A WATER BALANCE APPROACH TO GROUNDWATER RECHARGE ESTIMATION IN MONTAGU AREA OF THE WESTERN KLEIN KAROO Xianfeng Sun A thesis submitted in fulfillment of the requirements for the degree of Master in the Faculty of Natural Sciences, Department of Earth Sciences, University of the Western Cape, Bellville, South Africa. Supervisor: Prof. Yongxin Xu Co-supervisor: Dr. Shafick Adams May 2005 ABSTRACT A WATER BALANCE APPROACH TO GROUNDWATER RECHARGE ESTIMATION IN MONTAGU AREA OF THE WESTERN KLEIN KAROO Xianfeng Sun M Sc. Thesis Department of Earth Sciences Key words: Recharge estimation, Water Balance, TMG, Montagu, Modeling The Western Klein Karoo-Montagu area is located in the mid-eastern of the Western Cape Province , South Africa. In most of the study areas within semi-arid climatic zone , groundwater plays an important role in meeting both agricultural and urban water requirements. Developments of agriculture depend on more and more groundwater supply from Table Mountain Group (TMG) sandstone aquifer system in the study area. Groundwater recharge is considered as one of the most important factors governing the sustainable yield of groundw ater exploitation. There have been few studies on the recharge estimation of the TMG aquifer system in the Montagu area. Thus accurate and reliable recharge estimation of the TMG aquifer system in the Montagu area is important. The TMG aquifer in the Monta gu area comprises approximate 4,000m thick sequence of sandstone with an outcrop area of 3,124 km 2, which is recharge area. The outcrops are characterized by mountainous topography with sparse to dense vegetation, shallow and intermittent diverse soils and mean annual rainfall of 350-450 mm/yr. Based on detail analysis and interpretation of factors influencing recharge, water balance method is used to estimate recharge rates by using readily available data (rainfall, runoff, temperatures). Other estimate methods are difficult to be applied due to the limited information available in the study area. In this study, the water balance approach based on empirical evapotranspiration and runoff model is employed to determine and analyse long-term average water recharge. The long-term average recharge is modelled as a function of the regional interaction of the site conditions: i climate, soil, geology and topography. Modelling is performed according to the outlined procedure using long-term climatic and physical data from the different rainfall period of different gauge stations. As results, actual evapotranspiration, direct runoff and recharge have been quantified. The recharge ranges vary from 0.1 mm/yr to 38.0 mm/yr in the study area, and the values less than 20.0 mm/yr are predominant. Relatively low recharge rates coincide with low precipitation in most regions. Recharge is less than 5.0 mm/yr if mean annual precipitation (MAP) is less than 400 mm/yr. The ranges of 10.0-20.0 mm/yr of recharge occur in precipitation ranging from 600 mm/yr to 1,200 mm/yr. The recharge rates exceeding 20.0 mm/yr are more related to the precipitation with 800 mm/yr or more. The low recharge rates less than 2.0 mm/yr are related to single high rainfall event in the study area. The total recharge volume of the outcrop of the TMG in the study area is approximately 54.2× 106 m3/yr . Approximately 29.3% of the stream flow may be contributed by recharge in terms of baseflow. The recharge in the study area increases with increasing precipitation, but recharge percentage is non-linear relationship with the precipitation. Separate high rainfall events mainly contribute recharge if annual precipitation is extremely low in the study area. Spatial distribution of recharge is associated with the variations in precipitation, geological and geomorphologic settings in the study area. The method used yields a point estimate and then ext rapolate s to the whole study area. The ranges of recharge may be exaggerated or underestimated due to the finite numbe r of the rainfall stations in the outcrop of the TMG of the study area. After comparison to other recharge estimates from early studies in the area, the estimates are considered as reasonable and reliable. The feasibility of the water balance approach in semi-arid area is confirmed as well. The estimates based on the water balance model should be crosschecked before they are applied for management of groundwater resources. ii DECLARATION I declare the A WATER BALANCE APPROACH TO GROUNDWATER RECHARGE ESTIMATION IN MONTAGU AREA OF THE WESTERN KLEIN KAROO is my own work, that it has been submitted for any degree or examination in any other university, and that all the sources I have used or quoted have been indicated and acknowledge by complete references. Full name: Xianfeng Sun Date: 15th May 2005 Signed:???????????????????????????? ACKNOWLEDGEMENTS