Barbro Johansson

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.

Use of remote sensing to test and update simulated snow cover in hydrological models

Models of daily runoff from seasonal snowpacks and glaciers require knowledge or assumptions about the decline in snow covered area (SCA). Some semi-distributed models rely on satellite data as an input in addition to meteorological data but general purpose hydrological models with a snow component do not normally use earth observation (EO) data. EO data have the potential to verify or update SCA predictions generated by these models, but comparison is hampered by the unrealistic assumption in most models of spatially uniform snow water-equivalent (SWE) within entire zones, SO that SCA decline is stepped. Two possible solutions are either to allow a stepped SWE distribution within a sub-area, or to assume uniform melt over a non-uniform snowpack within a sub-area. In both approaches melt is converted into a reduction in SCA as well as SWE allowing snowpack depletion to be compared directly with EO data. Two examples are given in which EO data is used to verify (and in one case update) SCA. The HBV model is applied to a basin in Arctic Sweden and a recently developed glacier runoff model is applied to a basin in the Swiss Alps. Landsat TM data of both basins revealed considerably less snow than simulated by the models. TM data for the Swedish basin show that only glacier zones were 100% snow covered. Despite over-predicting SCA both models achieved very good discharge fits. It is argued that runoff models should correctly simulate the hydrological system state variables if they are to be transferred to different environments or new climate scenarios with confidence, and that EO data can play a valuable role in this.

Impacts of forest drainage on floods

Abstract Forest drainage networks are open ditches excavated for lowering the groundwater level in wetland areas to improve growing conditions for the trees. This procedure creates longer discharge surfaces for input to groundwater and shorter pathways for groundwater and overland flow to the channel system, which in turn, changes the basic runoff processes in the catchment. The hydrological effects of forest drainage have been the subject of studies for many years but the conclusions reached are conflicting because of the diversity of conditions as well as the constraints of data availability and the limited accuracy of the methods applied. This study applied three techniques (the control basin method, as well as conceptual and distributed models) for examining whether or not forest drainage increased peak flows. Small basins were investigated, and estimated parameters were transferred to a larger basin. This paper briefly presents the case study areas, the research concept and the methods applied. The c...