THE HBV STORY IN SWEDEN

Sten Bergström
Swedish Meteorological and Hydrological Institute
Norrköping, Sweden

Introduction

The story of the Swedish HBV model dates back to 1972 when the first successful run was made at the water balance section of the hydrological bureau of SMHI (Hydrologiska Byråns Vattenbalansavdelning, HBV; Bergström and Forsman, 1973). The aim was to come up with a hydrological model for operational use according to the following main principles (Bergström, 1991):
The model must be based on a sound scientific foundation
It must be possible to meet its data demands in most areas
Its complexity must be justified by its performance
It must be properly validated
The user must be able to understand the model

In 1975 the model was first tested for operational forecasts in the upper parts of River Ångermanälven. The same year the model was introduced in Norway. Since then the number of institutions involved in the development has increased and the scope of applications has widened. Today there exist a variety of model versions, with origins from different institutions, and applications have been made in more than 40 countries.

One important factor for the widespread use of the HBV model in Sweden was the development of personal computers. A strategic decision was taken to develop the Windows based Integrated Hydrological Modelling System, IHMS. This proved to be the right way and paved the way for more de-centralised use of the model. It became more and more evident that a model, no matter how good it is, is of little value unless it comes with a user-friendly interface. The ease of handling has improved further by the introduction of more reliable routines for automatic model calibration (Lindström, 1997).

Swedish HBV Models

The HBV model started as a very simple lumped hydrological model and has gradually been developed into a distributed model. With the latest release, HBV-96 (Lindström et al., 1997), full distribution into subbasins and statistical distribution of some properties within these have become the basic principle. It is therefore fair to say that the HBV model now is a distributed hydrological model.

The HBV-96 has also a new response function, which requires four instead of five parameters, and thus is less susceptible to overparameterisation (too many free coefficients to calibrate). This new routine gives a slightly better representation of peak flows.

Proper soil moisture accounting is a key to successful hydrological modelling. The HBV model was one of the firsts, if not the first, model to adopt a variability parameter in the soil moisture procedure. This was introduced in 1972 and there has not been any reason to change the concept since then. The technique has proved to be very efficient and has been copied into other hydrological models as well. Very recently it was also tested in the response function of the HBV model to cope with the problems with variable dynamics of winter and summer peaks.

In addition to different models, mainly developed for runoff modelling, a new branch of models appeared for water quality related research. In an attempt to modernise the name the PULSE model was introduced. It was, however, soon realised that this new name of a model, which in practise was a modified HBV, model created a lot of confusion. The name PULSE has therefore been abandoned in favour for the more established HBV.

Scope of Applications

Hydrological Forecasting

The HBV model was initially intended for runoff simulation and hydrological forecasting. The number of applications grew to cover most rivers in Sweden where flood forecasting and reservoir operation is an issue. Applications abroad became more frequent and a joint modelling project was carried out with countries in Central America among others (Häggström et al., 1990). Of great significance for the confidence in the HBV model was also the intercomparison of operational models for snowmelt runoff organised by WMO in the 1980s (WMO, 1986).

Today hydrological forecasting is probably still the most frequent type of application of the HBV model, both in Sweden and elsewhere. Research is still going on, in particular as concerns supplementary input from remote sensing and meteorological analysis techniques. More handy systems for real-time updating are also being developed.

Free Model Simulations

The traditional way of using a conceptual hydrological model is by first calibrating it, to find optimum values of its empirical parameters (coefficients). It was long felt that the need for calibration was an insurmountable limitation of the HBV model. Along with increasing experience, however, it was shown that the span of optimum values was not very dramatic. The idea of using the model without calibration (free simulations) grew from a need to relate long records of hydrochemistry to hydrological conditions in rivers, where runoff records simply do not exist. From a scientific point of view this application is still questioned, but not from a practical point of view. Free simulations are definitely better than not having any information at all.

The prerequisite for free model simulations is that that there is little variation in model coefficient or that we can find relationships between these and catchment characteristics. This was studied in the early 1990s with some success (Johansson, 1994). Today the HBV model is run with generalised coefficients in some 400 basins in Sweden.

Water Balance Mapping

Following surprising floods in southern Sweden in 1980 a synoptic hydrological map was developed as an automatic tool to give hydrologists a quick view of the hydrological situation (Bergström and Sundqvist, 1983). The mapping technique, based on hydrological modelling, developed further and it was decided to use it for the production of the volume of the National Atlas of Sweden dealing with climate, lakes and rivers. A gridded HBV model was developed with a resolution of 25 by 25 km and used for the production of the runoff map of Sweden launched in 1995 (SNA, 1995). Again the principle of generalised model coefficients was used.

At present a modified HBV soil model is used for real time mapping and assessment of the risks for forest fires in Sweden (Gardelin, 1996).

Design Floods

Analysis of Land Use Impacts

Groundwater and Soil Moisture

It was with some hesitation that we decided to try the HBV model for simulations of groundwater recharge. Nevertheless it could be shown that the storages of the response function of the HBV model could be used to describe at least the response of the unconfined aquifers of a catchment (Bergström and Sandberg, 1983). The model could not be used for the three dimensional flow of groundwater, but gave realistic recharge values.