I would like to thank the following people for their assistance and friendship: Prof. Yongxin Xu, my supervisor, introduced me to the research aspects in Hydrogeology and I have benefited greatly from him. Shafick Adams, my co-supervisor, has also critically read this thesis and his constructive comments and suggestions are greatfully appreciated. Dr. Rian Titus (The Council for Geosciences in Pretoria) for his excellent assistance and full support. Nebo Jovanovic, Chief of Department of Earth Sciences; Browen Honigwachs, Secretary of Department of Earth Sciences and Shaun Lawrence for the logistical support during research in South Africa. Caroline Barnard for her logistical support during the project. Many colleagues, friends have inspired me as well as given me moral support all these years. Special encouragement came from Lixiang Lin, Haili Jia, Andre oosthuizen and Vuyolwethu Sigonyela. Please, excuse me if your name is not mentioned here, because it is by no means deliberate, f or my pen has also run out of ink! Finally, I would like to thank my husband (Yong Wu) and my son (Zhenghao), my parents and family for their great support and encouragement, patience, understanding and love with all my heart. iii Abbreviations and Notations Abbreviations Abbreviations Description CAGE Cape Artesian Groundwater Exploration CFB Cape Fold Belt CN Curve number CNC Cape Nature Conservation CGS Council for geosciences CRD Cumulate Rainfall Departure DWAF Department of Water Affairs and Forestry Extended model for Aquifer Recharge and Moisture Transport through EARTH Unsaturated Hardrock GM Groundwater Modelling MAP Mean Annual Precipitation PRMS Precipitation-Runoff Modelling System SCS Soil Conservation Service SAWS South Africa Weather Service SHE System Hydrology European Stdev Standard deviation SVF Saturated Volume Fluctuation TMG Table Mountain Group USDA US Department of Agriculture WR90 Water Research Commission Flow data in 1990 (published in 1994) WRC Water Research Commission WTF Water Table Fluctuation Sn Nardouw Sub-group Oc Cedarberg Formation Ope Peninsular Formation Notations Notation Description Dimension or unit F latitude (radians , positive for north negative for south) Degree ? latent heat of vaporization MJkg-1 ? density of water ML-3 ? ? (t) change in volumetric water content in soils L or L3 A recharge area L2 Cc coverage of vegetation Dimensionless Cs soil factor Dimensionless iv Cv vegetation factor Dimensionless EC electrical conductivity mS/m E (t) evapotranspiration L or L3 3 Et (t) evapotranspiration L or L 3 Etr(t) evapotranspiration L or L -2 -1 GSC the solar constant with a value of 118.1 MJm d ? hi water level change during month i L H hydraulic head L I inflow L3 i, j principal coordinate directions L J the number of days since January 1 of the current year Dimensionless K hydraulic conductivity tensor L/F Kc (t) vegetation coefficient Dimensionless Lf lithological factor Dimensionless O outflow L or L3 Pi rainfall for month i L/T Pre precipitation L threshold value representing aquifer boundary P L t conditions P (t) precipitation L 3 Qa abstraction during period L /T 3 Qout natural outflow L Qp fluid sources or sinks per unit volume l/T Q (t) runoff L or L3 3 Qv annual recharge volume M -2 -1 RA extraterrestrial radiation MJm d Re recharge L or L3 RE annual recharge rate mm/yr 3 Rf variable recharge rate L or L R (t) recharge L or L3 S aquifer storativity Dimensionless 2 Sc catchment area [L ] Sf slope factor Dimensionless Ss specific storage l/L Sy specific yield l/L T time T T average monthly temperature °C inference between average monthly maximum and T °C D minimum temperatures ? t time increment T ? v change in saturated volume of the aquifer L3 x a space coordinate L v TABLE OF CONTENTS ABSTRACT......................................................................................................................i ACKNOWLEDGEMENTS ..........................................................................................iii ABBREVIATIONS AND NOTATIONS.....................................................................iv TAB LE OF CONTENTS ..............................................................................................vi LIST OF FIGURES........................................................................................................ x LITS OF TABLES........................................................................................................xii LIST OF APPENDICES.............................................................................................xiii CHAPTER 1 INTRODUCTION................................................................................... 1 1.1 BACKGROUND.......................................................................................................