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University of Southampton Research Repository Eprints Soton University of Southampton Research Repository ePrints Soton Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON SCHOOL OF CIVIL ENGINEERING AND THE ENVIRONMENT THE USE OF NUMERICAL GROUNDWATER MODEL TO IMPROVE EFFECTIVENESS OF SUBSURFACE DRAINAGE SYSTEM IN IRRIGATED FIELD By Edward Akwasi Ampofo B.Sc., M.Sc. A thesis submitted for the degree of Doctor of Philosophy October 2009 Abstract University of Southam pton ABSTRACT SCHOOL OF CIVIL ENGINEERING AND THE ENVIRONMENT Doctor of Philosophy THE USE OF NUMERICAL GROUNDWATER MODEL TO IMPROVE EFFECTIVENESS OF SUBSURFACE DRAINAGE SYSTEM IN IRRIGATED FIELD By Edward A. Ampofo The research demonstrates that Three-Dimensional Variable-Density Groundwater Flow models such as the SEAWAT model can be effectively used for design of subsurface drainage systems for controlling salt concentration in the root zone on salt affected irrigated land. The SEAWAT model was used to optimize subsurface drainage design to ensure that the salt concentration of the groundwater at the base of the root zone does not exceed pre determined levels instead of the conventional approach of maintaining the groundwater at a predetermined water table level. The study was carried out on a conceptual uniform homogenous block of irrigated flat field of shallow water table depth of 0.5 m and salt concentration of 7200 mg/l with impermeable layer at 20 m deep and impermeable field boundaries. Using the model, spacings were designed to be used as design criteria for subsurface drainage system to maintain salt concentrations of 6000, 5000 and 4000 mg/l at the base of the root zone and water table depth of 0.8 m from the soil surface. The results showed that over a wide range of irrigation water quality and aquifer hydraulic conductivity the optimum drain spacing using SEAWAT was, depending on irrigation water quality and aquifer hydraulic conductivity, wider by between 3 and 50 % and the amount of drain discharge reduced by 1 and 27 % than would be recommended using conventional design equations. It was concluded that Three-Dimensional Variable-Density Groundwater Flow models are better for designing effective drainage systems than Conventional drain spacing design equations such as Hooghoudt. ii Dedication University of Southampton DEDICATION Dedicated to my beloved mother, Obaa Yaa Aboraa , for persevering against all odds to give me education, and to the loving memories of Opanying Attah Kwasi (my late father) who did not live long to see me to this level of my education, and Mr. Yaw Appiah (my late father-in-law) who was alive when I began this research but is no more. iii Table of contents Un iversity of Southampton TABLE OF CONTENTS Abstract………………………………………………………………………………..….ii Dedication………………………………………………………………………………..iii Table of contents……..……………………………………………………………….….iv List of Figures……………………………………………………………………….…...vii List of Tables……………………………………………………………………………..ix Author’s declaration……………………………………………………………………...xi Acknowledgements………………………………………………………………………xii Definitions and abbreviations…………………………………………………………...xiii 1 INTRODUCTION…………………………………………………………………......1 1.1 General introduction..………………………………………………...………..……...1 1.2 Objective…………………………………………………………..………..….…… ..2 1.3 Scope of the study……………………………………………………………………..6 2. BACKGROUND AND LITERATURE REVIEW………………………………….7 2.1 Introduction and context of research………………………………….……………….7 2.2 Salinity………………………………………………………………….……………..7 2.3 Irrigation and agricultural production……………………………..…………………..8 2.4 Drainage……………………………………………………………………………….9 2.5 The need for subsurface drainage………………………..……….………………….11 2.6 Theory of groundwater model………………………………………………..……...16 2.6.1 Groundwater flow model……………………………...………………..……….19 2.6.2 Solute (Salt)- transport model………………………………………..……..…...21 2.7 Numerical analysis of groundwater model……………………………………….… 22 2.7.1 Discretisation…………………………………………….……………….……...22 2.7.2 Finite-Difference Approximation………………………………………….…….24 2.8 Subsurface drainage models…………………………………………………………25 2.8.1 The WAVE model………………………………………………………………26 2.82 The SWAP model……………………………………………………………….27 2.8.3 The DRAINMOD model…………………………….…………..……………..28 2.8.4 The SEAWAT model…...