EVALUATION OF A MONTHLY WATER BALANCE MODEL CONSIDERING RAINFALL STATION WEIGHTS AND PHYSICAL PARAMETERS IN NILWALA BASIN SRI LANKA Mukhtar Ahmad Masoud (189242M) Degree of Master of Science Department of Civil Engineering University of Moratuwa Sri Lanka September 2019 EVALUATION OF A MONTHLY WATER BALANCE MODEL CONSIDERING RAINFALL STATION WEIGHTS AND PHYSICAL PARAMETERS IN NILWALA BASIN SRI LANKA Mukhtar Ahmad Masoud (189242M) Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Water Resources Engineering and Management Supervised by Professor N.T.S. Wijesekera UNESCO Madanjeet Singh Centre for South Asia Water Management (UMCSAWM) Department of Civil Engineering University of Moratuwa Sri Lanka September 2019 ii ACKNOWLEDGEMENTS Prima facea, I am grateful to the God for the good health and wellbeing that were necessary to complete this thesis. I would like to express my sincere gratitude to my advisor Professor N.T.S. Wijesekera for the continuous support of my Postgraduate studies and related research, for his patience, motivation, and immense knowledge. His guidance helped me all the time of research and writing of this thesis. I could not have imagined having a better advisor and a mentor for my Postgraduate studies. My sincere thanks also goes to Dr.R.L.H Lalith Rajapakse for his support and guidance provided both in terms of academic and logistic welfare during my stay. I take this opportunity to express gratitude to all of the faculty members of the department of Civil Engineering for their help and support. I place on record, my sincere thank you to Madanjeet Singh for providing scholarship to pursue a Master’s degree in Water Resources Engineering and Management. I would like to thank Mr. H.W. Kumarasinghe for his kind assistance during my stay in Sri Lanka. Last but not the least; I would like to thank my family especially from my elder brother Doctor Basir Ahmad Masoud who encouraged and supported me from school days to pursue a Postgraduate degree. iii EVALUATION OF A MONTHLY WATER BALANCE MODEL CONSIDERING RAINFALL STATION WEIGHTS AND PHYSICAL PARAMETERS IN NILWALA BASIN SRI LANKA ABSTRACT Water Resources Management is key for economic growth and sustainable development. Monthly Water Balance Models are widely applied for its easy and simple structure characteristics. Many research efforts have been carried out using Two Parameter Monthly Water Balance Model for water resources management in Sri Lanka in which model estimation are influenced by rainfall and the approach for the selection of parameters which can be performed using rainfall station weights optimization ;on the contrary, the non-availability of gauged streamflow data in hydrological modelling for optimization remains one of the major challenges where many modelers suggests parameter estimation using physical characteristic of a watershed as solution. The objective of the study is to evaluate monthly water balance model incorporating optimization of rainfall station weights and physical parameters of the catchment for water resources planning and development. Two parameter model was used for monthly water resource estimation of Nilwala Ganga basin in Sri Lanka. The model was calibrated and verified for Pitabeddara (324km2) watershed using 24 years’ monthly rainfall, pan evaporation and streamflow data successfully. Initially, the model parameters values C and Sc estimated with Thiessen method later rainfall stations weights were optimized while keeping the model parameters C and Sc unchanged for calibration and verification. Secondly C and Sc parameters of two parameters monthly water balance model with station weights were optimized simultaneously where parameters were estimated using physical characteristics of the catchment taking into account rainfall, pan evaporation and landuse variables. Rainfall and pan evaporation relationship was utilized for estimation of C and Sc parameter was estimated using correlation of curve number (CN). After Two Parameter Monthly Water Balance Model Applied using Thiessen method on Pitabeddara watershed. The value for C and Sc were 1.5 and 1700 respectively with average MRAE of 0.22 and 0.31 during calibration and verification periods. Rainfall station weights optimization only resulted in values of 1.3 and 1600 for C and Sc parameters respectively with average MRAE of 0.22 during calibration and 0.27 during verification, stations weights of (0.47, 0.31, 0.07, 0.12, 0.03) for Deniyaya, Dampahala, Anningkanda, Goluwawatta, Kirama stations respectively. Obtained C and Sc values of 1.41 and 1550 while station weights are parameters are optimized simultaneously with average MRAE of 0.19 and 0.25 for calibration and verification respectively, stations weights of (0.12, 0.22, 0.32, 0.22,0.12) for mentioned stations respectively. Also, value of C and Sc parameters were 1.40 and 1500 were retrieved by accounting physical characteristics of catchment and MRAE of 0.23 and 0.28 for calibration and verification. The station weights optimization improved the MRAE results of model by (10%) which is significant with indication of better MRAE than conventional rainfall averaging method. Estimation using physical characteristics of model resulted in (5%) superior results than empirical approach. This research effort concludes that rainfall station weights optimization method results are superior then Thiessen Method and parameters estimation using physical characteristics of the catchment can be useful for ungauged catchments and it can provide acceptable results. Keywords: Ungauged streamflow estimation, Physical catchment characteristics, Spatial Variability of Rainfall, Water balance modelling, iv Contents DECLARATION .......................................................................................................... i ACKNOWLEDGEMENTS ......................................................................................... ii ABSTRACT ................................................................................................................ iii LIST OF FIGURES ................................................................................................... vii LIST OF TABLES ...................................................................................................... ix LIST OF ABBREVIATIONS ...................................................................................... x 1 INTRODUCTION ................................................................................................ 