GloFAS v2.2 forecast skill is evaluated using the Continuous Ranked Probability Skill Score (CRPSS) for ECMWF-ENS 20-year reforecasts against both persistence and climatology benchmark forecasts with respect to GloFAS-ERA5 river discharge reanalysis (also known as  forced simulations (sfo))  as proxy observations at n=5997 GloFAS diagnostic river points.  Further details on the evaluation methodology can be found here:  GloFAS forecast skill and in the following scientific paper (in review):

Ensemble forecast skill with lead time

Overall, GloFAS version 2.1/2.2 is skilful at the global scale for the majority of catchments across all lead times analysed (Figure 1). The CRPSS against persistence decays exponentially as a function of lead time out to around 15 days lead time, then stabilises reaching a minimum of 0.48 by day 22. The CRPSS at day 1 is 0.96 (interquartile range of 0.88 to 0.99), day 3 = 0.82 (0.62, 0.94), day 5 = 0.70 (0.50, 0.87), day 10 = 0.55 (0.37, 0.72). The CRPSS against climatology begins higher than persistence and decays continuously towards day 30. At day 15 is 0.47 (0.27, 0.68), day 20 = 0.37 (0.17, 0.58), day 25 = 0.30 (0.11, 0.49), day 30 = 0.25 (0.06, 0.43).

As the global median CRPSS against climatology becomes lower than against persistence (0.49 versus 0.50, respectively) from day 14, we present and discuss all forecast skill from short (1- to 3-day) to medium range (5- to 10-day) lead times calculated against the persistence benchmark forecast, and from extended lead times (15- to 30-days) calculated against the climatology benchmark forecast.

Figure 1: Skill of GloFAS 2.1/2.2 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against persistence (red line) and climatology (blue line) benchmarks from 1- to 30-day lead times with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. The interquartile range of CRPSS values at each lead time are shown by semi-transparent bands.

Spatial distribution of skill

Short to medium lead time (river discharge persistence as benchmark)

At short lead times (1 and 3 days), GloFAS is skilful compared to the persistence benchmark forecast in over 96 % of catchments (Figure 2). In the medium range at day 5, GloFAS remains skilful for 93 % of catchments. Regions with the highest skill (CRPSS >= 0.8) include South America, especially the Amazon basin, the US, southern Africa, central Asia, and eastern Australia. There are notable clusters of catchments with negative skill (i.e. CRPSS < 0) mainly located in higher northern latitudes above 55° N as well as in the Congo River Basin. By day 10, the strength of skill has decreased, but 89 % of catchments remain skilful (i.e. CRPSS > 0).

Figure 2:  Skill of GloFAS 2.1/2.2 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against the persistence benchmark for short to medium lead times (1- (a), 3- (b), 5- (c), and 10-days (d)) with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. Optimum value of CRPSS is 1. Blue (red) dots show catchments with positive (negative) skill.

Extended lead time (river discharge climatology as benchmark)

For extended lead times shown in Figure 3, GloFAS is skilful compared to the climatology benchmark forecast in 89 % of catchments by day 15, reducing to 86 %, 83 % and 80 % by lead times 20-, 25- and 30-days, respectively. The regions of highest skill are similar to those for short and medium range, with areas of negative skill expanding to tropical Africa, a large region in central and northern Asia, and western coast of South America.

Figure 3:  Skill of GloFAS 2.1/2.2 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against the climatology benchmark for extended lead times (15- (a), 20- (b), 25- (c), and 30-days (d)) with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. Optimum value of CRPSS is 1. Blue (red) dots show catchments with positive (negative) skill.