On the utility of operational precipitation forecasts to served as input for streamflow forecasting

This article studies the utility of quantitative forecast precipitation for the prediction of daily streamflow. Application is made over the Rhone basin, which was included in the Gewex–Rhone program. The precipitation forecasts of the two numerical weather prediction models operationally used in France, ARPEGE and ALADIN, are tested. The riverflow forecast is made using the precipitation forecast as input to the one-way atmosphere–hydrology coupled model SAFRAN–ISBA–MODCOU (SIM). Such a forecast is very sensitive to the initialisation of soil moisture and snow-pack. Therefore, two kinds of streamflow forecast were made: first, a plain forecast, for which the initial conditions are taken from the guess, and second, a re-initialised forecast, for which the initial conditions are set according to a reference run. This reference run is obtained using 1200 daily observed precipitation, interpolated in time and space by SAFRAN. First, the quality of the precipitation forecasts is checked over the Rhone basin for the period August 1997–July 1998 using the SAFRAN analysis as a reference. Then, the SIM system used to forecast riverflow is briefly presented. The predicted riverflows are compared both to the observations at 22 streamgage locations and to the reference run. The results show that the annual average of the average discharge errors at the 22 streamgages can reach 20% in the forecast without re-initialisation, but that this error is reduced significantly when the model is properly initialised. This is due to the fact the re-initialisation of the soil moisture and snow-pack according to the reference run minimizes influences of the previous precipitation forecast error. It is shown that the use of precipitation forecast as input of the SIM system can be of interest to forecast the progress of the long-duration floods of the main stations of the Rhone basin.