1 1.1 General .......................................................................................................... 1 1.2 Problem identification ................................................................................... 4 1.3 Study Objectives ............................................................................................ 4 1.3.1 Overall Objective ................................................................................... 4 1.3.2 Specific Objective .................................................................................. 4 1.4 Study Area Selection ..................................................................................... 4 2 LITERATURE REVIEW ..................................................................................... 7 2.1 General .......................................................................................................... 7 2.2 Current state of water balance model ............................................................ 7 2.3 Monthly water balance models comparison .................................................. 9 2.4 Two parameter water balance model ........................................................... 10 2.5 Model components ...................................................................................... 11 2.5.1 Rainfall ................................................................................................. 11 2.5.2 Actual Monthly evapotranspiration...................................................... 12 2.5.3 Streamflow ........................................................................................... 13 2.5.4 Soil water content ................................................................................. 13 2.6 Model Calibration and Verification ............................................................ 14 2.6.1 Objective Functions ............................................................................. 14 2.6.2 Parameter Optimization ....................................................................... 19 2.6.3 Warm up period.................................................................................... 20 2.7 Rainfall Spatial Variability .......................................................................... 21 2.7.1 Methods of areal averaging rainfall ..................................................... 21 v 2.7.2 Importance of rainfall spatial variability .............................................. 22 3 Methodology: ...................................................................................................... 24 4 Data collection .................................................................................................... 25 4.1 Rainfall and Streamflow .............................................................................. 26 4.2 Data Checking ............................................................................................. 26 4.3 Filling the missing data ............................................................................... 26 4.4 Thiessen Rainfall ......................................................................................... 28 4.5 Double Mass Curve ..................................................................................... 43 4.6 Annual Data Comparison ............................................................................ 45 4.6.1 Annual monthly rainfall comparison ................................................... 45 4.6.2 Annual Water Balance ......................................................................... 45 4.7 Identification of Missing Data ..................................................................... 49 5 ANALYSIS AND RESULTS ............................................................................. 50 5.1 Introduction ................................................................................................. 50 5.2 Model Development .................................................................................... 51 5.3 Warm up period ........................................................................................... 52 5.4 Model Calibration and Model Verification (Thiessen Rainfall) ................. 52 5.5 Selection of Objective function ................................................................... 53 5.6 parameter optimization ................................................................................ 53 5.6.1 Determination of Global Minimum ..................................................... 54 5.7 Model Calibration and Model Verification (Optimized Rainfall) ............... 65 5.8 Model Calibration and Model Verification with two parameters and station weights and optimization ........................................................................................ 74 5.9 Parameters estimation from physical characteristics of the catchment ....... 80 6 DISCUSSION ..................................................................................................... 93 6.1 Model selection ........................................................................................... 93 6.2 Data collection and checking ...................................................................... 93 6.3 Rainfall spatial variability ........................................................................... 94 6.4 Model Development .................................................................................... 96 6.4.1 Flow duration curve for High Medium and Low flows ....................... 