The application to ground water forced us to some re-interpretation of the model structure and gave very useful insights into the possibilities and limitations of the model. It became the starting point for further water quality oriented work. The same can be said about attempts to simulate the concentrations of the stable natural isotope oxygen-18. It seemed that we had reached the limit of the simple structure, when we wanted to model the fate of one molecule of water on its way from the top of the soil to the watercourse (Lindström and Rodhe, 1986). The introduction of a fair amount of extra water in the soil and ground helped overcome the problem. Proper retention times in various geochemical environments are of importance in detailed hydrochemical modelling. The modified model was thus used to provide input to geochemical models for studies of risks for groundwater acidification.

Parallel to the detailed studies of retention times more direct soil moisture simulations with the HBV model, or modifications thereof, were carried out by Andersson (1989A; 1989B)

Water Quality

It has become more and more evident that proper hydrological modelling is a prerequisite for water quality modelling. The latter type of modelling is also more difficult. Several attempts to expand the HBV model into water quality have been made over the years. The conclusion is that it can be made if the level of ambition is realistic. This means that simulations of climate induced variabilities, based on stationary conditions, are possible while it remains to be seen whether models can be developed for changing environmental pressures.

The climate induced variability of pH and alkalinity in forested rivers was modelled by the PULSE model in the 1980s (Bergström et al., 1985). Later focus has shifted to eutrophication and in the late 1990s the transport of nitrogen to the coastal waters from southern Sweden was modelled by some 4000 HBV models equipped with subroutines for nitrogen retention (HBV-N; Arheimer and Brandt, 1998).

Climate Change Studies

Climate change due to human activities is one of the greatest scientific issues today. In spite of all uncertainties in regional climate outlooks, hydrological models are in use for water resources impact studies since the early 1990s. The HBV model is no exception (Vehvilainen and Lohvansuu, 1991). A Nordic study on climate change and hydropower production was finalised in 1998 (Saelthun et al., 1998). The work was based on regional climate scenarios and a modified HBV model. This work will continue within the Swedish programme for regional climate modelling, SWECLIM, where the Rossby Centre is providing climate scenarios.

Proper modelling of evapotranspiration seems to be a general problem when using hydrological models for water resources scenarios. The issue is how the evapotranspiration routine shall be modified to realistically describe climate change conditions.

The climate issue has brought meteorologists and hydrologists closer together. A need for harmonisation of soil parameterisations has been identified as the energy and the water cycle will have to be solved simultaneously in the models. This debate will have strong impacts on future model development in meteorology as well as in hydrology. Of special interest is the scale problem. To bridge the scale gap between hydrological models and climate models the HBV model was applied to the land area of the entire catchment of the Baltic Sea (Graham, 1999). Regarded as a river basin it is the largest in Europe, some 1 700 000 km² excluding the Baltic Sea itself. The model has then been used for a review of the process descriptions in respective models. Already now some critical needs for model improvements have been identified (Graham et al., 1998).

The Future

It is realistic to believe that the HBV model will remain a standard hydrological tool for many years to come. There is a great need for simple techniques that link meteorology to hydrology and where human impacts can be distinguished from the effects of natural climate variability.Of special promise is the joint interest among climate modellers and hydrologists in better surface parameterisations (snow, soil and evapotranspiration) which is a key subject for the BALTEX research programme and other continental scale experiments within GEWEX. It seems that the scale problem, at least partly, can be overcome by a conceptual model of this type (Bergström and Graham, 1999)

Although the hydrological modelling technique is now well established in the Nordic countries, there is still some room for further development of the hydrological models. This is particularly the case for peak flow simulations. The greatest potential, however, lies in better representation of the input to the models. Work is in progress on introducing remote sensing as well as more advanced meteorological interpolation techniques to achieve this.

The hydrological water quality models in use are still relatively pre-mature. Better models, that link atmospheric deposition and hydrology to effects on the ecosystem, are urgently needed. For regional problems, like the eutrophication of the Baltic Sea, these models have to address the continental scale.

One common problem in all modelling is the risk for compensating errors. Models might perform well for the wrong reason. This might block further development, as improvement in one process description falsely may be interpreted as a failure, if we do not get rid of a compensating error simultaneously. To cope with this problem we have to pay more attention to internal process validation in our models in the future

The use of hydrological models requires up-to-date user-friendly computer systems and effective data collection and processing procedures. This has to be worked out in close co-operation with day-to-day users of the systems. For the continental scale applications proper data exchange between nations has to be secured.

References

The history of the HBV model can be followed in the scientific literature and in numerous technical reports and conference proceedings. The author has identified more than 400 references, where HBV model results or modelling systems are presented or used. The following list of references covers some of the key titles related to the above presentation.

Andersson, L. (1989A) Soil Moisture Deficits in South-Central Sweden, I - Seasonal and regional distributions. Nordic Hydrology, Vol. 20.

