Abstract. The GRACE satellites provide signals of total terrestrial water storage (TWS)
variations over large spatial domains at seasonal to inter-annual timescales. While the GRACE data have been extensively and successfully used to
assess spatio-temporal changes in TWS, little effort has been made to
quantify the relative contributions of snowpacks, soil moisture, and other
components to the integrated TWS signal across northern latitudes, which is
essential to gain a better insight into the underlying hydrological
processes. Therefore, this study aims to assess which storage component
dominates the spatio-temporal patterns of TWS variations in the humid regions
of northern mid- to high latitudes. To do so, we constrained a rather parsimonious hydrological model with
multiple state-of-the-art Earth observation products including GRACE TWS
anomalies, estimates of snow water equivalent, evapotranspiration fluxes,
and gridded runoff estimates. The optimized model demonstrates good
agreement with observed hydrological spatio-temporal patterns and was used
to assess the relative contributions of solid (snowpack) versus liquid
(soil moisture, retained water) storage components to total TWS variations.
In particular, we analysed whether the same storage component dominates TWS
variations at seasonal and inter-annual temporal scales, and whether the
dominating component is consistent across small to large spatial scales. Consistent with previous studies, we show that snow dynamics control
seasonal TWS variations across all spatial scales in the northern
mid- to high latitudes. In contrast, we find that inter-annual variations of
TWS are dominated by liquid water storages at all spatial scales. The
relative contribution of snow to inter-annual TWS variations, though,
increases when the spatial domain over which the storages are averaged
becomes larger. This is due to a stronger spatial coherence of snow
dynamics that are mainly driven by temperature, as opposed to spatially
more heterogeneous liquid water anomalies, that cancel out when averaged
over a larger spatial domain. The findings first highlight the effectiveness
of our model–data fusion approach that jointly interprets multiple Earth
observation data streams with a simple model. Secondly, they reveal that the
determinants of TWS variations in snow-affected northern latitudes are scale-dependent. In particular, they seem to be not merely driven by snow
variability, but rather are determined by liquid water storages on
inter-annual timescales. We conclude that inferred driving mechanisms of
TWS cannot simply be transferred from one scale to another, which is of
particular relevance for understanding the short- and long-term variability
of water resources.
Liquid water storage in snow and ice in 86 Eastern Alpine basins and its changes from 1970–97 to 1998–2006