Sten Bergström

Development and test of the distributed HBV-96 hydrological model

Abstract A comprehensive re-evaluation of the HBV hydrological model has been carried out. The objectives were to improve its potential for making use of spatially distributed data, to make it more physically sound and to improve the model performance. The new version, HBV-96, uses subbasin division with a typical resolution of 40 km z, although any resolution can be used. In addition, each subbasin is divided into elevation bands, vegetation and snow classes. Automatic weighting of precipitation and temperature stations was introduced and a new automatic calibration scheme was developed. The modifications led to significant improvements in model performance. In seven test basins the average value of the efficiency criterion R 2 increased from 86 to 89%, with improvements in both the calibration and verification periods.

Hydrological change - Climate change impact simulations for Sweden

Climate change resulting from the enhanced greenhouse effect is expected to give rise to changes in hydrological systems. This hydrological change, as with the change in climate variables, will vary regionally around the globe. Impact studies at local and regional scales are needed to assess how different regions will be affected. This study focuses on assessment of hydrological impacts of climate change over a wide range of Swedish basins. Different methods of transferring the signal of climate change from climate models to hydrological models were used. Several hydrological model simulations using regional climate model scenarios from Swedish Regional Climate Modelling Programme (SWECLIM) are presented. A principal conclusion is that subregional impacts to river flow vary considerably according to whether a basin is in northern or southern Sweden. Furthermore, projected hydrological change is just as dependent on the choice of the global climate model used for regional climate model boundary conditions as the choice of anthropogenic emissions scenario.

Runoff trends in Sweden 1807-2002

Abstract Abstract Time series of annual runoff volumes and annual and seasonal flood peaks in Sweden were analysed. The study included a total of 61 discharge series, with emphasis on the period 1901–2002. Three wet decades stand out in the 20th century: the 1920s, 1980s and 1990s, with a runoff anomaly of +8%. The 1970s were very dry. In a short perspective, both runoff volumes and flood magnitude increased substantially between 1970 and 2002, but similar conditions were experienced in the 1920s. The linear regression line for the average runoff from all of Sweden increased by 5% over the past century, but the trend was not statistically significant. The runoff in the 19th century was in fact even higher than in recent decades, although temperatures were lower. Flood levels increased slightly more than annual runoff volumes in northern Sweden. However, flood peaks in old data are probably underestimated. The largest increase was consequently found in less reliable data series. It is therefore difficult to conclude that flood levels are actually increasing.

On the scale problem in hydrological modelling

The problem of scales and particularly the modelling of macro or continental scale catchments in hydrology is addressed. It is concluded that the magnitude of the scale problem is related to the specific hydrologic problem to be solved and to the scientific approach and perspective of the modeller. A distributed modelling approach, based on variability parameters, is suggested for modelling of soil moisture dynamics and runoff generation. It is shown that the parameters of such an approach are relatively stable over a wide range of scales. An example of the application of a standard version of the Swedish HBV hydrological model to the continental scale catchment of the Baltic Sea is shown and its usefulness is discussed.

The Swedish Regional Climate Modelling Programme, SWECLIM: a review.

The Swedish Regional Climate Modelling Programme, SWECLIM, was a 6.5-year national research network for regional climate modeling, regional climate change projections and hydrological impact assessment and information to a wide range of stakeholders. Most of the program activities focussed on the regional climate system of Northern Europe. This led to the establishment of an advanced, coupled atmosphere-ocean-hydrology regional climate model system, a suite of regional climate change projections and progress on relevant data and process studies. These were, in turn, used for information and educational purposes, as a starting point for impact analyses on different societal sectors and provided contributions also to international climate research.

Spillway design floods in Sweden: I. New guidelines

The new Swedish guidelines for the estimation of design floods for dams and spillways are presented, with emphasis on high-hazard dams. The method is based on a set of regional design precipitation sequences, rescaled for basin area, season and elevation above sea level, and a full hydrological model. A reservoir operation strategy is also a fundamental component of the guidelines. The most critical combination of flood generating factors is searched by systematically inserting the design precipitation sequence into a ten year climatological record, where the initial snowpack has been replaced by a statistical 30-year snowpack. The new guidelines are applicable to single reservoir systems as well as more complex hydroelectric schemes, and cover snowmelt floods, rain floods and combinations of the two. In order to study the probabilities of the computed floods and to avoid regional inconsistencies, extensive comparisons with observed floods and frequency analyses have been carried out.

