Seasonal to interannual ensemble streamflow forecasts for Ceara, Brazil: Applications of a multivariate, semiparametric algorithm

A semiparametric approach for forecasting streamflow at multiple gaging locations on a river network conditional on climate precursors is developed. The strategy considers statistical forecasts of annual or seasonal streamflow totals at each of the sites and their disaggregation to monthly or higher resolution flows using a k nearest neighbor resampling approach that maintains space-time consistency across the sites and subperiods. An application of the approach to forecasting inflows at six reservoirs in the state of Ceara in northeastern Brazil is presented. The climate precursors used are the NINO3 index for the El Nino-Southern Oscillation and an equatorial Atlantic sea surface temperature index. Forecasts of January through December streamflow are made at three lead times: in January of the same year and in October and July of the preceding year. The skill of the ensemble forecasts generated is evaluated on subsets of the historical data not used for model building. Correlations with the equatorial Atlantic index and with NINO3 translate into useful streamflow forecasts for the next 18 months of reservoir operation and water management.