Streamflow intermittence in Europe: Estimating high-resolution monthly
time series by downscaling of simulated runoff and Random Forest
modeling
Abstract
Knowing where and when rivers cease to flow provides an important basis
for evaluating riverine biodiversity, biogeochemistry and ecosystem
services. We present a novel modeling approach to estimate monthly time
series of streamflow intermittence at high spatial resolution at the
continental scale. Streamflow intermittence is quantified at more than
1.5 million river reaches in Europe as the number of no-flow days
grouped into five classes (0, 1-5, 6-15, 16-29, 30-31 no-flow days) for
each month from 1981 to 2019. Daily time series of observed streamflow
at 3706 gauging stations were used to train and validate a two-step
Random Forest modeling approach. Important predictors were derived from
time series of monthly streamflow at 73 million 15 arc-sec
(~500 m) grid cells that were computed by downscaling
the 0.5 arc-deg (~55 km) output of the global
hydrological model WaterGAP, which accounts for human water use. Of the
observed perennial and intermittent station-months, 97.8% and 86.4%,
respectively, are correctly predicted. Interannual variations of the
number of intermittent months at intermittent reaches are satisfactorily
simulated, with a median Pearson correlation of 0.5. While the spatial
prevalence of intermittent reaches is underestimated, the number of
intermittent months is overestimated in dry regions of Europe where
artificial storage abounds. Our model estimates that 3.8% of all
European reach-months and 17.2% of all reaches were intermittent during
1981-2019, predominantly with 30-31 no-flow days. Although estimation
uncertainty is high, our study provides, for the first time, information
on the continent-wide dynamics of intermittent rivers and streams.