicators of habitat disturbance. Assemblages within this family also showed among-habitat distribution patterns. Corallivores were the most common trophic group within the chaetodontids and they were most abundant in rich coral areas and correlated positively with live coral cover. They also showed lower numbers in disturbed areas. Results therefore suggested that chaetodontids may be useful as indicators of disturbance. However, it could be questioned whether these results can be applied in different geographic regions and on different reefs. Habitat selection at settlement was investigated as a potential determinant of among-habitat distribution patterns. Settling larvae had the ability to choose among habitats and their preferences were in accordance with habitat associations of conspecific adults in the wild. A range of strategies were observed however, habitat choice was also modified by conspecific interactions If the fish fauna within a given habitat is depleted as a result of human disturbance it would be beneficial to find a method to restore former abundances. This thesis also presents a method which enhances recruitment to artificial reefs by using lightattractors.
item: Thesis-Full-text
Assessment and regionalization of hydrological model parameters in neighboring Pho Chhu and Mo Chhu basins in Bhutan :
(2019) Choden, P; Rajapakse, RLHL
In the cold regions because of harsh climates, there exists no or an inadequate number of monitoring stations. It is indeed a challenge to generate the hydrographs of ungauged basins with scanty information from limited gauged basins. As a result, it has important implications for existing water resources systems as well as for future water resources planning and management since high elevation mountains are all important sources of water to the billions in the lowlands in these climatic regions. The Mo Chhu and Po Chhu catchments in Bhutan are used in this study to assess the regionalization of hydrological model parameters from one catchment to the other neighbouring catchment having similar characteristics using ABCD hydrological model incorporating snowmelt parameter. The Mo Chhu catchment was considered as the gauged catchment and its hydrological parameters were simulated through model calibration and validation, and then transferred to the neighbouring Pho Chhu catchment. For the corresponding watersheds, precipitation, streamflow and temperature daily data were collected for the 11 years from 2006~2017 from the National Centre for Hydrology and Meteorology in Bhutan and checked by visual comparison, single and double mass curve analysis and annual water balance to ensure data reliability, consistency and to identify suitable data periods for model calibration and validation. For the model performance evaluation, Root Mean Square Error (RMSE), Pearson correlation coefficient (r) and Coefficient of determination (R2) were used as the objective functions. The Pearson correlation values for calibration and validation of Mo Chhu basin are 0.84 and 0.88, respectively. When the same model parameters were transferred to Pho Chhu basin, Pearson value for validation was found to be 0.82, indicating good inter-basin parameter transferability and effective model regionalization. Comparing and analyzing the results of ABCD model with and without snow parameter "m", it can be concluded that the model with snow parameter performs better due to proper simulation of the major contribution to basin flow from snowmelt. Approximately, over 52% of the basin flows can be attributed to snowmelt during summer and spring and the incorporation of snow processes in the monthly ABCD model has thus significantly improved model performance in snow-covered areas in Bhutan
item: Thesis-Abstract
Assessment of climate change impact on water availability in upper Mahaweli river basin, Sri Lanka
(2022) Musadiq F; De Silva PKC
Climate change, population increase, and economic development will all have an impact on future water availability for drinking water supply, agriculture, and recreation activities, with different effects in different regions. The present study investigates the potential impact of climate change on future water availability in the Peradeniya sub-catchment of the Upper Mahaweli river basin. The hydrological modeling of this study was performed by Hydrologic Engineering Centre Hydrological Modelling systems (HEC-HMS). In this study, the entire catchment area was divided into three sub-basins to simulate runoff at the outlet of the catchment and the model results were calibrated and validated using historical streamflow data. Future runoff based on calibrated parameters was estimated after bias correction of climate rainfall data for representative concentration pathways (RCP) 4.5 and RCP 8.5 scenarios. Further, an assessment of water availability based on annual and seasonal periods was carried out from the model results. The model calibration carried out from 1990 to 1994, indicated good model results in terms of objective functions where root mean square error (RMSE) is 0.60, Nash-Sutcliffe (NSE) is 0.62, and Percent Bias is -15%. Further, validation of model results from 1994 to 2000 yielded RMSE of 0.60, NSE of 0.52, and Percent Bias of 13.9 % indicating good model results. From the results obtained, it was identified that the water availability will increase for both scenarios RCP 4.5 and RCP 8.5 during the mid-century (2040-2060) and end-century (2080-2100) period. The annual water availability concerning the historical period will increase by 27.34 % during the mid-century period and will further increase by 42.06 % during the end-century period in the RCP 8.5 scenario. The seasonal water availability in mid-century compared to the historical period will be more affected during the first inter-monsoon (FIM) period with an average increase of 69 % and 83 % in RCP 4.5 and RCP 8.5 scenario, respectively. Whilst the seasonal water availability will decrease during the first inter-monsoon (FIM) in the endcentury compared to the mid-century period by 26 % and 27 % in RCP 4.5 and RCP 8.5 scenarios, respectively. The findings of this study can be useful for the water managers and stakeholders to manage future water needs in the basin and reduce the future vulnerabilities associated with the increasing water availability in the basin.
