VIDENSKABELIGE ARTIKLER: Henriksen HJ, Troldborg L, Nyegaard P, Sonnenborg TO, Refsgaard JC, Madsen B 2003:
Methodology for construction, calibration and validation of a national hydrological model for
Denmark
JOURNAL OF HYDROLOGY 280 (1-4): 52-71 SEP 10 2003
Abstract:
An integrated groundwater/surface water hydrological model with a I km(2) grid has been constructed for Denmark covering
43,000 km(2). The model is composed of a relatively simple root zone component for estimating the net precipitation, a comprehensive three-dimensional groundwater component for estimating recharge to and hydraulic heads
in different geological layers, and a river component for streamflow routing and calculating stream-aquifer interaction. The model was constructed on the basis of the MIKE SHE code and by utilising comprehensive
national databases on geology, soil, topography, river systems, climate and hydrology. The present paper describes the modelling process for the 7330 km(2) island of Sjaelland with emphasis on the problems experienced
in combining the classical paradigms of groundwater modelling, such as inverse modelling of steady-state conditions, and catchment modelling, focussing on dynamic conditions and discharge simulation. Three model
versions with different assumptions on input data and parameter values were required until the performance of the final, according to pre-defined accuracy criteria, model was evaluated as being satisfactory. The paper
highlights the methodological issues related to establishment of performance criteria, parameterisation and assessment of parameter values from field data, calibration and validation test schemes. Most of the parameter
values were assessed directly from field data, while about 10 'free' parameters were subject to calibration using a combination of inverse steady-state groundwater modelling and manual trial-and-effor dynamic
groundwater/surface water modelling. Emphasising the importance of tests against independent data, the validation schemes included combinations of split-sample tests (another period) and proxy-basin tests (another
area). Sonnenborg TO, Christensen BSB, Nyegaard P, Henriksen HJ, Refsgaard JC 2003:
JOURNAL OF HYDROLOGY 273 (1-4): 188-204 MAR 25 2003
Abstract:
Use of automatic calibration of large
physically based hydrological models running in a transient mode is severely constrained by the long simulation time. Here, the possibility of using a steady-state model, with fast model execution, as the basis for
automatic calibration of the involved parameters is examined. It is tested if the parameters estimated with the steady-state model can be used for the subsequent transient modeling of the same hydrological system. The
problem is analysed for the 5900 km(2) South Jutland area, being one of the 11 model areas of the National Water Resource Model of Denmark. The area is modelled using the NUKE SHE code while UCODE is used for
optimization. Hydraulic head data and stream discharge data are available for calibration. It is examined how the estimated parameters are affected by the choice of (a) recharge input to the steady-state model, and (b)
target data used for calibration. The reliability of the parameters is evaluated and the resulting ability of the optimized models to simulate the transient response of the system is assessed. The results show that the
estimated parameters are highly sensitive to the way that the steady-state model is conceptualized. The study suggests how the steady-state model should be conceptualized in order to obtain reliable parameter estimates
that produce acceptable transient model results. (C) 2003 Elsevier Science B.V. All rights reserved. Harrar WG, Sonnenborg TO, Henriksen HJ 2003:
HYDROGEOLOGY JOURNAL 11 (5): 536-548 OCT 2003
Abstract:
Six regional-scale flow models are compared to gain insight into how different representations of hydraulic-conductivity distributions affect model calibration and predictions. Deterministic
geological models were used to define hydraulic-conductivity distributions in two steady-state flow models that were calibrated to heads and baseflow estimates using inverse techniques. Optimized hydraulic-conductivity
estimates from the two models were used to calculate layer and model mean hydraulic-conductivity values. Despite differences in the two geological models, inverse calibration produced mean hydraulic-conductivity values
for the entire model domain that are quite similar. The layer and model mean hydraulic-conductivity values were used to generate four additional flow models and forward runs were performed. All of the models adequately
simulate the observed heads and total baseflow. The six flow models were used to predict the steady-state impact of a proposed well field, and the flow solutions were used in simulating particle tracking and solute
transport. Results of the predictive simulations show that, for this example, simple models of heterogeneity produce capture zones similar to more complex models, but with very different travel times and breakthroughs.
Inverse modeling combined with different geological models can provide a measure of capture zone and breakthrough reliability.
Henriksen, H.J., C. Knudby, P. Nyegaard, P. Rasmussen, M. Hansen and P.R. Jakobsen (1997): National water ressource modelling in Denmark: Application of national groundwater model for the Isle of Funen.
Operational Water Management. Editors: Refsgaard & Karalis. 1997 Balkema, Rotterdam, ISBN 90 5410 897 5. pp. 277-284.