Andersson, L. (1989B) Soil Moisture Deficits in South-Central Sweden, II -Trends and fluctuations. Nordic Hydrology, Vol. 20.

Arheimer, B. and Brandt, M. (1998) Modelling nitrogen transport and retention in the catchments of southern Sweden. Ambio, Vol. 27 No.6.

Bergström, S. (1991) Principles and confidence in hydrological modelling. Nordic Hydrology, Vol. 22, 123 - 136.

Bergström, S. (1995) The HBV model. Contribution to: Computer Models of Watershed Hydrology, Water Resources Publications.

Bergström, S., Carlsson, B., Sandberg, G., and Maxe, L. (1985) Integrated modelling of runoff, alkalinity and pH on a daily basis. Nordic Hydrology, Vol. 16, No. 2.

Bergström, S., and Forsman, A. (1973) Development of a conceptual deterministic rainfall-runoff model. Nordic Hydrology, Vol. 4, No. 3.

Bergström, S. and Graham, L.P. (1997) On the scale problem in soil moisture modelling.Journal of Hydrology, in press.

Bergström, S., Harlin, J., & Lindström, G. (1992). Spillway design floods in Sweden. I: New guidelines. Hydrological Sciences Journal, 37, 5, 505 - 519.

Bergström, S., and Sandberg, G. (1983) Simulation of groundwater response by conceptual models - Three case studies. Nordic Hydrology, Vol. 14, No. 2.

Bergström, S., and Sundqvist, B. (1983) Synoptic water balance mapping in Sweden. Contribution to the IAHS Workshop on New Approaches in Water Balance Computa-tions, Hamburg 1983, IAHS Publ. No. 148.

Brandt, M., Bergström, S., Gardelin, M. (1988). Modelling the effects of clearcutting on runoff - Examples from Central Sweden. Ambio, 17, 5: 307 - 313.

Gardelin, M. (1996). Brandriskprognoser med hjälp av en hydrologisk modell. R53-127/96, Statens Räddningsverk, Karlstad. ISBN 91-88890-02-3.

Graham, L.P. (1999) Modeling Runoff to the Baltic Sea. Ambio, in press.

Graham,L.P., Bergström, S. and Jacob, D. (1998) A discussion of land parameterization in hydrologic and climate models - example from the Baltic Sea Basin. Cont. to the Second International Conference on Climate and Water. Espoo, Finland, 17-20 August.

Häggström, M., Lindström, G., Cobos, C., Martfnez, J., Merlos, L., Monzo, R.D., Castillo, G., Sirias, C., Miranda, D., Granados, J., A1faro, R., Robles, E., Rodrfguez, M. and Moscote, R. (1990). Application of the HBV model for flood forecasting in six Central American rivers. SMHI, Hydrology, No. 27, Norrköping.

Johansson, B. (1994). The relationship between catchment characteristics and the parameters of a conceptual runoff model. A study in the south of Sweden. 2nd Int. Conf. on FRIEND, Braunschweig 11 - 15 Oct., 1993. IAHS Publication No. 221.

Johansson, B., & Seuna, P. (1994). Modelling the effects of wetland drainage on high flows. Aqua Fennica, Vol. 24, No. 1, 59-67.

Lindström, G. (1997) A simple automatic calibration routine for the HBV model. Nordic Hydrology, Vol. 28, No. 3, pp 153-168.

Lindström, G., & Harlin, J. (1992). Spillway design floods in Sweden. II: Application and sensitivity analysis. Hydrological Sciences Journal, 37, 5, 521 - 539.

Lindström, G., Johansson, B., Persson, M., Gardelin, M.and Bergström, S. (1997) Development and test of the distributed HBV-96 model. Journal of Hydrology 201, 272-288.

Lindström, G. and Rodhe, A. (1986) Modelling water exchange and transit times in till basins using oxygen-18. Nordic Hydrology, Vol. 17, 325 - 334.

Norstedt, U., Brandesten, C.-O., Bergstrom, S., Harlin, J., and Lindströrn, G. (1992). Re-evaluation of hydrological dam safety in Sweden. International Water Power and Dam Construction, June 1992.

Saelthun, N.R., Aittoniemi, P., Bergström, S., Einarsson, K., Jóhannesson, T., Lindström, G., Ohlsson, P-E. Thomsen, T., Vehviläinen, B. and Aamodt, K. O. (1998) Climate change impacts on runoff and hydropower in the Nordic countries. Final report from the project "Climate Change and Energy Production"Tema Nord 1988:552, Oslo.

SNA (1995) Climate, lakes and rivers. Swedish National Atlas. Bokförlaget Bra Böcker, Höganäs.

Vehvilainen, B., and Lohvansuu, J. (1991).The effects of climate change on discharges and snow cover in Finland. - Hydrological Sciences Journal, 36, 2, 4.

WMO (1986).Intercomparison of models of snowmelt runoff. Operational Hydrology Report No. 23, WMO-No. 646, WMO, Geneva

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