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Assessment of deforestation and land cover change impacts on flood peak discharge in Maduru oya basin, Sri Lanka
(2022) Nab AW; Ratnasooriya AHR
Population growth raises demand and competition for water resources and food stocks while it changes the landuse types by anthropogenic activities to adopt applicable measures for supplying water for domestic, agricultural, and industrial purposes. These changes alter the hydrological response of the river basins and can impose the communities to severe environmental risks like floods and landslides. Therefore, understanding of landuse change is crucial to study river basins’ behavior and take mitigatory measures. The study presented here quantifies and analyzes the historical deforestation and landuse/landcover (LULC) change impacts on flood peak discharge of the Maduru Oya river basin, Sri Lanka using Hydrologic Engineering Centre-Hydrologic Modeling System (HEC-HMS) and remote sensing techniques. The Landsat Multispectral Scanner (MSS), Thematic Mapper (TM), and Operational Land Imager-thermal Infrared Sensor (OLI-TIRS) images are acquired in 1976, 1994, 2009, 2021 and classified using maximum likelihood algorithm of supervised classification. The analysis of LULC change revealed that LU change was faster and in high magnitude from 1976 to 1994 compared to the remaining period to 2021. The LULC change quantification by analyzing each scenario revealed a 24.9% deforestation while a 2.2%, 9.8%, 8.4%, and 4.5% increase in homestead/garden, paddy, scrubland, and water body between 1976 to 1994, respectively. The deforestation further continued to a rate of 4.1% and a 2.0% decrease in water bodies was also found in 2009 while homestead/garden, paddy, and scrubland continued to increase by 3.5%, 1.4%, and 1.5% compared to 1994 landuse scenario, respectively. In contrast, the 2021 landuse scenario indicated a 7.6% decrease in scrubland while 3.6%, 0.5%, 1.5%, and 1.8% increase in forests, homestead/garden, paddy, and water bodies. The classified images were subjected to accuracy assessment. The overall accuracy of 82%, 84%, 88%, and 91% are found for 1976, 1994, 2009, and 2021 LU scenarios while having kappa coefficients of 0.78, 0.80, 0.85, and 0.89 for respective years. The Normalized Difference Vegetation Index (NDVI) assessment of scenarios corresponds to the landuse classified images. An event-based HEC-HMS model is used to simulate the flood events in the Welikanda catchment of the Maduru Oya river basin. The model is calibrated and validated using the 1976 landuse and then the subsequent landuses are applied to study LU change impact on flood peak discharge. For model performance evaluation, the Nash-Sutcliffe, RMSE Observations Standard Deviation Ratio (RSR) Percent Bias (PBIAS), and the Coefficient of determination (R 2 ) were exploited. The average NSE, RSR, PBIAS, and R 2 values of 0.92, 0.25, 17.60, and 0.94 achieved in calibration and 0.73, 0.50, -3.03, and 0.78 are found in the validation which all can be rated very good performance except for PBIAS as satisfactory in calibration and NSE as good in the validation. The land cover change resulted in an increase (22.3%) in flood peak from 842 m 3 /s in 1976 to 1,030 m 3 /s in 2021. As a result of the landcover changes, the volume is also increased (42.3%) from 178.16 MCM in 1976 to 253.52 MCM in 2021. This study provides useful information for land and water managers, forests conservation units, and hydrologist to understand the LULC change impacts on floods and paves the way for broad LU and hydrological studies in Sri Lanka which are rarely conducted. The same approach can be applied in different parts of Sri Lanka which are exposed to severe LU changes.
