Downloaded from orbit.dtu.dk on: Sep 30, 2021 Groundwater recharge and capillary rise in a clayey till catchment Schrøder, Thomas Morville Publication date: 2003 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Schrøder, T. M. (2003). Groundwater recharge and capillary rise in a clayey till catchment. Environment & Resources DTU. Technical University of Denmark. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. 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Groundwater Recharge and Capillary Rise in a Clayey Till Catchment Thomas Morville Schrøder Groundwater Recharge and Capillary Rise in a Clayey Till Catchment Ph.D. Thesis Thomas Morville Schrøder 2003 Environment & Resources DTU Technical University of Denmark Copenhagen, Denmark Groundwater Recharge and Capillary Rise in a Clayey Till Catchment Cover: Birte Brejl Printed by: DTU tryk Environmental & Resources DTU ISBN 87-89220-73-0 The thesis will be available as a downloadable pdf-file from the department's homepage on: www.er.dtu.dk Environment & Resources DTU Library Bygningstorvet, Building 115, Technical University of Denmark DK-2800 Kgs. Lyngby Phone: Direct (+45) 45 25 16 10 (+45) 45 25 16 00 Fax: (+45) 45 93 28 50 E-mail: [email protected] iii Preface The present thesis is the outcome of my research in 1997-2001 towards the Ph.D. degree at the Technical University of Denmark, Environment & Resources DTU. Funding has been granted by the University and supplemented for the experimental part by the Geological Survey of Denmark and Greenland. My advisor and mentor in the final phase was Professor Dan Rosbjerg, who is thanked for fruitful discussions and support to what I believe can be a contribution to hydrology. I wish to thank Professor Eggert Hansen for emphasizing the pre-drainage regime, Professor Flemming Bo Pedersen for weir design guidelines, and Associate Professor Hans Lund for his model of global radiation. My former colleagues at the Danish Geotechnical Institute tested wells with a long screen, while Ole Larsen at University of Copenhagen provided equipment for electrical sounding and borehole logging. Per Rasmussen, Per Nyegaard, and Frants von Platen at the Survey helped on the first applied pressure gauges and initiated the geological and topographical modeling. Bent Jørgensen, Danish Meteorological Institute, gave me access to archived atmospheric data. Jesper Skovdal Christiansen, formerly at DHI–Water & Environment, assisted me on the MIKESHE code. I appreciate that Kurt Jensen and Ivan Boserup accepted the heavy disturbance of their property and took care of my well being, especially when a railway line cut through cables laid out, farm machines torpedoed wells, water voles undermined one weir, and car batteries flattened on dark winter eves. I am indebted to the technicians Jan Larsson and deceased Klaus Fæster Hansen for fabrication of equipment. Klaus was the key person for implementation of the experimental setup, and Per Jensen from the Survey became an immense relief on operating the fragile monitoring network. I must not forget my assistants, the former students Niels–Kristian Terkildsen, Tore Stamp Nielsen, Jacob Gudbjerg, Inge Pedersen, and Haraldur Hannesson who assisted on installation and campaigns of fieldwork. Thomas Morville Schrøder 27 December 2002 iv Abstract Quantification of recharge rates and patterns is essential for sustainable groundwater abstraction. On northern mid- to high latitudes, the common weather is humid and a common geological unit is clay till. Mapping of the tortuous flow through clay till has an unrealistically high cost and models integrating the processes in the surface-groundwater contact have only been developed over the past decade. One traditionally disregarded process is the capillary rise from groundwater in response to surface drying. Diversion via the artificial subsurface drainage can be presented by a linear reservoir, and this contribution dominates the primary calibration target for hydrological models: streamflow. The discharge from deep aquifers to streams is approximately linear and can accordingly be modeled by a number of linear reservoirs. The tempting conclusion is that groundwater recharge is equally linear, so the capillary rise should not be present at all. Moreover, many have speculated whether removal of the artificial subsurface drainage would enhance the aquifer replenishment and thereby the level of