96 6.4.2 Initial soil water content ....................................................................... 96 vi 6.4.3 Objective functions selection ............................................................... 96 6.4.4 Calibration and verification.................................................................. 96 6.5 Overall comparison of models performance ............................................... 97 7 CONCLUSIONS & RECOMMENDATIONS ................................................... 98 7.1 Conclusions ................................................................................................. 98 7.2 Recommendations ....................................................................................... 99 REFERENCES ......................................................................................................... 100 ANNEX A – DATA checking ................................................................................. 105 vii LIST OF FIGURES Figure 1-1: Pitabeddara watershed ............................................................................... 6 Figure 3-1:Methodology flow chart ........................................................................... 24 Figure 4-1:Thiessen polygon Pitabeddara watershed ................................................ 29 Figure 4-2:Land use Map of Pitabeddara watershed.................................................. 31 Figure 4-3:Soil Map Pitabeddara watershed .............................................................. 32 Figure 4-4:Dampahala Streamflow response to rainfall from(1993-2005 ................. 33 Figure 4-5:Dampahala Streamflow response to rainfall from (2005-2017)............... 34 Figure 4-6:Anningkanda Streamflow response to rainfall from (1993-2005) ........... 35 Figure 4-7:Anningkanda Streamflow response to rainfall from (2005-2017) ........... 36 Figure 4-8:Goluwawtta streamflow response rainfall from (1993-2005) .................. 37 Figure 4-9:Goluwawatta Streamflow response to rainfall (2005-2017...................... 38 Figure 4-10:Deniyaya Streamflow response to rainfall from (1993-2005) ............... 39 Figure 4-11:Deniyaya Streamflow response to rainfall from (2005-2017) ............... 40 Figure 4-12:Kirama Streamflow response to rainfall from (1993-2005) ................... 41 Figure 4-13:Kirama Streamflow response to rainfall from (2005-2017) ................... 42 Figure 4-14:Double Mass Curve for Rainfall Data of Pittabeddara .......................... 43 Figure 4-15::Monthly Flow Duration Curve .............................................................. 44 Figure 4-16:Annual Rainfall Pattern .......................................................................... 45 Figure 4-17:Annual water balance ............................................................................. 47 Figure 4-18:Runoff coefficient .................................................................................. 47 Figure 4-19:Stream flow response vs rainfall ............................................................ 48 Figure 4-20:Annual water balance difference ........................................................... 48 Figure 5-1:Model Warm-up Period for Initial Soil Water Content............................ 52 Figure 5-2:Coarser Resolution Surface for Pitabeddara ............................................ 54 Figure 5-3:Hydrographs from Model calibration -Thiessen rainfall (1993-2005) ..... 56 Figure 5-4:Hydrographs from model verification -Thiessen rainfall (2005-2017) .... 57 Figure 5-5:Hydrographs from model calibration (Thiessen rainfall) on both normal and log scale ............................................................................................................... 58 Figure 5-6:Hydrographs from model verification (Thiessen rainfall) on both normal and log scale ............................................................................................................... 59 Figure 5-7:Annual Water Balance comparison calibration (Thiessen rainfall) ......... 60 Figure 5-8:Annual Water Balance comparison verification (Thiessen rainfall) ........ 61 Figure 5-9:Water Balance for Calibration period (Thiessen rainfall) ........................ 62 Figure 5-10:Water Balance for Verification period (Thiessen rainfall).................... 63 Figure 5-11:Flow duration curve – calibration period (Thiessen rainfall) ................. 64 Figure 5-12:Flow duration curve – Verification period (Thiessen rainfall) ............. 64 Figure 5-13:Optimized station weights ...................................................................... 65 Figure 5-14:Hydrographs from model calibration using Optimized rainfall on both normal and log scale................................................................................................... 67 Figure 5-15: Hydrographs from model Verification using Optimized rainfall on both normal and log scale................................................................................................... 68 Figure 5-16:Annual Water Balance Comparison Calibration (Optimized Rainfall) . 69 viii Figure 5-17:Annual Water Balance Comparison Verification (Optimized Rainfall) 70 Figure 5-18:Annual Water Balance Comparison Calibration (Optimized Rainfall) . 71 Figure 5-19:Annual Water Balance Comparison Verification (Optimized Rainfall) 72 Figure 5-20:Flow duration curve – Calibration period (Optimized rainfall) ............ 73 Figure 5-21:Flow duration curve – Verification period (Optimized rainfall) ............ 73 Figure 5-22:Annual Water Balance Comparison calibration two parameter- station weights ....................................................................................................................... 77 Figure 5-23:Annual Water Balance Comparison two parameter verification two parameter - station weights ........................................................................................ 78 Figure 5-24:Flow duration curve – Calibration period two parameters and station weights ....................................................................................................................... 