Groundwater Recharge Modelling A case study in the Central Veluwe, The Netherlands Hiwot Ghiday Gebreyohannes February, 2008 Groundwater Recharge Modelling A case study in the Central Veluwe, The Netherlands by Hiwot Ghiday Gebreyohannes Thesis submitted to the International Institute for Geo-information Science and Earth Observation in partial fulfilment of the requirements for the degree of Master of Science in Geo-information Science and Earth Observation, Specialisation: Groundwater Assessment and Modelling. Thesis Assessment Board Chairman Dr.Ir. M.W. Lubczynski WRS, ITC, Enschede External Examiner Dr.Ir.P.Droogers Future Water, Wageningen First Supervisor Dr.A.S.M.Gieske WRS, ITC, Enschede Second Supervisor Dr.Ing.T.H.M.Rientjes WRS, ITC, Enschede INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION ENSCHEDE, THE NETHERLANDS Disclaimer This document describes work undertaken as part of a programme of study at the International Institute for Geo-information Science and Earth Observation. All views and opinions expressed therein remain the sole responsibility of the author, and do not necessarily represent those of the institute. Dedicated to My Dearest mother with Love and Gratitude ‘Who encouraged me to knowledge’ Abstract Quantitative understanding of the process of groundwater recharge is fundamental to the sustainable management of groundwater resources since the recharge magnitude directly affects the amount of water that can be extracted from aquifers. The objective of this study was to assess the effect of meteorological forcing on groundwater recharge and water table fluctuations in the Central Veluwe (The Netherlands) which is characterized by deep phreatic groundwater and dense vegetation. Two models were used to simulate soil moisture flow in the unsaturated zone, namely: the Soil- Water-Atmosphere-Plant system (SWAP) and the Extended model for Aquifer Recharge and soil moisture Transport through the unsaturated Hardrock (EARTH). Both models make use of daily precipitation and potential evapotranspiration data to simulate soil water content, actual evapotranspiration, percolation, recharge and groundwater level fluctuations. The precipitation data was obtained by spatial interpolation of daily precipitation records from four stations within the study area and the potential evapotranspiration was calculated using Makkink and Penman-Monteith equations. Land cover classification for this study was done by considering LANDSAT ETM images, topographic maps and ground truth data as collected during field trips, while vegetation height was determined with LIDAR derived AHN data. Soil moisture, actual evapotranspiration, percolation, recharge and groundwater level fluctuations were simulated for a period of twenty years (1973-1992) and the simulated groundwater levels were compared with the observed levels. Both models appear to simulate the slow groundwater level fluctuations of the study area with high accuracy in the first 15 years of the simulation period. However, systematic deviations occurred in the last 5 years of the simulations probably as a result of increased groundwater abstractions in the area. In this research the effects of the abstractions cannot be quantified further since these are not considered in the design of the model structures. In this study, the SWAP and EARTH approaches have nearly identical results. The long-term mean annual total evapotranspiration that also includes evaporation from tree interception is found to be 515 mm while the groundwater recharge amounts to 345 mm. Groundwater recharge is only 39% of the mean annual precipitation and implies that 61% of precipitation is lost by evapotranspiration. This study also reveals that the highest recharge fluxes are from the area covered by mixed forest, light coniferous forest, heath and Molinia grass. The annual actual evapotranspiration is nearly constant throughout the modelling period. In contrast, the recharge rate shows a high temporal variability and follows a pattern similar to precipitation. The overall conclusion of this study is that groundwater level fluctuations in the Central Veluwe are affected by natural climatic variations and anthropogenic influences. Key words: Soil water flow modelling, groundwater recharge and groundwater level fluctuations i Acknowledgements First and foremost I would like to acknowledge the enabling environment created by the World Bank (International Bank for reconstruction and development) through the Joint Japan/World Bank Graduate Scholarship program (JJ/WBGSP). I am grateful to my organization for granting my leave of absence to pursue further studies and for all the help they have rendered this far. Special thanks go to my first supervisor Dr. A.S.M. Gieske for his guidance, un-reserving support, encouragement and suggestions from the commencement of this work. Not forgetting his ability of instilling hope when things appeared going in the negative direction. I greatly thank to my second supervisor Dr.ing.T.H.M.Rientjes for his support, encouragement, invaluable suggestions and comments to improve my research work. I thank the Veluwe water board province of Gelderland for providing me with groundwater abstraction data for the research work. I would like to acknowledge the Dinoshop subsurface data archive of The Netherlands for letting me to use the groundwater level data. I wish to express my appreciation to all the staff and support members of the Water Resources and Environmental Management (WREM) division for their willingness to help when ever called upon at any time and not failing to mention their love and concern to my entire being at ITC. I can not end without acknowledging the support received from my fellow classmates. I really enjoy being with you. I will remember you for making my life more pleasant in The Netherlands. My sincerest thanks to my family for always being there for me. Last but not least to all ITC Ethiopian community for providing home feeling environment. ii Table of contents Abstract .................................................................................................................................................... i Acknowledgements ................................................................................................................................. ii 1. Introduction ......................................................................................................................................1 1.1. Rationale .................................................................................................................................1 1.2. Problem statement...................................................................................................................1 1.3. Objectives, research questions and research hypothesis ........................................................2 1.3.1. General objective................................................................................................................2 1.3.2. Specific objectives..............................................................................................................2 1.3.3. Research Questions ............................................................................................................2 1.3.4. Research Hypotheses..........................................................................................................2 1.4. General methodology..............................................................................................................3 1.5. Thesis outline..........................................................................................................................5 2. Literature Review.............................................................................................................................6 2.1. Concepts of Groundwater Recharge.......................................................................................6 2.2. Factors that affect groundwater recharge ...............................................................................6 2.3. Groundwater recharge estimation techniques.........................................................................7 2.3.1. Direct measurement - Lysimeter ........................................................................................7 2.3.2. Water balance methods/soil moisture balance ...................................................................7 2.3.3. Hydrological models ..........................................................................................................7 2.3.4. Tracer methods ...................................................................................................................8 2.4. Water Dynamics in the Unsaturated Zone..............................................................................8 2.4.1. Differential equation of unsaturated flow ..........................................................................9 2.4.2. Numerical solution of soil water flow equation.................................................................9 2.5. Soil physical properties...........................................................................................................9 3. Description of the study area..........................................................................................................12 3.1. Location and topography ......................................................................................................12 3.2. Climate and