WP3: Hydrological change
When dealing with data poor catchments typically found in third world countries remote sensing data are often the best data source both with respect to precipitation and evapotranspiration. However, a serious problem when using remote sensing based precipitation data is that these data are only available at large spatial and temporal scales, typically daily values at 25 km resolutions. In mountainous areas the precipitation is controlled by orographic effects resulting in large inhomogeneities in actual rainfall intensity within the resolution of the remote sensing image. Therefore, downscaling is required, but statistical downscaling methods like those suggested below for application in Denmark are not useful, because time series of precipitation measurement within the catchment is inadequate and because they do not directly address orographic effects. To enable use of hydrological models to assess hydrological change in data poor areas the study will investigate new remotely sensed data products for topography and new data products for precipitation that can be obtained for most parts of the world. A methodology will be developed for downscaling precipitation in areas with significant orographic effects based on digital elevation models and a limited number of ground rainfall stations. Additionally, the impact of climate change will be quantified using results from climate models and applying the methodologies developed in the project.
To improve the description of the exchange of water vapour and energy between the land surface and the atmosphere the national DK hydrological model will be coupled to the regional climate model HIRHAM. A coupled climate-hydrological model at a national scale will be novel for the scientific community and is expected to move the ability to carry out accurate hydrological impact assessment significantly forward. A full coupling between the two models will ensure the best possible estimates of water exchange (especially precipitation from the climate model and evapotranspiration from the hydrological model) between the two spheres. However, bias-correction of the climate model results, especially precipitation, might be required for the coupled model. An advanced geostatistical downscaling procedure of the low resolution (25 x 25 km) climate model results will be developed based on rainfall observations from rain gauges and radar measurements. The analyzed precipitation events will be subdivided into classes according to the weather system that generated the precipitation (e.g. frontal rain, convection rain). It is expected that the spatial distribution of precipitation and hence the downscaling procedure will be different for the alternative weather systems. Additionally, it will be examined if the bias-correction depends on weather type.
Finally, it will be investigated if the algorithm developed specifically to handle orographic effects can be utilised to improve the above statistical downscaling method to be developed for use over Denmark.
GEUS is in charge of this work package.
Contact: Jens Christian Refsgaard
Precipitation (Post doc project)
Contact: Mads Olander Rasmussen
International test case
Contact: Flemming Fogh Pedersen
Coupled model for Denmark (PhD project)
Contact: Morten Andreas Dahl Larsen
Conversion of DK model to coupled model
Contact: Jens Christian Refsgaard