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Calibration and verfication of a-two parameter monthly water balance model and its application potential for evaluation of water resources -a case study of kalu and mahaweli rivers of sri lanka
Sharifi, MB; Wijesekera, NTS
Water balance is a method by which we can account for the hydrological cycle of a specific area, with the emphasis on plants and soil moisture. One of the main purposes of a water balance study is to evaluate the net available water resources, both on the surface and in the subsurface. Understanding the behavior of a catchment from a hydrological point of view is necessary when planning and activities needed to be done in the watershed. A-two parameter monthly water balance model for two basins was calibrated and verified using 30 years monthly rainfall, observedflow and pan evaporation data. Kalu Ganga at Ellagawa and Mahaweli Ganga at Morape were selected to estimate the streamflow. The model was calibrated and verified and a good performance was shown for both catchments. The C coefficient for Kalu Ganga at Ellagawa and Mahaweli Ganga at Morape were found as 1 and 1.1 respectively while the SC parameter was found as 800 and 1200 respectively. The MRAE value for calibration period for Kalu Ganga at Ellagawa and Mahaweli Ganga at Morape showed a very good fitting with value of 0.145 and 0.152 respectively. The same for verification period was also very good with value of 0.153 and 0.157 respectively. During the calibration and verification periods value of the Nash–Sutcliffe efficiency for Kalu Ganga at Ellagawa was found as a 93.6% and 92.4% respectively. 93.6% and 94.1% were the Nash–Sutcliffe values for Mahaweli Ganga at Morape respectively. The two parameter monthly water balance model produced a better fitting of MRAE in annual and seasonal values when compared with monthly time series. The two-parameter monthly water balance model with the simple structure and two parameters proved as a very efficient model when simulating the monthly, seasonal and annual runoff. Due to its simplicity and high efficiency in performance, this two-parameter monthly water balance model can be easily and efficiently used for the water resources planning and management.
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Climate extremes and precipitation trends in Kelani river basin, Sri Lanka and impact on streamflow variability under climate change
Dissanayaka, KDCR; Rajapakse, RLHL
The study region comprises a major river basin in the West of Sri Lanka namely Kelani River basin. The hydrological regime of this river differs significantly from that of the others because the basin features great geographical and climatic diversities over its latitudinal and longitudinal extent. Kelani River is the second largest river in Sri Lanka that originates from the central hills and flows to the west coast through Colombo city. The river basin is bound by northern latitudes from 6°47' to 7°05' and eastern longitudes from 79°52' to 80°13'. The river originates approximately 2,250 m above mean sea level and passes 192 km to reach the Indian Ocean. The river basin experiences an annual average rainfall of about 3,450 mm corresponding to a volume of about 7,860 MCM out of 43% discharges into the sea. However, changes in precipitation and temperature due to the climate change can cause more frequent extremes with extended droughts and floods with further impact to the reservoir storage resulting a significant threat to water resources. Therefore, the present study focuses on climate extremes with reference to the past, present and future behavior of rainfall, temperature and streamflow at watershed scale to identify climate change impact on the spatial and temporal variations of streamflow in the Kelani River Basin. For this research, basin-wide future hydrology is simulated by using downscaled temperature and precipitation outputs according to RCP Scenarios of the Canadian Earth System Model - version 2 (CanESM2), Statistical Downscaling Model (SDSM) and the Hydrologic Engineering Centre’s Hydrologic Modeling System (HEC-HMS). The case study further evaluates the long-term behaviour and trends of the climate extremes based on the observed historical temperature and precipitation data. The findings suggest that the temperature and precipitation extremes are on the rise while the annual average precipitation in the river basin is declining. It is also predicted with the application of statistical downscaling that temperature may rise annually for representative concentration pathways of RCP2.6, RCP4.5 and RCP8.5. The mean explained variance are 67, 86 and 13% for temperature maximum, temperature minimum and precipitation respectively, for calibration with NCEP predictors. During calibration, the R2 value of the monthly and seasonal sub-model of RCP 2.6, RCP 4.5 and RCP 8.5 scenarios are lies between 80.1% and 99.4% for both maximum and minimum temperature and 50 to 90% for precipitation. During validation, R2 value for both monthly