In the framework of the Continental Scale Experiment BALTEX (Baltic Sea Experiment) the Odra drainage basin was chosen for more detailed hydrological model investigations. The implementation of a distributed hydrological model to the Odra drainage basin and runoff simulations for the years 1992 and 93 and a extreme flood event in 1997 are presented. The hydrological model is set up on a rotated grid with a resolution of 1/6 degree (about 18 km) like the atmospheric regional scale model (REMO) used in BALTEX. It combines the land surface scheme SEWAB and a horizontal water transport scheme.
SEWAB describes the vertical energy and water fluxes between atmosphere, vegetation and soil and calculates the amount of runoff for each grid box. The soil column is divided into a variable number of soil layers, here 5 layers are assumed. Different schemes (e. g. VIC, saturation access) are available for runoff generation simulation. For base flow simulation two linear reservoirs with different half times are attached to the lowest soil layer. In this application the model is run on a 30 minute time step. To provide atmospheric forcing such as precipitation, air temperature, air pressure, wind speed, relative air humidity and solar radiation for each grid box spatial and temporal interpolation procedures are applied.
The large scale horizontal water transport scheme describes both the time runoff takes to reach the boundary of a grid box and the water transport in the river network. By this calculated runoff can be routed to the outlet of subcatchments and compared with measured discharge. The horizontal transfer processes from runoff into streamflow are described by a Unit Hydrograph within a grid box and a routing scheme in the river network, which based on a kinematic wave solution.
The Odra drainage basin is typically for Central Europe. The predominant part of the drainage basin is lowland with a precipitation of 550 mm/year. Only in the mountainous regions the precipitation can reach values around 1000 mm/year. As a result the runoff rate for the Odra drainage basin is about 145 mm/year. The modelled catchment area is 110000 sqkm. Runoff data from 26 gauging stations are available to carry out distributed parameter determination and model calibration. The meteorological data base consists of 350 precipitation, 70 climate and 22 synop stations.
The flood event in July 1997 in the Odra drainage basin was one of the biggest and most devasting floods during the past centuries. It was caused by heavy precipitation over the mountainous part of the drainage basin. Measured discharge and water stages reached maximum values at many gauging stations since the start of the observations.
The hydrological model is calibrated for the time period 1992-93. The computations for the flood event are done with these calibrated model parameters. The model results for this two time periods will be discussed with regard to hydrological processes. Because of the complex flood conditions during the extreme hydrological event in 1997 the linear routing scheme is improved to simulate also flooding processes.