Modelling catchment discharge in Finland in 1951–2099 using transient climate scenarios

Veijalainen, N., Vehviläinen, B., Koivusalo, H. & Korhonen, J.

Climate change impacts on discharge were estimated using hydrological modelling and transient climate scenarios for 1951-2099. The future changes in annual and seasonal discharge and annual mean floods were projected for four catchments in different parts of Finland and with different hydrological characteristics. The assessment was carried out using daily air temperature and precipitation series simulated by four Regional Climate Models (RCMs) as input to a conceptual hydrological model Watershed Simulation and Forecasting System (WSFS). The daily data from RCMs was bias corrected with the quantile-quantile mapping method to remove systematic biases in the meteorological series. Without bias correction the simulated discharge differed significantly from the observed discharge, but the bias correction improved the results considerably. The results showed that seasonal changes, especially increase in winter runoff, were the most consistent hydrological impacts of climate change with all the climate scenarios. Changes in annual mean discharge and high and low flows were only significant for some individual scenarios. The use of bias corrected RCM data as input to the hydrological model enables transient simulations and uses more information from the RCMs compared to the traditional methods, which use changes of monthly mean values of temperature and precipitation as the climate change signal for the hydrological model. However, the simulation results aggregate considerable uncertainties from the climate modelling, bias correction and the hydrological model. Further development and evaluation the bias correction methods are part of an ongoing project at the Finnish Environment Institute and the Finnish Meteorological Institute.