Simulation of runoff and nitrogen leaching from two fields in southern Sweden

ABSTRACT A conceptual model for the simultaneous computation of runoff and inorganic nitrogen leaching is described. The model is semi-empirical with coefficients that are calibrated against observed data. Two applications of the model, using well-controlled data from small fields of arable land in southern Sweden, are demonstrated. These show that a major part of the variations of concentrations of inorganic nitrogen in runoff water can be explained by this model approach. Finally, the sensitivity and limitations of this type of model are discussed.

Modelling Climate Change Impacts on Water Resources in the Swedish Regional Climate Modelling Programme

The Swedish Regional Climate Modelling Programme (SWECLIM) focuses on interpretation of climate scenarios for the Nordic Region. Water resources studies include hydrological model simulations both at the large scale to simulate trends for the entire Baltic Basin and at smaller basin scales to simulate local impacts in Sweden. Global climate model simulations (GCMs) provide lateral boundary conditions to drive the finer resolution Rossby Centre regional atmospheric climate model (RCA) in dynamical downscaling. Two different GCMs—HadCM2 and ECHAM4/OPYC3—have thus far been used. Analyses of future climates are created from differences in 10-year time slices between RCA control runs of the present climate and RCA scenario climate runs with transient greenhouse gas simulations. These differences drive the offline hydrological impacts assessment models. Both of the RCA climate scenarios show overall increases in temperature and precipitation for the Nordic Region, although spatial and temporal distribution varies between them. Hydrological model scenario simulations show a strong decrease in snowmelt peak river discharge. Modelled changes to average annual freshwater inflow to the Baltic Sea vary from +8% to 21% from present day conditions. The interface between atmospheric models and hydrological impact models is a weak link in the process, as is representation of evapotranspiration in the hydrological models for a future climate.

Climate change and water resources in Sweden — analysis of uncertainties

Simulations of the impacts of climate change on water resources in Sweden are produced within the Swedish Regional Climate Modelling Programme, SWECLIM. The impact studies are based on a combination of global climate models (GCMs), a regional climate model and a hydrological runoff model. The two different GCMs used so far are the UKMO HadCM2 from the Hadley Centre and the ECHAM4/OPYC3 of the Max Planck Institute for Meteorology. The regional climate model, RCA, was developed at the Rossby Centre of the Swedish Meteorological and Hydrological Institute (SMHI) and is a modified version of the international HIRLAM meteorological forecast model. The RCA model performs downscaling from GCM scenarios on a time horizon of 50 to 100 years. Based on the RCA scenarios, water resources scenarios were produced with the HBV hydrological runoff model developed at the SMHI. Two different methods for estimation of evapotranspiration in the hydrological model were used. Neither of the methods takes into account the possible feedback from changing land-use, vegetation dynamics or changing plant use of water at increasing CO2 concentrations in the atmosphere. The impacts on water resources were simulated from differences between control runs and scenario runs of the RCA model for a number of selected test basins covering the major climate regions in Sweden. Changes in runoff totals, runoff regimes and extreme values were analysed with focus on the uncertainties introduced by the choice of global climate model, routines for estimation of evapotranspiration in the hydrological model and methods applied in the interface between the models. It was further analysed how these choices affect the statistical return periods of future extremes in a design situation.

Measurement of areal water equivalent of snow by natural gamma radiation—experiences from northern Sweden

ABSTRACT The Kultsjon basin in northern Sweden has been the subject of detailed studies of the potential of airborne γ-ray spectrometry for snow mapping and hydrological forecasting since the spring of 1980. A brief introduction to the theory behind this technique is given. Results from five melt seasons are presented, and problems and uncertainties are discussed. A verification against ground “truth” based on snow courses, runoff measurements, and hydrological models is given. Finally it is shown how the data can be used for updating and improving more conventional forecasting models.