New runoffmap for Norway by combining a precipitation runoff model with geostatistical interpolation

We present a new runoff map for Norway for the reference period 1991-2020. A new framework combing precipitation runoff-modelling with geostatistical interpolation was used. The precipitation-runoff model WASMOD was used to simulate runoff on a 1x1 km grid covering all Norway and nearby catchments in Sweden and Finland. The parameters of WASMOD were conditioned on land-use and climate classes and calibrated globally using data from 215 streamflow stations. Figure 1a) shows the runoff estimates from WASMOD that are biased when compared to observations (Figure 2a) .

To correct the biases in the gridded simulations we used runoff observations from 732 locations of which 198 had data covering the whole period. A geostatistical approach was used to estimate the 30 year mean annual runoff from short records (1-29 years) for 482 catchments. For 45 catchments with substantial glacier coverage or regulation capacity, a manual extension of mean annual runoff was performed. Another 7 stations with between 25 and 30 years of data were included.

The biases from WASMOD were corrected by simple linear regression using the raw runoff map as a covariate and the runoff observations as the dependent variable. The regression coefficients were modelled as spatial fields using a geostatistical Bayesian approach where SPDE and INLA ensured fast Bayesian inference. The mean annual runoff after the correction is shown in Figure 1b), and in Figure 1c) the difference between the two previous maps is shown. Figure 2b shows a scatter plot of corrected and observed runoff. The scatter is now much smaller.

Figure 1. Mean annual runoff from a) the Wasmod model and b) after the correction. The difference in runoff between a) and b) is shown in c)

The new framework was evaluated by cross-validation. On average this new approach outperformed a WASMOD when predicting runoff for ungauged and partially gauged catchments and reduced the RMSE by 42% . Scatter plot of predicted runoff is shown in Figure 2c).

Figure 2. Scatterplot of mean annual runoff for a) WASMOD, b) corrected values, and c) predicted runoff , versus observations in 146 catchments in a split sample cross-validation test