sustainable groundwater abstraction. A literature survey of methods for determination of groundwater recharge clarified that usually only the percolation at some level above the watertable is considered, and the subsequent capillary rise is neglected. Monitoring the very watertable by a number of wells captures the lag phase of recharge, its spatial distribution, and allows for model interpretation of also the capillary rise. Though the number of wells was limited to eleven, they covered two- third of the 16 km2 study catchment in terms of terrain level. Adopting the variable source concept, they sufficed for a monthly distinction between the dry and wet state in six terrain intervals. From the wet stat area, the uniformly distributed subsurface drainage diverts recharge to the stream. A sequential parameterization of existing modeling tools was unfolded with the assistance of electrical sounding, a minor equilibrium model for determination of the proper boundary condition between the unsaturated zone and groundwater, and tools for mathematical inversion. This study finds that watertable monitoring render calibration of a groundwater model’s surface contact superfluous apart from drain time; vertical conductivity of weathered soil, and snow storage. We have found a first guess of the drain time in 1D by streamflow separation, and calibrated in 3D by MIKE SHE modeling. Preferential conduits are merged into the soil by 2D modeling, and snow storage parameterised by comparison of an automatic and a manual precipitation gauge. The capillary rise accounts for an uphill increase in the long-term net recharge from 200 to 260 mm/y, implying that local topography is mirrored into the aquifer circulation faster than found by regional modeling without this feedback process. Perhaps the capillary rise ties annual evapotranspiration to the North Atlantic Oscillation better than thought of for Denmark, implying that hydrological modeling of clayey catchments can establish area integrated values, which may contribute to the debate on the oscillation’s spatial coverage. Moreover, subsurface drainage reduces evaporation from waterlogged areas by an increase in streamflow that is one magnitude larger than the 2 mm/y decrease in aquifer replenishment. v Sammendrag Bestemmelse af grundvandsdannelsens hastighed og arealmæssige fordeling er essentiel for den bæredygtige oppumpning af grundvand. Det fremherskende vejr på den nordlige halvkugles midt- til høje breddegrader har et overskud af nedbør i forhold til fordampning. Samtidigt er moræneler en almindelig geologisk forekomst, også i det østlige Danmark. Men kortlægning af det stærkt uens moræneler er urealistisk dyr, og hydrologiske modeller som inkluderer processerne mellem jordoverfladen og grundvandet er kun blevet færdige i løbet af halvfemserne. En normalt udeladt proces er grundvandets tilbagekobling til fordampning, kaldet den kapillære stigning. Den almindelige underjordiske afledning via drænrør kan beskrives som et lineært magasin, og dette bidrag dominerer det primære mål for model kalibrering: vandløbsafstrømningen. Grundvandets udstrømning til vandløb fra dybe magasiner er tilstrækkelig lineær til at den kan beskrives med det tilsvarende antal lineære magasiner. Man kunne forledes til at tro, at grundvandsdannelsen også er lineær og der derfor ikke er nogen kapillær stigning. Med henblik på at øge indstrømningen til de pumpede magasiner under moræneleret har det været foreslået at fjerne noget af dræn-lægningen. Et litteraturstudium af metoder til bestemmelse af grundvandsdannelsen gjorde det klart, at som regel undersøges kun nedsivningen i et bestemt niveau, mens kapillær stigning ikke opgøres særskilt. Derimod vil måling af det frie vandspejl i et antal boringer vise forsinkelsen mellem infiltration og grundvandsdannelse, dens arealmæssige fordeling, og muliggøre model tolkning af den kapillære stignings størrelse. Elleve boringer dækkede to tredjedele af tærren niveauet i det undersøgte opland på 16 km2. Et koncept for stormafstrømning til vandløb gav mulighed for en månedlig skelnen i seks højdeintervaller mellem tilhør til den tørre kontra den våde del af oplandet, hvor grundvandsdannelsen gav anledning til drænudstrømning. En sekventiel parametrisering af eksisterende modeller er udfoldet med assistance af en kortlægning af jordens elektriske ledningsevne, en ligevægtsmodel til at bestemme randbetingelsen