………………………………………………………32 iv Table of contents University of Southampton 3. ASSESSING THE APPLICABILIY OF SEAWAT MODEL TO IRRIGATED FIELD AS SUBSURFACE DRAINAGE MODEL……………………………….35 3.1 Introduction………………………………………………………………………….35 3.2 SEAWAT model construction……………………………………………………….35 3.2.1 Input data………………………………………………………………………...38 3.3 Verification of SEAWAT model performance on irrigated field……………………42 3.4 Confirming the effectiveness of the model on irrigated field…...…………………...47 3.4.1 Changes in applied recharges, drain discharges and salt balance in the aquifer…48 3.4.2 Hydraulic head distribution………………………………………………………48 3.4.3 Advective velocity vectors……………………………………………………….50 3.4.4 Groundwater salt concentration………………………………………………….51 3.4.5 Discharges when using applied recharge salt concentration of 300 mg/l………..52 3.4.6 Salt concentration dynamics in the aquifer………………………………………53 3.4.7 Mid-drain salt concentration dynamics…………………………………………..56 3.5 Discussions and conclusion………………………………………………………….59 4. MODEL SIMULATION OF DRAINAGE AND LEACHING IN IRRIGATED FIELDS..……………………………………………………………………………...60 4.1 Introduction…………………………………………………………………………..60 4.2 Methodology…………………………………………………………………………60 4.2.1 Spatial and temporal discretisation………………………………………............60 4.3 Results and discussion……………………………………………………………….66 Case (a): The no evapotranspiration case………….…………………………………….66 4.3.1 Water table and drain discharge characteristics ………….………………………66 4.3.2 Salt remaining in aquifer and leached salt when no evapotranspiration was included in the model……………………………………………………………68 4.3.3 Mid-drain salt concentration distribution at the base of the root zone…………..69 4.4 Case (b): Evapotranspiration included in the model………….………….…………..71 4.4.1 The effect of drain spacing on water table and drain discharges ….……………71 4.4.2 Salt remaining in the aquifer and leached salt …………..………………………72 4.4.3 Mid-drain salt concentration dynamics at the base of the root zone……………..74 v Table of contents University of Southampton 4.4.4 Salt dynamics within rooting zone for applied recharge concentrations of 1000 mg/l and 700 mg/l………………………………………………………………77 4.4.5 Performance of the model in response to different aquifer hydraulic conductivities………………………………………………………………..…...82 4.5 Discussion and conclusions………………………………………………………….85 5. EFFECTIVENESS OF NUMERICAL MODELLING IN IMPROVING DRAINAGE SYSTEMS DESIGNED FOR SALT CONTROL..………………..87 5.1 Introduction…………………………………………………………………………..87 5.2 Methodology…………………………………………………………………………87 5.3 Results and discussion……………………………………………………………….89 5.3.1 Drain spacing to maintain desired salt concentration at the base of the root zone…………………………………………………….…………………………89 5.3.2 Drain spacing design……………………………………………………………..91 5.3.3 Comparison of simulated and conventional design spacings and drain discharges………………………………………………………………………...98 5.4 Discussion and conclusions………………………………………………………...103 6. GENERAL DISCUSSIONS AND CONCLUSIONS……………..………………105 6.1 Introduction………………………………………………….……….……………..105 6.2 General discussions………………………………………………….……………...107 6.3 Conclusions…………………………………………………………………………113 6.4 Recommendations…………………………………………………………………..116 APPENDICES………………………………….………………..………………….A-1 I Sensitivity analysis of SEAWAT model……….…………………………………….A-1 II Model simulation of leaching with changing applied recharge qualities…………....A-3 III Supporting information to Chapter 5……….………………………………………A-6 REFRENCES…………………………………………………..……………….. ..R-1 vi List of figures University of Southampton LIST OF FIGURES 2-1 Worldwide cropped lands equipped with or without irrigation and/or drainage system………………………………………………………………………………10 2-2a No drainage system: Water table near soil surface and water ponding in surface depressions………………………………………………..………………11 2-2b Introduction of surface drainage system: Water table lowered, unsaturated root zone created………………………………………………….……………………..…….12 2-2c Subsurface drainage system introduced: Water table lowered, larger unsaturated root zone created ………………………………………………………………..………12 2-3 Schematic representation of homogenous soil underlain by an impervious boundary that is drained by parallel equally placed drains, two of which are shown….….…..13 2-4 Discretisation of an aquifer system with cell dimensions…………………………...23 2-5 Block-centred grid system…………………………………….……………………..24 3-1 Conceptual site plan…………………………………….……………………………36 3-2 Finite-difference grid for the transect showing various cell sizes …………………..37 3-3 Hydraulic-head distribution: (a) Year 0.08 and (b) Year 20…………………..…….49 3-4 Flow velocity
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