Abstract:
Development and application of a national groundwater resources
model for Denmark based on the MIKE SHE modelling system is to be implemented during the four year period: 1996-99. The project has three main components: 1) Development of a National Water Resource Model (NWRM), 2)
Development of additional MIKE SHE code and 3) Reorganisation of the national groundwater level monitoring network. The NWRM will simulate the major flow processes of the entire land phase of the hydrological cycle with
emphasis on the groundwater flow system including interaction with surface water. A grid size of 1000 m and a vertical discretization into 7 calculation layers was chosen. Topography, land use, soil and river network
data was transferred into the model assisted by GIS features. The paper presents development and validation of the conceptual model, model setup, calibration and preliminary results of the first step of the creation of
the NWRM – the implementation of MIKE SHE for the Isle of Funen. Henriksen, H.J. (1998): Climate change and available groundwater resources – the National Water Resource Model for Denmark.
Proceedings of The Second International Conference on Climate and Water. Edited by Risto Lemmelä and Nea Helenius. Espoo, Finland, 17-20 August 1998. Volume 3, pp. 1459-1470.
Abstract:
In Denmark
the viable exploitable groundwater resource is greater than the total quantity of abstracted water under average climate conditions. In some parts of the country there is a water shortage due to significant regional
variations in net precipitation. Influence of long term climate variations was not incorporated in the last comprehensive estimate of the national water resources made in 1992. However, the still more widespread
contamination of groundwater resources with pesticides and degradation products has highlighted the need for an assessment including both a proper quantification of contaminated groundwater resources and incorporation
of long term climate variations. The Geological Survey of Denmark and Greenland therefore took the initiative to develop and apply a National Water Resources Model (NWRM) for the whole country. Evaluation of effects of
climate variability on groundwater recharge and exploitable groundwater resources require a "holistic" approach, in order to describe the major flow processes of the entire land phase of the hydrological
cycle, including interaction between groundwater and surface water. Furthermore, traditional numerical groundwater models commonly used to evaluate the response of groundwater systems to groundwater exploitation may not
be appropriate for analysing climate change impacts due to the normally simplified representation of groundwater recharge processes in such groundwater models. In stead, a hydrological model is needed which include the
time dependent variation and regional distribution as well as an estimation of both the possible future water consumption and climate change scenarios. In this paper focus is directed toward the NWRM model set up
("DK-model") and assumptions regarding calculation of net precipitation and groundwater recharge input for the model, which is based on daily precipitation and evaporation for a 40x40 km grid. T. V. Jacobsen og M. Hansen (1998): Geological modelling and editing in GIS environment. Hydroinformatics'98, Babovic Larsen (eds). Balkema, Rotterdam, ISBN 90 5410 983 1. pp. 525- 530 (volume 1)
.
Abstract:
Geological and hydrogeological data constitutes the basis for assessment of groundwater resources and pollution risk. The amount of available data and the heterogenity and complexity of
geological formations makes it difficult to establish a general view of the main geological features and subsequently derive a plausible geological model. This paper presents a graphical tool (GeoEditor) which provides
facilities to develop and test geological models based on borehole data and geophysical data. The GeoEditor provides a close link between basic geological and geophysical data, conceptual interpretation and model
representation. The GeoEditor is based on an inherent methodology, which leads the user through a systematic definition of a geological model. Based on experience of geologists, hydrogeologists and modellers two
alternative approaches based on specifying either overall geological structures or zonation of characteristic aquifer properties have been implemented.
Madsen, B., H.J. Henriksen, and C. Knudby (1998): The National Water Ressource Model – Developed for the assessment of the distribution and protection of the Danish groundwater resources.
IAH/AIH joint conference: Gambling with Groundwater – Physical, Chemical, and Biological Aspects of Aquifer – Stream Relations. Ed. Brahana et al., pp. 219-226.
Abstract:
Previously, it was
thought that the groundwater supplies in Denmark were well protected from pollution due to the depth of the groundwater reservoirs and the thickness of overlying clays. However, the weaknesses of this philosophy have
during recent years been heavily exposed, primarily caused by the detection of an increasing amount of contaminants like pesticides etc. being the result of systematic national monitoring. In present-day Denmark,
groundwater contamination has reached a proportion that constitutes a threat against the traditional, decentralised water supply based upon simple water treatment methods. The limitations that the increasing contaminant
load sets on the exploitation of the groundwater resources put at the same time additional focus on the size and distribution of the resources, and especially on the time dependent development controlled by change in
climate. The complexity of the problems has elucidated the need for a more advanced, flexible, and exact tools to be used on national level for water resources assessment than the simple and rough calculations performed
earlier. The Survey therefore took the initiative to develop and apply a water resources model for the whole country (45,000 km2) during the four year period 1996-99. The National Water Resources Model is based on all
available information on groundwater, land use, and recharge/discharge conditions, and the extensive amount of information is processed by the physically based MIKE SHE hydrological modelling system using a grid size of
1 km2 and a vertical discretisation into 7-9 layers. The major flow processes of the entire land phase of the hydrological system, including interaction between groundwater and surface water, can be simulated to assess
the distribution and protection of the groundwater on a regional and national scale. Dahl, M., W.G. Harrar, H.J. Henriksen, and C. Knudby (1998): Integrated hydrological modelling of freshwater
resources in Denmark – Distribution of aquifer – river exchange parameters.