79 Figure 5-25:Flow duration curve – Verification period two parameters and station weights ....................................................................................................................... 79 Figure 5-26:Annual Water Balance Comparison Calibration (Physical Parameters) 84 Figure 5-27:Annual Water Balance Comparison Verification (Physical Parameters) .................................................................................................................................... 85 Figure 5-28:Flow duration curve – Calibration period (Physical Parameters) .......... 85 Figure 5-29:Flow duration curve – Verification period (Physical Parameters) ........ 86 Figure 5-30:Hydrographs from model calibration using physical parameter on both normal and log scale................................................................................................... 87 Figure 5-31:Annual Water Balance Comparison Calibration (Physical Parameters) 88 Figure 5-32:Hydrographs from Model Verification using Physical Parameter on both normal and log scale................................................................................................... 89 Figure 5-33:Annual Water Balance Comparison Verification (Physical Parameters) .................................................................................................................................... 90 Figure 6-1:Thiessen and Optimized Rainfall Comparison......................................... 95 Figure 9-4:Variation of Maximum, Mean and average monthly rainfall, streamflow & evaporation ........................................................................................................... 109 ix LIST OF TABLES Table 4-1:Data source and Data Resolution of Nilwala ganga at Pitabeddara ..................... 25 Table 4-2:Rain Gauging Station Details of Nilwala ganga at Pitabeddara ............................ 26 Table 4-3:Distribution of Gauging Stations in Pittabeddara at Nilwala gaga ........................ 26 Table 4-4:Missing value for each month ............................................................................... 28 Table 4-5:Each station Thiessen polygon area and Thiessen weight ..................................... 28 Table 4-6:Annul water balance for Pittabeddara watershed .................................................. 46 Table 5-1:Comparison of Model Performance Calibration (Thiessen Rainfall) .................... 55 Table 5-2:Comparison of Model Performance Calibration (Thiessen Rainfall) .................... 55 Table 5-3:Water Balance Estimation Calibration Period (Thiessen rainfall) ........................ 62 Table 5-4:Water Balance Estimation Verification Period (Thiessen rainfall) ....................... 63 Table 5-5:Comparison of Model Performance Calibration (Optimized Rainfall) ................. 66 Table 5-6:Comparison of Model Performance Verification (Optimized Rainfall) ............... 66 Table 5-7:Water Balance Estimation Calibration Period (Optimized Rainfall) .................... 71 Table 5-8:Water Balance Estimation Verification Period (Optimized Rainfall) ................... 72 Table 5-9:Two parameters and station weights optimization ................................................ 74 Table 5-10:Comparison of Model Performance Calibration with two parameter and station weights ................................................................................................................................... 75 Table 5-11:Comparison of Model Performance Verification with two parameter and station weights ................................................................................................................................... 75 Table 5-12:Hydrographs from Model calibration –two parameter & station weights optimization (1993-2005) ...................................................................................................... 76 Table 5-13:Water Balance Estimation Calibration Period two parameters- station weights . 77 Table 5-14:Water Balance Estimation Verification Period two parameters- station weights 78 Table 5-15:Comparison of Model Performance Calibration (Physical based parameters) .... 82 Table 5-16:Comparison of Model Performance Verification (Physical based parameters) .. 83 Table 5-17:Water Balance Estimation Calibration Period (Physical Parameters) ................. 83 Table 5-18:Water Balance Estimation Verification Period (Physical Parameters) ................ 84 Table 5-19:Mean Rainfall computation for parameter C ....................................................... 91 Table 5-20:Mean Evaporation computation for parameter .................................................... 92 Table 6-1: Rainfall Average by Thiessen and Optimized station weights ............................. 95 Table 6-2:Overall Summary sheet of all models results ........................................................ 97 Table A-1:Thiessen Average Rainfall Data ......................................................................... 106 Table A-2:Evaporation Data ................................................................................................ 107 Table A-3:Streamflow Data ................................................................................................. 108 x LIST OF ABBREVIATIONS Abbreviation Description c Parameter c C Runoff Coefficient DSD Divisional Secretary Divisions E Nash–Sutcliffe coefficient E (t) Actual Evapotranspiration EP (t) Pan Evaporation IPCC Intergovernmental Panel on Climate Change K Pan Coefficient MAR Mean Annual Rainfall MRAE Mean Ratio of Absolute Error MSE Mean Square Error NEM North East Monsoon P (t) Rainfall Q (t) Runoff RAEM Ratio of Absolute Error to Mean RE Relative Error RMSE Root Mean Square Error S (t) Soil Moisture Content SC Field capacity of the catchment SWM South West Monsoon TPMWBM Two Parameter Monthly Water Balance Model WMO World Meteorological Organization