seasonal sub-model followed by bias correction was between 76.9% and 99.2% for both maximum and minimum temperature, and 55% to 95.2% for precipitation. A detailed modelling approach is incorporated to Hanwella sub-watershed (1799.67 km2) of the Kelani River basin, to study the subsequent water resource management options with the varying streamflow of the Kelani River basin under the effect of the future (2020’s, 2050’s and 2080’s) rainfall and temperature as impending climate change impacts for RCP scenarios. The paper reviews the current state of the catchment as well as the suitability of applying the GCM’s rather than RCM’s to Sri Lanka to assess this river basin, according to monthly, seasonal and annual variations of the climatology. Apart from the water resources management, a quantitative analysis was conduct to assess the change in the amount of surface water within the selected river basin as a function of the expected variations in precipitation and temperature. This study will set the baseline for commencing and continuing quantitative studies incorporating the behaviour of the basin-wide climatology and streamflow variability with the use of general circulation models
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Continuous hydrological modelling using soil moisture accounting for water resources assessment in Kelani river basin, Sri Lanka
(2018) Nasimi, MN; Rajapakse, RLHL
The assessment of water resources in a river basin for fulfilling various needs in the present and future requires a proper estimation of water availability. This is possible through hydrological modelling. The Kelani river basin in Sri Lanka experiences water stress under the current water uses, development, and urbanization effects. It requires a continuous hydrological model for the assessment of its water resources, focusing on impending climate change impacts. Continuous hydrological models, unlike event-based models, simulate longer periods that include both dry and wet conditions. Soil moisture accounting (SMA) model in the Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) is chosen to simulate the streamflow. However, the SMA loss model requires precise and updated soil and land use data for parameter estimation, which is not available for the study area. In addition, the lumped nature of the model comparing to distributed models is also in question. This research discusses the development, parametrization and calibration methodologies for the 14 parameters of the HEC-HMS model with the SMA algorithm by considering a catchment divided into several sub-catchments. This division is based on the maximum drainage area method to improve the model accuracy in a scarce soil data situation. The SMA loss model requires 14 parameters to be set. Among these, the impervious percentage is calculated from a land use map; the groundwater 1 and 2 storage as well as the groundwater 1 and 2 coefficients are calculated through the streamflow recession analysis. The maximum infiltration, soil storage, tension storage, and soil percolation rate are calculated from the similar studies; and the groundwater 1 and 2 percolation with four initial parameters are calculated only through a calibration procedure. The model is calibrated using daily data from 2007 to 2012 and validated from 2012 to 2017. The mean ratio of absolute error (MRAE) is used as a primary objective function. The coefficient of determination (R2), percent volume error (PVE), and Nash-Sutcliffe efficiency (NSE) are also used to compare and evaluate the model performance. The results indicate that the performance of the rainfall-runoff model significantly improves when the basin is subdivided into three to eight sub-catchments and the optimum result is found with the five sub-catchments. For the calibration period, the performance of the model is adequate with a R2 of 0.83, a NSE of 0.82, a PVE of 5.3%, and a MRAE = of 0.38. Similarly, adequate results are also retrieved for the validation period, with a R2 of 0.81, a NSE of 0.80, a PVE of 13.1%, and a MRAE of 0.36. The results of the statistical analysis indicate that the simulated and observed flows are reasonably well correlated. The parameter analysis shows that the soil percolation and tension zone storage rates are the most sensitive and second storage of ground water (GW2) is the least sensitive parameters. Furthermore, for the Kelani river basin up to the Hanwella catchment, the simple surface, simple canopy, ModClark, recession and Muskingum methods are found to be the most suitable methods alongside the SMA model. The model performance can potentially be improved through further calibration using hourly climatic input data instead of daily data and with using multiple gauging stations instead of single gauge station. In the future, the validated HEC-HMS model can be employed with seasonal climate forecasts under long-range land use and climate projections. Besides, radarbased precipitation data can be used to represent the climatic variability on a grid-based scale.