IAH/AIH joint conference: Gambling with Groundwater – Physical, Chemical, and Biological Aspects of Aquifer – Stream Relations. Ed. Brahana et al., pp. 607-616.
Abstract:
Numerical models are
commonly used to evaluate the response of groundwater – surface water systems to groundwater exploitation. The objective of this paper is to examine the importance of distributing the stream bed leakage coefficients in
simulating the spatial distribution of baseflow on the regional scale. On the Isle of Funen Quaternary deposits consists of up to 150 meters thick primarily clayey till with lenses and alternating more or less regional
layers of outwash sand. A conceptual model for the baseflow discharge was developed based on 600 synchronous discharge measurements made during extended dry periods. The stream reaches between adjacent measurements were
combined into 50 stream areas comprising similar magnitudes of net baseflow gain per unit stream length. The stream areas were classified as low (< 1 l/s/km) and high (> 7 l/s/km) baseflow areas. The low baseflow
areas are generally located in the till uplands, whereas the high baseflow areas are located in the valley bottoms directly overlying a regional aquifer. Medium baseflow areas are underlain by till and sand. The MIKE
SHE model was used to evaluate the response of baseflow simulation to three different representations of the stream bed leakage coefficients, encompassing one of constant value (1e-7s-1) and two in which the leakage
coefficients were varied according to the conceptual model. The values of leakage coefficients, providing the best simulated baseflow results, were for low and high baseflow areas estimated from the average thickness
and vertical hydraulic conductivity of the till and sand layer containing the streams, respectively. For medium baseflow areas a value between the two others was applied. The assigned values were 2e-10s-1, 2e-8s-1 and
2e-6s-1 for low (till), medium (till and sand) and high (sand) baseflow areas, respectively. The main conclusion drawn from this study is, that the spatial distribution of water exchange between groundwater and surface
water systems obtained from numerical models can be improved by distributing leakage coefficients of the stream bed based on geology of the layers in which the streams are imbedded. To achieve a good tool for
groundwater management purposes on a regional scale it requires, that the model is calibrated to groundwater heads and distributed baseflow in the streams. It is also concluded, that it was equally important to
represent in the model the exchange of water between the groundwater and surface water systems for both low, medium and high baseflow areas on Funen, as they each contribute approximately one third of the cumulated
baseflow from the island. Rasmussen, P. and Henriksen, H.J. (1998): MODFLOW versus MIKE SHE: Significant differences in recharge areas to selected pumping wells?
MODFLOW'98 Conference. Golden, Colorado USA. October 4-8, 1998.
Abstract:
Implementation of a multi-layer heterogenous
hydrogeological conceptual model into different groundwater codes may imply different modifications of the geological layers depending on the capability and guidance of pre-processors as well as the solvers ability to
handle heterogenous geology. The two codes MODFLOW and MIKE SHE have been compared for the delineation of recharge areas to selected pumping wells in the same 660 km2
large Nyborg area in Denmark. The MIKE SHE system is a deterministic fully distributed and physically based finite difference modelling system for describing the major flow processes of the entire land phase of the hydrological cycle. The Visual MODFLOW package was used for the MODFLOW / MODPATH simulations. Simulated flow and particle tracking was compared for the two applied models. The conceptual hydrogeological model consists of 9 geological layers (melt water sand, clayey till and limestone) each of varying thickness throughout the model area. The model parameters, sinks, sources and calibration criteria's were practically identical for the two calibrated models. The simulations have shown significant differences in the location of recharge areas. Which may be crucial when it comes to delineation of groundwater protection areas, stressing the importance and sensitivity of geological interpretations as well as the need for improved pre-processing tools to facilitate importing and analysing different conceptual hydrogeological models.
Henriksen, H.J. and Stockmarr, J. (1999, submitted): Groundwater Resources in Denmark. Modelling and monitoring. IWSA world congress, Buenos Aires, September 1999.
Abstract:
Drinking water in Denmark is produced exclusively from groundwater. The exploitable resource is greater than the present abstraction but increasing contamination, climate variation and changes in land use may
adversely affect the future groundwater resources. Changes in water quality are systematically being registrated in the Danish groundwater monitoring programme, described in this paper. The groundwater quality in
Denmark is generally good, but locally shallow aquifers are contaminated by nitrate, pesticides, heavy metals and in urban areas by organic micro-pollutants. The National Water Resource Model is under development for
use as a tool for the assessment of the size and distribution of the future exploitable groundwater resources. |
