Investigation of hydrometeorological forecasting scenarios over Peloponnese

The combined use of weather forecasting and hydrological models in flood risk estimations is an established technique, with many successful applications worldwide. However, most known flood forecasting systems have been established in large rivers with perpetual flow. In Greece there are mostly small basins with variable river flow, which are often affected by flash floods. In this paper we investigate the use of hydrometeorological forecasting in these basins by emphasizing in two issues: (a) which modelling approach can credibly represent the complex dynamics of basins with highly variable runoff; and (b) which transformation of point-precipitation forecasts provides the most reliable estimations of spatially aggregated data, to be used as inputs to semi-distributed hydrological models. The basins we investigate are Nedontas and Lousios basins, in the region of Peloponnese and the two simulated episodes of rainfall take place in 17/1/13 and 13/11/13. This employs conjunctive modelling of surface and groundwater flows and their interactions (percolation, infiltration, underground losses), which are key processes in river basins with significant variability of runoff. The model was calibrated against hourly flow data by the hydrometric stations of the basins for a 3-year period (2011-2014). The forecast scenarios of the two studied episodes of rainfall are produced with 6-hour time step and provided by the National Observatory of Athens with the numerical weather prediction model WRF. The forecast scenarios are spatial aggregated with two methods: a) through the optimization of the participation of the four nearest grid points to the stations of the basins and b) through the use of the average of all the grid points within the area of the two basins (surface approach). The above aggregated data are used as inputs in the hydrological model, in order to produce the runoff time series in the output of the basins. These time series of the runoff are compared with the observed values and this comparison is used to indicate the best method of runoff forecast.