Rivers under pressure: Satellite data filtering techniques may underestimate Europe's water storage changes
Research in the International Journal of Hydrology Science and Technology has shown that conventional approaches to measuring water storage across Europe's complex river systems may significantly underrepresent the scale and severity of changes linked to climate change.
Earth's gravitational field is not uniform; it changes slightly depending on the presence of mountains, where the oceans are, even levels of groundwater. Indeed, when large amounts of water move through heavy rainfall, melting glaciers, or groundwater depletion, they change the local gravitational field by a small amount.
Conventionally, these changes have been detected by a technique known as satellite gravimetry. NASA's GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (Follow-On, its successor), use satellites flying in tandem to measure the tiny gravitational changes. In turn, the technique can then be used to monitor changes in natural water storage in a region, such as Europe.
However, the new research has compared traditional and data-driven filtering techniques used to interpret data from GRACE-FO. The researchers found that newer, model-independent approaches offer more accurate results, particularly in capturing droughts and floods in Europe's complex system of rivers.
The main issue with the old approach is that the data from the satellites has low spatial resolution and is affected by signal interference from neighboring regions. This latter issue, known as leakage, makes interpreting the raw data a technical challenge. To reduce noise and clear up the signals, researchers apply mathematical filters, such as Gaussian smoothing. The new study shows that this conventional method can introduce errors in regions with irregular geography, such as coastlines and densely interlaced river basins, characteristics common across the continent of Europe.
The researchers have evaluated two data-driven techniques: the "method of scale" and the "method of deviation," which use only the satellite data rather than putatively biased external hydrological models. They showed that method of scale reduces measurement uncertainty to less than 15%.
This improvement has practical implications for understanding three European river basins: the Rhone in France, the Neman on the Belarus-Lithuania border, and the Vuoksi-Neva in Finland and Russia. The new approach provides a clearer and more precise view of how water levels respond to extreme weather events.
Europe is already experiencing more frequent and severe droughts, with disproportionate effects on its smaller river basins. These catchments are limited in water storage and often highly sensitive to climatic fluctuations, but nevertheless form the backbone of many regional water systems. More accurate monitoring is critical to policymakers attempting to respond to changes.