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Development of a rainfall-runoff-inundation model and flood monitoring system based on satellite imagery for Kalu ganga basin, Sri Lanka
(2022) Sultana T; Rajapakse RLHL
Floods are getting severe due to climate change and anthropogenic activities which neeimmediate response to lower the risk and decrease the human and financial losses. Floodinundation mapping for flood risk preparedness using satellite data has been widely used imany recent studies. However, satellite imageries may contain some uncertainties. Thereforflood inundation maps from satellite data need to be verified with flood inundation mapgenerated by hydrological models from observed data for accurate estimation of flood risk.Although satellite-generated flood maps are widely used to determine the inundation extentthere are certain challenges to their use such as inaccessibility of imagery due to satellite orbior cloud cover, which hampers accurate measurement of inundation risk. In this study, the rainfall-runoff inundation (RRI) model for the Kalu Ganga basin wasdeveloped, and its applicability to evaluate the discharge and flood inundation areas wasdiscussed. The RRI model could estimate discharge, water levels, and inundation areasimultaneously based on two-dimensional diffusion wave equations. The results and statisticaanalysis indicate that the RRI model could efficiently estimate extreme flood events. Formodel calibration, the R 2 value ranges from 0.72-0.80 and for model validation, the R valueranges from 0.75-0.90, which shows good performance of the model. The simulated inundation extents were verified and compared with Sentinel 1A SA(Synthetic Aperture Radar) satellite imagery data for 2016 and 2017 flood events. Sentinel 1AGRD-IW (Ground Range Detected - Interferometric Wide swath) mode of VV co-polarization,with a spatial resolution of 20 m was acquired and pre-processed using the SentineApplication Platform (SNAP) software toolbox. The pre-processed images were correcteand maximum likelihood supervised classification was performed to produce the floodinundation maps of the study area. The actual flooded area from RRI is found to be 291.9km 2 and that from satellite image is found to be 201.7 km 2 for the 2016 flood event. For th2017 flood event, the actual flooded area from RRI is found to be 371.14 km and that frosatellite image is found to be 297.42 km 2 . Hence, the flooded area difference was found to be35.54 % for 2016 and 22.13 % for 2017 flood events from the total area selected from thmodel. Most of the floodplains from the RRI model and satellite images were along the mairiver in the basin, including the city of Ratnapura (upstream), the city of Kalutar(downstream), and the areas in between. These results with an accuracy level of ~25 % - 30 % are deemed to be within an acceptable range for emergency evacuation and rapid flood damageassessment purposes. Future studies should further investigate and validate the flooinundation mapping ability of Sentinel 1A SAR using ground-based reference flood maps orother satellite data. This study reveals that satellite imagery can be one of the most coseffective ways to capture the flood disaster footprints, identify flood-prone areas, anunderstand the flooding problem in a better way. This methodology can be effectively usefor disaster risk management, where the time factor is very critical. 2 2
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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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The Effect of antecedent moisture condition on HEC-HMS model performance : a case study in Kelani river basin, Sri Lanka
(2018) Dorji, KY; Rajapakse, RLHL
Among all observed natural hazards, water-related disasters are the most frequent and they pose major threats to people and while hindering socio-economic development. Flood forecasting is one the most challenging and difficult problems in hydrology. However, it is also one of the most important problems in hydrology due to its critical contribution in reducing economic damages and loss of life losses. In many regions of the world, flood forecasting is one among the few feasible options to manage floods. In Soil Conservation Service Curve Number (SCS-CN) method, Antecedent Moisture Condition (AMC) of the soil plays a very consequential role because the curve number varies according to the soil, land cover and soil moisture content, and that is considered while estimating runoff depth. Soil water represents only a minimal part of the water on our planet, but it is certainly one of the most imperative factors when it comes to flood forecasting since soil saturation directly affects runoff generation. Kelani river basin was selected for the study because of the nature of the basin with respect to the vulnerability to floods and availability of data at finer resolution. Ten years of daily rainfall, streamflow and evaporation data from 2007 to 2017 water year were used for the study. Events separation was carried out using Minimum Inter-event Time (MIT) method. There are 38 selected events, out of which the first half events were used for model calibration and the second half events were used for model verification. The univariate gradient search method was applied to optimize the parameters by minimizing the Sum of Absolute Residual Error (SARE) objective function. Manual calibration was carried out using Nash-Sutcliffe model efficiency coefficient (NASH) as an objective function for comparison. The average NASH value in model calibration and validation were 0.63 and 0.62 while the lowest Root Mean Square Error (RMSE) obtained in model calibration and validation were 1.31 and 2.82 respectively. The closer the model efficiency is to NASH value of 1, the more accurate the model is. The calibration data set performed better than the model verification data set as depicted by lower RMSE value. Random events were selected to incorporate different soil moisture conditions to check the model performances. It has been observed that the events that falls in Maha season performs better when AMC III is applied whereas the model performance neither improves nor deteriorate when the events falls in Yala season. The present work reveals and confirms that while conducting event rainfall-runoff modelling for flood management using HEC-HMS, AMC should be considered in order to improve the model efficiency and performance. The study findings are applicable to other hydrologically similar basins in the same region or elsewhere and the findings from model sensitivity analysis are useful for fine tuning model performance and opting for better flood management strategies.